Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/PhysRevB.104.214413

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.104.214413/fulltext

Authorship：Lead author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.90.044713

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.103.l100408

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.103.L100408/fulltext

Authorship：Last author,　Corresponding author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

DOI： 10.7566/jpsj.89.084701

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Authorship：Last author   Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society (APS)  

DOI： 10.1103/physrevb.102.075118

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.102.075118/fulltext

Language：English   Publishing type：Research paper (scientific journal)   Publisher：Springer Science and Business Media LLC  

DOI： 10.1038/s41467-020-17776-3

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Other Link： http://www.nature.com/articles/s41467-020-17776-3

Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.3390/app9010070

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.7566/JPSJ.87.124703

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1088/1361-6455/aad40a

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

Photoinduced dynamics in an excitonic insulator is studied theoretically by using a two-orbital Hubbard model on the square lattice where the excitonic phase in the ground state is characterized by the BCS-BEC crossover as a function of the interorbital Coulomb interaction. We consider the case where the order has a wave vector Q=(0,0) and photoexcitation is introduced by a dipole transition. Within the mean-field approximation, we show that the excitonic order can be enhanced by the photoexcitation when the system is initially in the BEC regime of the excitonic phase, whereas it is reduced if the system is initially in the BCS regime. The origin of this difference is discussed from behaviors of momentum distribution functions and momentum-dependent excitonic pair condensation. In particular, we show that the phases of the excitonic pair condensation have an important role in determining whether the excitonic order is enhanced or not.

DOI： 10.1103/PhysRevB.97.115105

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：Physical Society of Japan  

Photoinduced charge dynamics in dimerized systems is studied on the basis of the exact diagonalization method and the time-dependent Schrödinger equation for a one-dimensional spinless-fermion model at half filling and a two-dimensional model for κ-(bis[ethylenedithio]tetrathiafulvalene)2X [κ-(BEDT-TTF)2X] at three-quarter filling. After the application of a one-cycle pulse of a specifically polarized electric field, the charge densities at half of the sites of the system oscillate in the same phase and those at the other half oscillate in the opposite phase. For weak fields, the Fourier transform of the time profile of the charge density at any site after photoexcitation has peaks for finite-sized systems that correspond to those of the steady-state optical conductivity spectrum. For strong fields, these peaks are suppressed and a new peak appears on the high-energy side, that is, the charge densities mainly oscillate with a single frequency, although the oscillation is eventually damped. In the two-dimensional case without intersite repulsion and in the one-dimensional case, this frequency corresponds to charge-transfer processes by which all the bonds connecting the two classes of sites are exploited. Thus, this oscillation behaves as an electronic breathing mode. The relevance of the new peak to a recently found reflectivity peak in κ-(BEDT-TTF)2X after photoexcitation is discussed.

DOI： 10.7566/JPSJ.87.044708

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Language：English   Publishing type：Research paper (scientific journal)  

DOI： 10.1038/s41566-018-0194-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Using a high-frequency expansion in periodically driven extended Hubbard models, where the strengths and ranges of density-density interactions are arbitrary, we obtain the effective interactions and bandwidth, which depend sensitively on the polarization of the driving field. Then, we numerically calculate modulations of correlation functions in a quarter-filled extended Hubbard model with nearest-neighbor interactions on a triangular lattice with trimers after monocycle pulse excitation. We discuss how the resultant modulations are compatible with the effective interactions and bandwidth derived above on the basis of their dependence on the polarization of photoexcitation, which is easily accessible by experiments. Some correlation functions after monocycle pulse excitation are consistent with the effective interactions, which are weaker or stronger than the original ones. However, the photoinduced enhancement of anisotropic charge correlations previously discussed for the three-quarter-filled organic conductor alpha-(bis[ethylenedithio]-tetrathiafulvalene)(2)I-3 [alpha-(BEDT-TTF)(2)I-3] in the metallic phase is not fully explained by the effective interactions or bandwidth, which are derived independently of the filling.

DOI： 10.7566/JPSJ.86.064702

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Polarization selectivity of light-field-induced charge localization was investigated in an organic metal alpha-(BEDT-TTF)(2)I-3 with a triangular lattice. Dependences of transient reflectivity spectra on polarizations of the 7-fs pump and probe lights indicate that a short-range charge order (CO) is induced efficiently from the metallic phase for the pump polarization perpendicular to the 1010-type CO axis. A numerical solution of a time-dependent Schrodinger equation clarified that the 1010 CO is induced by field-induced re-distribution of charges cooperating with competing intersite Coulomb repulsions in the triangular lattice.

DOI： 10.1103/PhysRevB.95.201105

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

To explore nontrivial photoinduced modulations of charge correlations, we theoretically study photoinduced dynamics in quarter-filled extended Hubbard models with competing intersite repulsive interactions on triangular lattices with trimers, where the end points are crystallographically equivalent. The exact diagonalization method is used and the time-dependent Schrdinger equation is numerically solved during and after photoexcitation. Time-averaged double occupancy and intersite density-density correlations can be interpreted as due to effective on-site and intersite repulsive interactions, respectively, relative to transfer energies. In the case where the intersite repulsive interactions compete with each other, the anisotropy of their effective interactions can be enhanced with the help of the trimers, irrespective of whether the trimers are linear or bent. In particular, in the case where the arrangement of the trimers is close to that in alpha-(bis[ethylenedithio]-tetrathiafulvalene)(2)I-3 [alpha-(BEDT-TTF)(2)I-3] in the metallic phase, the effective on-site repulsion is enhanced relative to the transfer energies. The relevance of this theoretical finding to the experimentally observed optical freezing of charge motion is discussed.

DOI： 10.7566/JPSJ.86.024711

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OSA  

DOI： 10.1364/up.2016.utu4a.26

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：OSA  

DOI： 10.1364/up.2016.utu4a.24

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DOI： 10.1103/PhysRevB.93.165126

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.71.2.0_1233

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.71.2.0_1240

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Many-electron dynamics induced by a symmetric monocycle electric-field pulse of large amplitude is theoretically investigated in one-and two-dimensional half-filled extended Hubbard models on regular lattices (i.e., without dimerization) using the exact diagonalization method for small systems and the Hartree-Fock approximation for large systems. The formation of a negative-temperature state and the change from repulsive interactions to effective attractive interactions are shown to be realized for a wide region of the field amplitude and the excitation energy. For a nonnegligible intersite repulsive interaction, the numerical results are consistent with the fact that the phase separation between charge-rich and charge-poor regions is caused by the corresponding effective attraction.

DOI： 10.7566/JPSJ.84.094705

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

The excited state dynamics of correlated electron and electron-phonon systems triggered by an oscillating electric-field pulse of large amplitude are theoretically investigated. A "negative-temperature" state and inversion of electron-electron and electron-phonon interactions are induced even by a symmetric monocycle pulse. This fact is numerically demonstrated, using the exact diagonalization method, in a band-insulator phase of one-dimensional three-quarter-filled strongly dimerized extended Peierls-Hubbard and Holstein models. When the total-energy increment is maximized as a function of the electric field amplitude, the occupancy of the bonding and antibonding orbitals is inverted to produce a negative-temperature state. Around this state, the dependences of time-averaged electron-electron and electron-phonon correlation functions on interaction parameters are opposite to those in the ground state.

DOI： 10.7566/JPSJ.84.054702

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Springer Science and Business Media, LLC  

Ultrafast optical response of organic metal (TMTTF)2AsF6 induced by 1.5 cycle (7 fs) infrared pulse was investigated. Intense coherent oscillation of correlated charge (18 fs) grows in the time scale of 50 fs, reflecting the spontaneous- formation of the electronic coherence before the electronic thermalization.

DOI： 10.1007/978-3-319-13242-6_59

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.1.0_3062

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.1.0_1432

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.2.0_1208

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

DOI： 10.11316/jpsgaiyo.70.1.0_1521

CiNii Books

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

A negative differential resistance (NDR) in a one-dimensional band insulator attached to electrodes is investigated. We systematically examine the effects of an electrode bandwidth and a potential distribution inside the insulator on current-voltage characteristics. We show that, in uncorrelated systems, the NDR is generally caused by a linear potential gradient as well as by a finite electrode bandwidth. In particular, the former reduces the effective bandwidth of the insulator for elastic tunneling by tilting its energy band, so that it brings about the NDR even in the limit of large electrode bandwidth.

DOI： 10.7566/JPSJ.83.124704

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：NATURE PUBLISHING GROUP  

Dynamical localization, that is, reduction of the intersite electronic transfer integral t by an alternating electric field, E(omega), is a promising strategy for controlling strongly correlated systems with a competing energy balance between t and the Coulomb repulsion energy. Here we describe a charge localization induced by the 9.3MVcm(-1) instantaneous electric field of a 1.5 cycle (7 fs) infrared pulse in an organic conductor alpha-(bis[ethylenedithio]-tetrathiafulvalene) I-2(3). A large reflectivity change of >25% and a coherent charge oscillation along the time axis reflect the opening of the charge ordering gap in the metallic phase. This optical freezing of charges, which is the reverse of the photoinduced melting of electronic orders, is attributed to the similar to 10% reduction of t driven by the strong, high-frequency (omega >= t/(h) over bar) electric field.

DOI： 10.1038/ncomms6528

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The photoinduced phase formation in a strongly correlated crystal (EDO-TTF)(2)PF6 (EDO-TTF: 4,5-ethylenedioxytetrathiafulvalene) is investigated using a 12 fs laser pulse. The formation time is determined as 40 fs with observation of coherence of electron-phonon coupled excited states prior to formation. The temperature-independent dephasing time is determined as similar to 22 fs up to 180 K and the frequency of phonon oscillation is similar to 38 THz, corresponding to the intramolecular vibrations in EDO-TTF. The phase formation is coherently controlled by relative-phase-controlled two-pulse excitation.

DOI： 10.1103/PhysRevB.89.161102

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Publishing type：Research paper (international conference proceedings)   Publisher：Journal of the Physical Society of Japan  

DOI： 10.7566/JPSCP.1.012044

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We theoretically study the field-intensity dependence of charge transfers driven by continuous-wave and pulsed lasers in a two-dimensional crystal consisting of molecular dimers by numerically solving the time-dependent Schrodinger equation for an extended Hubbard model. Generally, electronic transfer integrals are regarded as renormalized when an oscillating external field is applied, as far as the electronic dynamics averaged over the oscillation period is concerned. The cases where effective transfer integrals vanish are known as dynamic localization. Interdimer charge transfers driven by continuous-wave lasers are thus governed by effective interdimer transfer integrals. After the field is switched off, the dynamic localization is no longer relevant. Then, we show that interdimer charge transfers driven by pulsed lasers of energy resonant with an intradimer transition are governed by an effective intradimer transfer integral. The total-energy increment depends on how the intradimer transfer integral is renormalized. The same holds for interdimer charge transfers. This interdimer dynamics governed by effective intradimer parameters is evident even for one-and two-cycle pulses, suggesting possible control of photoinduced charge-order melting.

DOI： 10.7566/JPSJ.83.024706

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We theoretically investigate charge order and nonlinear conduction in the quasi-two-dimensional organic conductor β-(meso-DMBEDT-TTF) 2PF6 (DMBEDT-TTF=dimethylbis(ethylenedithio) tetrathiafulvalene). Within the Hartree-Fock approximation, we study the effects of structural distortion on the experimentally observed checkerboard charge order and its bias-induced melting by using an extended Hubbard model with Peierls- and Holstein-types of electron-lattice interactions. The structural distortion is important in realizing the charge order. The current-voltage characteristics obtained by a nonequilibrium Green's function method indicate that a charge-ordered insulating state changes into a conductive state. Although the charge order and lattice distortions are largely suppressed at a threshold voltage, they remain finite even in the conductive state. We discuss the relevance of the results to experimental observations, especially to a possible bias-induced metastable state. © 2013 IOP Publishing Ltd.

DOI： 10.1088/0953-8984/25/46/465603

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

To elucidate interconnections of intra- and interdimer electronic and phonon degrees of freedom during the photoinduced melting of charge order in the quasi-two-dimensional metal complex Et2Me2Sb[Pd(dmit)(2)](2), we study the photoinduced charge dynamics and transient C C and intermonomer vibration frequencies in an extended Peierls-Hubbard model. The charge order consisting of neutral and divalent dimers is realized by strong intradimer electron-phonon interactions. After photoexcitation inducing intradimer electronic transitions, the structures of the neutral and divalent dimers are changed into that of monovalent dimers by couplings between electrons and intermonomer vibrations. Then, the electronic energy levels of these dimers resonate with each other and a large charge transfer occurs between them. Some C=C vibrations are selectively coupled with intradimer charge transfer, while other C=C vibrations with interdimer charge transfer. During the charge-order melting, the frequencies of the C=C vibrations are modulated and deviate from the charge-density-phonon-frequency relations in the equilibrium state. In the hierarchical structure involving the intra-and interdimer electron-phonon degrees of freedom, their concerted action promotes the charge-order melting.

DOI： 10.7566/JPSJ.82.094716

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The current-voltage characteristics and mechanism of bias-induced breakdown in one-dimensional models for band and Mott insulators are investigated theoretically by using nonequilibrium Green's functions. We attach the models to metallic electrodes, the effects of which are incorporated into the self-energy. For the models of both the band and the Mott insulators with lengths L C, the bias voltage induces a breakdown of the insulating state, and the breakdown mechanism shows a crossover depending on L C. When L C is smaller than the correlation length ξ = W/Δ, the threshold is determined basically by the bias V th ∼ Δ, where W and Δ are the bandwidth and the energy gap, respectively. For systems with L C ≫ ξ, the threshold is governed by the electric field, V th/L C, which is consistent with a Landau-Zener-type breakdown V th/L C ∝ Δ2/W. © 2013 The Korean Physical Society.

DOI： 10.3938/jkps.62.2164

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER/PLENUM PUBLISHERS  

We investigate the spatial distribution of temperature induced by a dc current in a two-dimensional electron gas (2DEG) subjected to a perpendicular magnetic field. We numerically calculate the distributions of the electrostatic potential I center dot and the temperature T in a 2DEG enclosed in a square area surrounded by insulated-adiabatic (top and bottom) and isopotential-isothermal (left and right) boundaries (with I center dot (left)< I center dot (right) and T (left)=T (right)), using a pair of nonlinear Poisson equations (for I center dot and T) that fully take into account thermoelectric and thermomagnetic phenomena, including the Hall, Nernst, Ettingshausen, and Righi-Leduc effects. We find that, in the vicinity of the left-bottom corner, the temperature becomes lower than the fixed boundary temperature, contrary to the naive expectation that the temperature is raised by the prevalent Joule heating effect. The cooling is attributed to the Ettingshausen effect at the bottom adiabatic boundary, which pumps up the heat away from the bottom boundary. In order to keep the adiabatic condition, downward temperature gradient, hence the cooled area, is developed near the boundary, with the resulting thermal diffusion compensating the upward heat current due to the Ettingshausen effect.

DOI： 10.1007/s10909-012-0852-8

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Language：English   Publishing type：Part of collection (book)   Publisher：Springer-Verlag Wien  

Mechanisms of a variety of charge and lattice ordered phases observed in MMX compounds are theoretically studied by using a one-dimensional two-band three-quarter-filled extended Peierls-Hubbard model. In R4[Pt2(pop)4I]nH2O [R = Na, K, NH4, (CH3(CH2)7)2NH2, etc., pop = P2O5H 2 2- ] containing charged MMX chains, three electronic phases are suggested by experiments. We find that the variation of the electronic phases originates not only from competition between site-diagonal electron-lattice and electron-electron interactions but also from competition between short-range and long-range electron-electron interactions. On the other hand, in Pt2(RCS2)4I (R = CH3, n-C4H9) containing neutral MMX chains, a site-off-diagonal electron-lattice interaction and the absence of counterions are found to be crucial to produce the alternate-charge-polarization phase. The optical conductivity spectra are also studied, which directly reflect the electronic phases. A photoinduced transition has been found in a MMX compound, R4[Pt2(pop)4I]nH2O (R = (C2H5)2NH2). Its mechanism is theoretically studied by solving the time-dependent Schrödinger equation. Above a threshold in the photoexcitation intensity, a transition takes place from the charge-density-wave phase to the charge-polarization phase. The threshold-intensity dependence on the relative stability of these phases is explained qualitatively by their diabatic potentials. However, the transition in the opposite direction is hardly realized and needs careful consideration of different charge transfer processes.

DOI： 10.1007/978-3-7091-1317-2_12

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We have theoretically studied the ground-state properties of the molecular compound, Et2Me2Sb[Pd(dmit)(2)](2), which shows a dimer-Mott character in Pd(dmit)(2) layers at high temperature and a charge order mainly stabilized by an electron-phonon interaction at low temperature. An effective extended Peierls-Hubbard model is constructed with intra- and intermolecular electronic and phonon degrees of freedom. Using mean-field approximation, the energies and optimized structures are calculated for isolated neutral, monovalent, divalent Pd(dmit)(2) dimers and their two-dimensional crystallized states. The optical conductivities of the crystallized states are calculated by a single-configuration interaction method. For intramolecular vibrations, normal-mode analysis is performed, which is useful to explain the recent experimental results. Molecular vibrations with different symmetries are coupled to different combinations of electrons and holes within a molecule, so that their frequencies depend on the molecular charge in different manners. This information will be useful to analyze photoinduced transient states, where the relation between their frequencies and the molecular charge is modified from the equilibrium counterpart.

DOI： 10.7566/JPSJ.82.024701

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

The dependence of the current-induced cooling effect on the electron mobility (e) is explored for a two-dimensional electron gas (2DEG) subjected to a perpendicular magnetic field. We calculate the distributions of the electrochemical potentials and the temperatures under a magnetic field, fully taking account of thermoelectric and thermomagnetic phenomena. Whereas the electrochemical potential and the electric current remain qualitatively unchanged, the temperature distribution exhibits drastic mobility dependence. The lower-mobility system has cold and hot areas at opposite corners, which results from the heat current brought about by the Ettingshausen effect in the vicinity of the adiabatic boundaries. The cooling effect is intensified by an increase in (e). Intriguingly, the cold and hot areas change places with each other as the mobility (e) is further increased. This is because the heating current on the adiabatic edges due to the Righi-Leduc effect exceeds that due to the Ettingshausen effect in the opposite direction.

DOI： 10.1007/s11664-011-1850-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-V C H VERLAG GMBH  

The roles of molecular vibrations during photoinduced phase transitions in strongly correlated electron systems are theoretically studied on the basis of the time evolution of the exact many-electron and many-electronphonon wave functions in extended HolsteinPeierlsHubbard models. For the transition from a Coulomb-driven charge-ordered insulator to a metal, molecular vibrations interfere with charge transfers between neighboring molecules. For the transition from a neutral paraelectric state to an ionic ferroelectric state, they enhance the amplitude of ionicity modulation. They stabilize the initial phase in both cases, but their quantum nature is pronounced only in the former case.

DOI： 10.1002/pssb.201100533

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IOP PUBLISHING LTD  

We propose a two-pulse excitation to efficiently form a novel sp(3)-bonded nanosize domain with frozen shear in a graphite crystal. This sp(3) structure is well stabilized by shear displacement between two neighboring graphite layers. The shearing motion is induced transiently by the first laser pulse, and is frozen by the second pulse before disappearing, resulting in the efficient formation of the sp(3)-bonded domain with frozen shear. We show this dynamical process qualitatively by molecular dynamics calculations.

DOI： 10.1088/0953-8984/24/20/205402

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Effects of lattice dimerizations on charge dynamics in one-dimensional (1D) half-filled Mott insulators were studied using an organic compound, Rb-tetracyanoquinodimethane (TCNQ). First, we investigated the presence of the dimeric molecular displacements by the measurements of polarized Raman spectra and the time evolutions of photoinduced reflectivity changes. The results indicate that Rb-TCNQ shows a spin-Peierls-like structural phase transition and molecular dimerization occurs below 220 K. Second, we performed femtosecond reflection spectroscopy from visible to infrared regions down to 0.1 eV on Rb-TCNQ in both the low-temperature phase with dimerization and the high-temperature phase without dimerization. The results revealed that the molecular dimerization causes splitting of midgap absorption due to photocarriers. Theoretical calculations by the density-matrix renormalization group method reveal that low- and high-energy midgap absorptions are due to pure charge excitation and spin-charge-coupled excitation, respectively, of polarons stabilized by the electron-lattice interaction. This indicates that dimerization breaks the spin-charge separation characteristic of 1D Mott insulators with large electron correlation. © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.85.125112

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Language：English   Publisher：MDPI AG  

Dynamics of photoinduced phase transitions in molecular conductors are reviewed from the perspective of interplay between correlated electrons and phonons. (1) The charge-transfer complex TTF-CA shows a transition from a neutral paraelectric phase to an ionic ferroelectric phase. Lattice phonons promote this photoinduced transition by preparing short-range lattice dimerization as a precursor. Molecular vibrations stabilize the neutral phase so that the ionic phase, when realized, possesses a large ionicity and the Mott character; (2) The organic salts theta-(BEDT-TTF)(2)RbZn(SCN)(4) and alpha-(BEDT-TTF)(2)I-3 show transitions from a charge-ordered insulator to a metal. Lattice phonons make this photoinduced transition hard for the former salt only. Molecular vibrations interfere with intermolecular transfers of correlated electrons at an early stage; (3) The organic salt kappa-(d-BEDT-TTF)(2)Cu[N(CN)(2)]Br shows a transition from a Mott insulator to a metal. Lattice phonons modulating intradimer transfer integrals enable photoexcitation-energy-dependent transition pathways through weakening of effective interaction and through introduction of carriers.

DOI： 10.3390/cryst2010056

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：POLISH ACAD SCIENCES INST PHYSICS  

At an early stage of the photoinduced transition from an insulator to a metal in quasi-two-dimensional organic conductors, a coherent motion of electrons is observed in a charge-ordered insulator, but not so far in a Mott insulator. The mechanisms of these different photoinduced charge dynamics are theoretically studied by numerical solutions to the time-dependent Schrodinger equation for exact many-electron-phonon wave functions on small clusters of model systems. We use two-dimensional three-quarter-filled extended Holstein-Hubbard models on anisotropic triangular lattices. For a charge-ordered insulator on a lattice simplified from the structure of alpha-(BEDT-TTF)(2)I-3, we indeed find a low-energy collective electronic motion coupled with quantum phonons even if the energy of photoexcitation is away from this energy. For a Mott insulator on a lattice simplified from the structure of kappa-(BEDT-TTF)(2)X, however, such a collective motion does not appear, and quantum phonons are hardly excited.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

We have theoretically studied photoinduced melting in a charge-separated (CS) phase of the dimeric radical anion salt, Et2Me2Sb[Pd(dmit)(2)](2), by using an extended Peierls-Hubbard model with intra-molecular degrees of freedom and the mean-field approximation. Model parameters are determined to reproduce the ground-state properties of neutral, monovalent, and divalent dimers, as well as their crystallized states.
We have calculated the dynamics of a photoexcited state in the CS phase by solving the time-dependent Schrodinger equation. Neutral and divalent dimers are changed into a monovalent state after photoexcitation, resulting in the melting of charge order. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201100602

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：ELSEVIER SCIENCE BV  

We have performed a variational Monte Carlo simulation on a two-dimensional Hubbard model with first-and second-neighbor hopping terms in order to study the coexistence state of a static stripe state and a modulated d-wave superconductivity in the under-doped cuprates. In addition to a Gutzwiller, a Jastrow and a doublon-holon correlation effects, the band-renormalization effect was considered in the trial wave function. The condensation energies of an 8-period stripe state was computed as a function of a Coulomb energy under the hole-density x=1/8. Our results reveal that the renormalization of higher hopping parameters due to the strong correlation effect enhances the one-dimensional hole motion on a quarter-filled band in the stripe state, and brings quasi-Fermi surface close to the magnetic zone boundary in the coexistence state. (C) 2012 Published by Elsevier B. V. Selection and/or peer-review under responsibility of ISS Program Committee

DOI： 10.1016/j.phpro.2012.03.411

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

Nonlinear conduction in quasi-two-dimensional organic conductors partial derivative-(BEDT-TTF)(2)X [BEDTTTF= bis(ethylenedithio)tetrathiafulvalene] is investigated theoretically. By using a nonequilibrium Green's function method, an extended Peierls-Hubbard model attached to metallic electrodes is studied within the mean-field approximation. We show that in a coexistent state of stripe-type and nonstripe 3-fold charge orders, the former charge order is completely suppressed by the applied bias, whereas the latter charge order survives. This leads to selective melting of charge order of different origins. The disappearance of the stripe-type charge order largely alters the conduction behavior, which is consistent with the experimental findings. (C) 2012 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201100642

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

We theoretically discuss the mechanism for the peculiar nonlinear conduction in quasi-two-dimensional organic conductors theta-(BEDT-TTF)(2)X [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] through the melting of stripe-type charge order. An extended Peierls-Hubbard model attached to metallic electrodes is investigated by a nonequilibrium Green's function technique. A novel current-voltage characteristic appears in a coexistent state of stripe-type and nonstripe 3-fold charge orders, where the applied bias melts mainly the stripe-type charge order through the reduction of lattice distortion, whereas the 3-fold charge order survives. These contrastive responses of the two different charge orders are consistent with the experimental observations.

DOI： 10.1143/JPSJ.80.103702

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We theoretically study pump-photon-energy-dependent pathways of a photoinduced dimer-Mott-insulator-to-metal transition, on the basis of numerical solutions to the time-dependent Schrodinger equation for the exact many-body wave function of a two-dimensional three-quarter-filled extended Peierls-Hubbard model. When molecular degrees of freedom inside a dimer are utilized, photoexcitation can weaken the effective interaction or increase the density of photocarriers. In the organic dimer Mott insulator, kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br, the intradimer and the interdimer charge-transfer excitations have broad bands that overlap with each other. Even in this disadvantageous situation, the photoinduced conductivity change depends largely on the pump photon energy, confirming the two pathways recently observed experimentally. The characteristic of each pathway is clarified by calculating the modulation of the effective interaction and the number of carriers involved in low-energy optical excitations. The pump-photon-energy-dependent pathways are confirmed to be realized from the finding that, although the effective interaction is always and slowly weakened, the introduction of carriers is sensitive to the pump-photon energy and proceeds much faster.

DOI： 10.1143/JPSJ.80.084710

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For electronic states and photoinduced charge dynamics near the neutral-ionic transition in the mixed-stack charge-transfer complex tetrathiafulvalene-p-chloranil (TTF-CA), we review the effects of Peierls coupling to lattice phonons modulating transfer integrals and Holstein couplings to molecular vibrations modulating site energies. The former stabilizes the ionic phase and reduces discontinuities in the phase transition, while the latter stabilizes the neutral phase and enhances the discontinuities. To reproduce the experimentally observed ionicity, optical conductivity and photoinduced charge dynamics, both couplings are quantitatively important. In particular, strong Holstein couplings to form the highly-stabilized neutral phase are necessary for the ionic phase to be a Mott insulator with large ionicity. A comparison with the observed photoinduced charge dynamics indicates the presence of strings of lattice dimerization in the neutral phase above the transition temperature.

DOI： 10.1143/JPSJ.80.084707

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：SPRINGER  

We consider thermoelectric effects in a pseudo-one-dimensional electron gas (P1DEG) with a spin-orbit interaction (SOI). The SOI splits the dispersion relation of the P1DEG into subbands with an energy gap. We find quantum oscillations in transport coefficients, which coincide with the locations of the subband edges, as a function of the electrochemical potential.

DOI： 10.1007/s11664-010-1470-3

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Two-dimensional electron gases having an electrochemical potential gradient under a magnetic field are numerically examined using the finite-difference method. The temperature, voltage, electric current, and heat flux are calculated from transport equations describing thermoelectric and thermomagnetic effects, namely the Hall, Nernst, Ettingshausen, and Righi-Leduc effects. The results show that a magnetic field distorts equipotential lines and generates an uneven temperature distribution. In particular, a part of the system is found to become colder than the temperature of the heat baths. The cooling effect under a strong magnetic field is due primarily to the Ettingshausen and Hall effects.

DOI： 10.1007/s11664-010-1427-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Nonequilibrium states induced by an applied bias voltage (V) and the corresponding current-voltage characteristics of one-dimensional models describing band and Mott insulators are investigated theoretically by using nonequilibrium Green's functions. We attach the models to metallic electrodes, the effects of which are incorporated into the self-energy. Modulation of the electron density and the scalar potential coming from the additional long-range interaction are calculated self-consistently within the Hartree approximation. For both models of band and Mott insulators with length L-C, the bias voltage induces a breakdown of the insulating state, the threshold of which shows a crossover depending on L-C. It is determined basically by the bias V-th similar to Delta for L-C smaller than the correlation length xi = W/Delta, where W denotes the bandwidth and Delta denotes the energy gap. For systems with LC >> xi, the threshold is governed by the electric field V-th/L-C, which is consistent with a Landau-Zener-type breakdown V-th/L-C proportional to Delta(2)/W. We demonstrate that the spatial dependence of the scalar potential is crucially important for this crossover by showing the case without the scalar potential, where the breakdown occurs at V-th similar to Delta regardless of the length L-C.

DOI： 10.1103/PhysRevB.83.085113

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WILEY-BLACKWELL PUBLISHING, INC  

Photoinduced transition dynamics is studied in extended Peierls-Hubbard models for organic BEDT-TTF salts on anisotropic triangular lattices. For theta- and alpha-type salts, the lattice distortion pattern in the charge-ordered ground state governs the easiness of the insulator-to-metal transition. For K-type salts, the energy of photoexcitation in the dimer-Mott phase determines whether the insulator-to-metal transition is controlled by the bandwidth or the band filling. (C) 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssb.201000567

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

We investigate photoinduced dynamical properties of the one-dimensional two-orbital degenerate Hubbard model by calculating the time-dependent many-electron wave function. The transient optical response spectra around the charge-transfer peak show characteristic coherent oscillations, which can be attributed to the Raman-active orbital excitations (orbitons). An analysis for the oscillating component shows that the photoinduced oscillations are caused by the quantum interference between photogenerated states. (c) 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.201000648

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Ultrafast dynamics of the light-matter interaction in a charge-ordered molecular insulator alpha-(BEDT-TTF)(2)I(3) were studied by pump-probe spectroscopy using few-optical-cycle infrared pulses (pulse width 12 fs). Coherent oscillation of the correlated electrons and subsequent Fano destructive interference with intramolecular vibration were observed in time domain; the results indicated a crucial role for electron-electron interplay in the light-matter interaction leading to the photoinduced insulator-to-metal transition. The qualitative features of this correlated electron motion were reproduced by calculations based on exact many-electron-phonon wave functions.

DOI： 10.1103/PhysRevLett.105.246402

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We study photoinduced ultrafast coherent oscillations originating from orbital degrees of freedom in the one-dimensional two-orbital Hubbard model. By solving the time-dependent Schrödinger equation for the numerically exact many-electron wave function, we obtain time-dependent optical response functions. The calculated spectra show characteristic coherent oscillations that vary with the frequency of probe light. A simple analysis for the dominant oscillating components clarifies that these photoinduced oscillations are caused by the quantum interference between photogenerated states. The oscillation attributed to the Raman-active orbital excitations (orbitons) clearly appears around the charge-transfer peak. © 2010 The American Physical Society.

DOI： 10.1103/PhysRevB.82.161105

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan (JPS)  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan (JPS)  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ELSEVIER SCIENCE BV  

Quasi-two-dimensional organic conductors theta-(BEDT-TTF)(2)RbZn(SCN)(4) and alpha-(BEDT-TTF)(2)I-3 [BED-TTF- bis(ethylenedithio)tetrathiafulvalene] are known to have quite similar charge-ordered ground states with so-called horizontal stripes Their photoinduced dynamics toward conductive states are quite different and their mechanisms are theoretically studied by numerical solutions to the time-dependent Schrodinger equation for the exact many-electron wave functions coupled with classical phonons in extended Peierls-Hubbard models on anisotropic triangular lattices The theta-type salt has a highly symmetric crystal structure and its triangular lattice Induces charge frustration which is relieved by molecular rotations in the charge-ordered state The lattice effect is thus strong and the large lattice stabilization energy makes the charge order hard compared with that in the a- type salt The different structural symmetries are reflected in their photoinduced evolutions of ground-state populations and charge distributions (C) 2009 Elsevier B V All rights reserved

DOI： 10.1016/j.physb.2009.11.105

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The current-voltage characteristics and charge distribution of charge-ordered electron systems at quarter-filling under an applied bias voltage (V) are investigated theoretically by using nonequilibrium Green s functions We consider an extended Hubbard model with long-range Coulomb interactions on a square lattice which describes a checkerboard-type charge order in the absence of the bias V The effects of metallic electrodes are incorporated Into the self-energy The electron density and a scalar potential that satisfies the Poisson equation with a suitable boundary condition are calculated self-consistently within the Hartree approximation A first-order transition is observed from the charge-ordered insulating state to a conductive state with increasing V In the former state the charge distribution is almost unchanged by V whereas the charge order disappears so that the charge distribution is basically uniform in the latter state (C) 2009 Elsevier B V All rights reserved

DOI： 10.1016/j.physb.2009.12.065

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：Bussei Kenkyu  

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Other Link： https://projects.repo.nii.ac.jp/?action=repository_uri&item_id=474580

Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan (JPS)  

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To elucidate the different photoinduced melting dynamics of charge orders observed in the quasi-two-dimensional organic conductors theta-(BEDT-TTF)(2)RbZn(SCN)(4) and alpha-(BEDT-TTF)(2)I-3 [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene], we theoretically study the photoinduced time evolution of charge and spin correlation functions on the basis of exact many-electron wave functions coupled with classical phonons in extended Peierls-Hubbard models on anisotropic triangular lattices. In both salts, the so-called horizontal-stripe charge order is stabilized by nearest-neighbor repulsive interactions and electron-lattice interactions. In theta-(BEDT-TTF)(2)RbZn(SCN)(4) (abbreviated as theta-RbZn), the stabilization energy due to lattice distortion is larger, so that a larger quantity of energy needs to be absorbed to melt charge and lattice orders. The photoinduced charge dynamics shows a complex behavior owing to a substantial number of nearly degenerate eigenstates involved. This is related to the high structural symmetry when the lattice is undistorted. In alpha-(BEDT-TTF)(2)I-3 (abbreviated as alpha-I-3), the lattice stabilization energy is smaller, and a small quantity of energy is sufficient to melt charge and lattice orders leading to a metallic phase. The photoinduced charge dynamics shows a sinusoidal oscillation. In alpha-I-3, the low structural symmetry ensures nearly spin-singlet bonds between hole-rich sites, where the spin correlation survives even after photoexcitation.

DOI： 10.1143/JPSJ.79.034708

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Photoinduced melting, of horizontal-stripe charge orders in quasi-two-dimensional organic conductors theta-(BEDT-TTF)(2)RbZn(SCN)(4) [BEDT-TTF = bis(ethylenedithio)tetrathiafulvalene] and alpha-(BEDT-TTF)(2)I-3 is investigated theoretically. By numerically solving the time-dependent Schrodinger equation, we study the photoinduced dynamics in extended Peierls-Hubbard models on anisotropic triangular lattices within the Hartree-Fock approximation. The inching of the charge order needs more energy for theta-(BEDT-TTF)(2)RbZn(SCN)(4) than for alpha-(BEDT-TTF)(2)I-3, which is a consequence of the larger stabilization energy in theta-(BEDT-TTF)(2)RbZn(SCN)(4). After local photoexcitation in the charge ordered states, the growth of a photoinduced domain shows anisotropy. In theta-(BEDT-TTF)(2)RbZn(SCN)(4), the domain hardly expands to the direction perpendicular to the horizontal-stripes. This is because all the molecules on the hole-rich stripe are rotated in one direction and those on the hole-poor stripe in the other direction. They Modulate horizontally connected transfer integrals homogeneously, stabilizing the charge order stripe by stripe. In alpha-(BEDT-TTF)(2)I-3, lattice distortions locally stabilize the charge order so that it is easily weakened by local photoexcitation. The photoinduced domain indeed expands in the plane. These results are consistent with recent observation by femtosecond reflection spectroscopy.

DOI： 10.1143/JPSJ.79.024712

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We study photoinduced dynamical properties of the one-dimensional two-orbital degenerate Hubbard model. By using the Lanczos diagonalization method, we calculate optical conductivity spectra of the lowest photoexcited state. The optical conductivity has the Drude-like low-energy component caused by photogenerated carriers, a holon and a triplet doublon. We have also found an additional low-energy component in the spectra, which demonstrates a transition from the triplet doublon to singlet doublons.

DOI： 10.1088/1742-6596/200/1/012109

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：The Physical Society of Japan (JPS)  

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The suppression of rectification at metal-Mott-insulator interfaces, which was previously shown by numerical solutions to the time-dependent Schrodinger equation and experiments on real devices, is reinvestigated theoretically using nonequilibrium Green's functions. The one-dimensional Hubbard model is used for a Mott insulator. The effects of attached metallic electrodes are incorporated into the self-energy. A scalar potential originating from work-function differences and satisfying the Poisson equation is added to the model. For electron density, we decompose it into three parts. One is obtained by integrating the local density of states over energy to the midpoint of the electrodes' chemical potentials. The others, obtained by integrating lesser Green's functions, are due to couplings with the electrodes and correspond to an inflow and an outflow of electrons. In Mott insulators, incoming electrons and holes are extended over the whole system, avoiding further accumulation of charges relative to that in the case without bias. This induces collective charge transport and results in the suppression of rectification.

DOI： 10.1143/JPSJ.78.054705

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：American Physical Society  

Photoexcited states are relaxed by transferring energy to the environments. In order to study which coupling allows fast energy transfer to lattice vibrations in correlated electron systems, we calculate the time evolutions of the kinetic energies of different types and frequencies of lattice vibrations. The one-dimensional half-filled Hubbard model is augmented with electron-lattice couplings that modulate transfer integrals, site energies, and Coulomb repulsion strengths. The time-dependent Schrödinger equation is solved for exact many-electron wave functions, and the classical equation of motion for the lattice displacements. In order to transfer energy to classical lattice vibrations that modulate transfer integrals or site energies, the translational invariance must be broken to give optical activity to an electronic excitation with wave number π and to these lattice vibrations. On the other hand, a certain amount of energy is always transferred to lattice vibrations that modulate Coulomb repulsion strengths, irrespective of the symmetry of the ground state, as long as the corresponding electron-lattice couplings are present. In strongly correlated electron systems, these couplings can be strong, although they are usually insignificant because their effects on the equilibrium properties can be absorbed into redefinition of Coulomb repulsion strengths. We will discuss competition or collaboration between energy-transfer pathways through different types of electron-lattice couplings. © 2009 The American Physical Society.

DOI： 10.1103/PhysRevB.79.125118

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DOI： 10.1143/JPSJ.78.018002

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：WILEY-V C H VERLAG GMBH  

In order to study relaxation rates of photoexcited states in correlated electron systems, we calculate the time evolution of phonons' kinetic energies after photoexcitation of a one-dimensional half-filled Hubbard model coupled with different types of phonons, which modulate transfer integrals, site energies, and on-site repulsion strengths. Without or with weak dimerization, the energy transfer to phonons that modulate on-site repulsion strengths is the largest. It increases rapidly as a function of the corresponding displacement. This suggests that the rapid relaxation of photoexcited states in one-dimensional correlated electron systems is allowed without change in symmetry by phonons that modulate Coulomb repulsion strengths. (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

DOI： 10.1002/pssc.200879904

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We discuss photogenerated midgap states of a one-dimensional dimerized Mott insulator potassium-tetracyanoquinodimetbane (K-TCNQ). We take account of two types of phonons, the intermolecular and the intramolecular vibrations. We treat these phonon modes adiabatically and analyse a theoretical model by using the density matrix renormalization group (DMRG). Our numerical results demonstrate that the midgap state of K-TCNQ can be reproduced by the intermolecular lattice distortion.

DOI： 10.1088/1742-6596/148/1/012005

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Effects of spin fluctuations, charge fluctuations and lattice distortions on charge orders in theta-(BEDT-TTF)(2)RbZn(SCN)(4) and alpha-(BEDT-TTF)(2)I-3 axe investigated theoretically in a two-dimensional extended Peierls-Hubbaxd model. By using exact diagonalization, we have calculated hole-density distributions and transfer modulations with transfer integrals based on the corresponding high-temperature structures as a function of electron-phonon couplings. The results clearly show the origin of their lattice-effect differences, namely, the lattice effect on theta-(BEDT-TTF)(2)RbZn(SCN)(4) is much larger than that on alpha-(BEDT-TTF)(2)I-3. This finding is systematically explained by the strong-coupling perturbation theory. It is found that spin fluctuations induce lattice distortions in theta-(BEDT-TTF)(2)RbZn(SCN)(4), but their effects axe partially cancelled by charge fluctuations in alpha-(BEDT-TTF)(2)I-3.

DOI： 10.1088/1742-6596/148/1/012006

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Photoinduced charge dynamics in one- and two-dimensional organic conductors are studied theoretically in extended Peierls-Hubbard models. For quasi-one-dimensional (EDO-TTF)(2)PF6, photoinduced change in the charge order pattern from (0110) to (1010) is accompanied by probe-energy-dependent oscillations of conductivity. This is caused by coexistence of charge order and delocalized electrons. For quasi-two-dimensional alpha-(BEDT-TTF)(2)I-3 and theta-(BEDT-TTF)(2)RbZn(SCN)(4), photoinduced melting of the horizontal-stripe charge order proceeds easier in the a-type salt than in the theta-type salt. This is because the charge order in the theta-type salt is more strongly stabilized by electron-phonon interactions.

DOI： 10.1088/1742-6596/148/1/012054

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Charge ordering with structural distortion in quasi-two-dimensional organic conductors theta-(ET)(2)RbZn(SCN)(4) (ET=BEDT-TTF) and alpha-(ET)(2)I(3) is investigated for an extended Hubbard model with Peierls-type electron-lattice interactions within the Hartree-Fock approximation. It is found that the lattice effects stabilize the experimentally observed charge order substantially in theta-(ET)(2)RbZn(SCN)(4), whereas the energy gain by a lattice distortion is small and its band structure and Coulomb interactions play an important role to realize the charge order in alpha-(ET)(2)I(3). The results are consistent with experimental observations and show contrastive roles of lattice degrees of freedom on charge orders in these compounds.

DOI： 10.1088/1742-6596/150/4/042204

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

Photoinduced melting of charge order in quasi-two-dimensional organic conductors alpha-(ET)(2)I-3 (ET=BEDT-TTF) and theta-(ET)(2)RbZn(SCN)(4) is investigated theoretically. By solving the time-dependent Schrodinger equation numerically within the Haxtree-Fock approximation for an extended Peierls-Hubbaxd model, we study the photoinduced dynamics in each compound. The obtained charge, spin and lattice dynamics are considered to reflect the different natures of charge ordered states in these systems. In particular, the melting of charge order needs more energy for theta-(ET)(2)RbZn(SCN)(4) than for alpha-(ET)(2)I-3, which is a consequence of large lattice distortion and the essential role of electron-phonon coupling in stabilizing the charge order in theta-(ET)(2)RbZn(SCN)(4).

DOI： 10.1088/1742-6596/148/1/012063

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IOP PUBLISHING LTD  

We investigate the ground-state properties of the one-dimensional two-band Hubbard model with different bandwidths. The density-matrix renormalization group method is applied to calculate the averaged electron occupancies n as a function of the chemical potential mu. Both at quarter and half fillings, "charge plateaux" appear in the n-mu plot, where d mu/dn diverges and the Mott insulating states are realized. To see how the orbital polarization in the one-quarter charge plateau develops, we apply the second-order perturbation theory from the strong-coupling limit at quarter filling. The resultant Kugel-Khomskii spin-orbital model includes a magnetic field coupled to orbital pseudo-spins. This field originates from the discrepancy between the two bandwidths and leads to a finite orbital pseudo-spin magnetization.

DOI： 10.1088/1742-6596/150/4/042128

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Nonequilibrium dynamics is studied near the quantum phase transition point in the one-dimensional quantum Blume-Emery-Griffiths model. Its pseudospin component S-z represents an electric polarization, and (S-z)(2) corresponds to ionicity, in mixed-stack charge-transfer complexes that exhibit a transition between neutral quantum-paraelectric and ionic ferroelectric (or antiferroelectric) phases. The time-dependent Schrodinger equation is solved for the exact many-body wave function in the quantum-paraelectric phase. After impact force is introduced on a polarization locally in space and time, polarizations and ionicity coherently oscillate. The oscillation amplitudes are large near the quantum phase transition point. The energy supplied by the impact flows linearly into these oscillations, so that the nonequilibrium behavior is uncooperative.

DOI： 10.1103/PhysRevB.78.205102

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DOI： 10.1088/1742-6596/132/1/012019

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The effects of spin and charge fluctuations and electron-phonon couplings on charge ordering in alpha(BEDT-TTF)(2)I-3 [BEDT-TTF = bis(ethylenedithio)-tetrathiafulvalence] are investigated theoretically for all anisotropic triangular lattice at 3/4 filling. By the exact-diagonalization method. we have calculated the hole density distributions and the modulations of transfer integrals from high-temperature values as it function of electron-phonon coupling strength. The results clearly show that the lattice effect on alpha-(BEDT-TTF)(2)I-3 is weak compared with that on theta-(BEDT-TTF)(2)RbZn(SCN)(4) as previously found by experiments. This finding. which is also consistent with recent mean-field results, is systematically explained by strong-coupling perturbation theory: the effects of spin fluctuations tire partially canceled by charge fluctuations in alpha-(BEDT-TTF)(2)I-3.

DOI： 10.1143/JPSJ.77.094712

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The quasistable state in the photoinduced phase transition for the quasi-one-dimensional quarter-filled organic conductor (EDO-TTF)(2)PF6 has been examined by ultrafast reflective measurements and time-dependent model calculations incorporating both electron-electron and electron-phonon interactions. The transient optical conductivity spectrum over a wide probe photon-energy range revealed that photoexcitation induced a new type of charge-disproportionate state. Additionally, coherent and incoherent oscillations dependent on probe photon energies were found, as predicted by the calculation.

DOI： 10.1103/PhysRevLett.101.067403

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Language：English   Publisher：ELSEVIER SCIENCE BV  

Theoretical progress in the research of photoinduced phase transitions is reviewed with closely related experiments. After a brief introduction of stochastic evolution in statistical systems and domino effects in localized electron systems, we treat photoinduced dynamics in itinerant-electron systems. Relevant interactions are required in the models to describe the fast and ultrafast charge-lattice-coup led dynamics after photoexcitations. First, we discuss neutral-ionic transitions in the mixed-stack charge-transfer complex. TTF-CA. When induced by intrachain charge-transfer photoexcitations, the dynamics of the ionic-to-neutral transition are characterized by a threshold behavior, while those of the neutral-to-ionic transition by an almost linear behavior. The difference originates from the different electron correlations in the neutral and ionic phases. Second, we deal with halogen-bridged metal complexes, which show metal, Mott insulator, charge-density-wave, and charge-polarization phases. The latter two phases have different broken symmetries. The charge-density-wave to charge-polarization transition is much more easily achieved than the reverse transition. This is clarified by considering microscopic charge-transfer processes. The transition from the charge-density-wave to Mott insulator phases and that from the Mott insulator to metal phases proceed much faster than those between the low-symmetry phases. Next, we discuss ultrafast, inverse spin-Peierls transitions in an organic radical crystal and alkali-TCNQ from the viewpoint of intradimer and interdimer charge-transfer excitations. Then, we study photogenerated electrons in the quantum paraelectric perovskite, SrTiO3, which are assumed to couple differently with soft-anharmonic phonons and breathing-type high-energy phonons. The different electron-phonon couplings result in two types of polarons, a "super-paraelectric large polaron" with a quasi-global parity violation, and an "off-center-type self-trapped polaron" with only a local parity violation. The former is equivalent to a charged and conductive ferroelectric domain, which greatly enhances both the quasi-static electric susceptibility and the electric conductivity. Finally, we outline the development of time-resolved X-ray diffraction experiments, which directly accesses the dynamics of electronic, atomic and molecular motions in photoexcited materials. They are extremely useful when a three-dimensional structural long-range order is established and changes the symmetry. (C) 2008 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physrep.2008.04.008

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We discuss photogenerated midgap states of a one-dimensional (1D) dimerized Mott insulator, potassium-tetracyanoquinodimethane (K-TCNQ). Two types of phonon modes are taken into account: intermolecular and intramolecular vibrations. We treat these phonon modes adiabatically and analyze a theoretical model by using the density-matrix renormalization group (DMRG). Our numerical results demonstrate that the intermolecular lattice distortion is necessary to reproduce the photoinduced midgap absorption in K-TCNQ. We find two types of midgap states. One is a usual polaronic state characterized by a localized elementary excitation. The other is superposition of two types of excitations, a doped-carrier state and a triplet-dimer state, which can be generally observed in 1D dimerized Mott insulators, not limited to K-TCNQ.

DOI： 10.1143/JPSJ.77.074713

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Language：Japanese   Publisher：The Laser Society of Japan  

Theoretical understanding of photoinduced phase transitions and their dynamics is reviewed from the viewpoint of coherent oscillations and transient states in condensed molecular materials with quasi-one-dimensional electronic conduction. In the mixed-stack organic charge-transfer compound TTF-CA (TTF: tetrathiafulvalene, CA: chloranil), the photoinduced transition from the ionic phase to the neutral phase is accompanied with coherent motion of a neutral-ionic domain boundary. Its high cooperativity is realized by the sufficiently strong, interchain Coulomb coupling in the ionic phase. In the quarter-filled-band organic salt (EDO-TTF) ₂PF₆ (EDO: ethylenedioxy), the photoinduced melting of charge order is accompanied with oscillations in transient reflectivity. The degree of coherence in these oscillations depends on the probe photon energy. It is caused by different space and time scales of observed electronic motion and relevant phonon motion.

DOI： 10.2184/lsj.36.343

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

Charge ordering with structural distortion in quasi-two-dimensional organic conductors theta-(ET)(2)RbZn(SCN)(4) (ET = BEDT-TTF) and alpha-(ET)(2)I-3 is investigated theoretically. By using the Hartree-Fock approximation for an extended Hubbard model which includes both on-site and intersite Coulomb interactions together with Peierls-type electron-lattice couplings, we examine the role of lattice degrees of freedom on charge order. It is found that the experimentally observed, horizontal charge order is stabilized by lattice distortion in both compounds. In particular, the lattice effect is crucial to the realization of the charge order in theta-(ET)(2)RbZn(SCN)(4), while the peculiar band structure whose symmetry is lower than that of theta-(ET)(2)RbZn(SCN)(4) in the metallic phase is also an important factor in alpha(ET)(2)I-3 together with the lattice distortion. For alpha-(ET)(2)I-3, we obtain a phase transition from a charge-disproportionated metallic phase to the horizontal charge order with lattice modulations, which is consistent with the latest X-ray experimental result.

DOI： 10.1143/JPSJ.77.034708

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Charge transport through metal-Mott insulator interfaces is studied and compared with that through metal-band-insulator interfaces. For band insulators, rectification has been known to occur owing to a Schottky barrier, which is produced by the work-function difference. For Mott insulators, however, qualitatively different current-voltage characteristics are obtained. Theoretically, we use the one-dimensional Hubbard model for a Mott insulator and attach to it the tight-binding model for metallic electrodes. A Schottky barrier is introduced by a solution to the Poisson equation with a simplified density-potential relation. The current density is calculated by solving the time-dependent Schrodinger equation. We mainly use the time-dependent Hartree-Fock approximation and also use exact many-electron wave functions on small systems for comparison. Rectification is found to be strongly suppressed even for large work-function differences. We show its close relationship with the fact that field-effect injections into one-dimensional Mott insulators are ambipolar. Experimentally, we fabricated asymmetric contacts on top of single crystals of quasi-one-dimensional organic Mott and band insulators. Rectification is strongly suppressed at an interface between metallic magnesium and Mott-insulating (BEDT-TTF)(F(2)TCNQ) [BEDT-TTF=bis(ethylenedithio)tetrathiafulvalene; F(2)TCNQ=2,5-difluorotetracyanoquinodimethane].

DOI： 10.1103/PhysRevB.76.235118

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Photoinduced melting of charge order is calculated by using the exact many-electron wave function coupled with classically treated phonons in the one-dimensional quarter-filled Hubbard model with Peierls and Holstein types of electron-phonon couplings. The model parameters are taken from recent experiments on (EDO-TTF)(2)PF6 (EDO-TTF=ethylenedioxy-tetrathiafulvalene) with a (0110) charge order, where transfer integrals are modulated by molecular displacements (bond-coupled phonons) and site energies by molecular deformations (charge-coupled phonons). The charge-transfer photoexcitation from (0110) to (0200) configurations and that from (0110) to (1010) configurations have different energies. The corresponding excited states have different shapes of adiabatic potentials as a function of these two phonon amplitudes. The adiabatic potentials are shown to be useful in understanding differences in the photoinduced charge dynamics and the efficiency of melting, which depend not only on the excitation energy but also on the relative phonon frequency of the bond- and charge-coupled phonons.

DOI： 10.1103/PhysRevB.76.075105

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We investigate the optical properties of one-dimensional (1D) dimerized Mott insulators using the 1D dimerized extended Hubbard model. Numerical calculations and a perturbative analysis from the decoupled-dimer limit clarify that there are three relevant classes of charge-transfer (CT) states generated by photoexcitation: interdimer CT unbound states, interdimer CT exciton states, and intradimer CT exciton states. This classification is applied to understanding the optical properties of an organic molecular material, 1,3,5-trithia-2,4,6-triazapentalenyl (TTTA), which is known for its photoinduced transition from the dimerized spin-singlet phase to the regular paramagnetic phase. We conclude that the lowest photoexcited state of TTTA is the interdimer CT exciton state and the second lowest state is the intradimer CT exciton state.

DOI： 10.1143/JPSJ.074713

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To find characteristic properties of relaxation process in the neutral-ionic, paraelectric-ferroelectric phase transition in the charge-transfer complex, tetrathiafulvalene-p-chloranil, we investigate stochastic process in the spin 1 anisotropic Blume-Emery-Griffith model [Phys. Rev. A 4, 1071 (1971)]. We assume that spins obey the Markov process which is described by the master equation with the transfer probability 1/tau exp[-(beta/2)Delta H], where Delta H is the energy difference for the transfer. We derive time-evolution equations from two standpoints. One is the mean-field approximation (MFA) and the other is an extension of the Saito-Kubo treatment to the spin 1 and anisotropic case, which improves MFA. Solving the equations numerically, we have found highly anisotropic relaxations during the neutral-ionic phase transition: the interchain ordering develops much more slowly than the intrachain one. In contrast, the ionic-neutral phase transition proceeds rather isotropically.

DOI： 10.1103/PhysRevB.75.235125

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Combined effects of electron correlations and lattice distortions on the charge ordering in theta-(BEDT-TTF)(2)RbZn(SCN)(4) are investigated theoretically in a two-dimensional 3/4-filled extended Hubbard model with electron-lattice couplings. It is known that this material undergoes a phase transition from a high-symmetry metallic state to a low-symmetry insulating state with a horizontal-stripe charge order (CO) by lowering temperature. By means of the exact-diagonalization method, we show that electron-phonon interactions are crucial to stabilize the horizontal-stripe CO and to realize the low-symmetry crystal structure.

DOI： 10.1103/PhysRevB.75.245112

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Charge ordering accompanied by lattice distortion in quasi-two dimensional organic conductors θ-(ET) 2X (ET = BEDT-TTF) is studied by using an extended Hubbard model with Peierls-type electron-lattice couplings within the Hartree-Fock approximation. It is found that the horizontal-stripe charge-ordered state, which is experimentally observed in θ-(ET) 2RbZn(SCN) 4, is stabilized by the self-consistently determined lattice distortion. Furthermore, in the presence of the anisotropy in nearest-neighbor Coulomb interactions V ij, the horizontal charge order becomes more stable than any other charge patterns such as diagonal, vertical and 3-fold-type states. At finite temperatures, we compare the free energies of various charge-ordered states and find a first-order transition from a metallic state with 3-fold charge order to the insulating state with the horizontal charge order. The role of lattice degrees of freedom in the realization of the horizontal charge order and the relevance to experiments on θ-(ET) 2X are discussed. ©2007 The Physical Society of Japan.

DOI： 10.1143/JPSJ.76.053708

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We study an incommensurate long-range order induced by an external magnetic field in a quasi-one-dimensional bond-alternating spin system, F5PNN, focusing on the role of the frustrating interaction which can be enhanced by a high-pressure effect. On the basis of the density matrix renormalization group analysis of a microscopic model for F5PNN, we present several H-T phase diagrams for typical parameters of the frustrating next-nearest-neighbour coupling and the interchain interaction, and then discuss how the field-induced incommensurate order develops by the frustration effect in such phase diagrams. A magnetization plateau at half the saturation moment is also mentioned.

DOI： 10.1088/0953-8984/18/20/006

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Motivated by recent experiments, which give strong evidence for an excitonic insulating phase in TmSe0.45Te0.55, we developed a scheme to quantitatively construct, for generic two-band models, the phase diagram of an excitonic insulator. As a first application of our approach, we calculated the phase diagram for an effective mass two-band model with long-range Coulomb interaction. The shielded potential approximation is used to derive a generalized gap equation controlling for positive (negative) energy gaps the transition from a semi-conducting (semi-metallic) phase to an insulating phase. Numerical results. obtained within the quasi-static approximation, show a steeple-like phase diagram in contrast to long-standing expectations. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physb.2006.01.099

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Dynamics of ionic-to-neutral and neutral-to-ionic phase transitions induced by intrachain charge-transfer photoexcitations are studied in a quasi-one-dimensional extended Hubbard model with alternating potentials and an electron-lattice coupling for mixed-stack charge-transfer complexes. For interchain couplings, we use electron-electron interactions previously estimated for TTF-CA (TTF=tetrathiafulvalene, CA=chloranil). Photoexcitation is introduced by a pulse of oscillating electric field. The time-dependent Hartree-Fock (HF) approximation is used for the electronic part, and the classical approximation for the lattice part. In the ionic-to-neutral transition, the transferred charge density is a strongly nonlinear function of the photoexcitation density, which is characterized by the presence of a threshold. With substantial interchain couplings comparable to those in TTF-CA, the interchain correlation is strong during the transition. Neutral domains in nearby chains simultaneously grow even if their nucleation is delayed by reducing the amplitude of the electric field. With weaker interchain couplings, the growing processes are in phase only when the amplitude of the electric field is large. Thus, the experimentally observed, coherent motion of a macroscopic neutral-ionic domain boundary is allowed to emerge by such substantial interchain couplings. In the neutral-to-ionic transition, by contrast, the transferred charge density is almost a linear function of the photoexcitation density. Interchain electron-electron interactions make the function slightly nonlinear, but the uncooperative situation is almost unchanged and consistent with the experimental findings.

DOI： 10.1103/PhysRevB.73.155120

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Effects of interchain electron-electron interactions on the photoinduced ionic-to-neutral and neutral-to-ionic transition dynamics are studied in a quasi-one-dimensional extended Peierls-Hubbard model with alternating potentials for mixed-stack charge-transfer complexes. The ionic-to-neutral transition dynamics depend on the strengths of interchain couplings. For weak couplings, the interchain coherence is destroyed For strong couplings such as those corresponding to TTF-CA, once neutral domains are nucleated above an increased absorption threshold, they grow spontaneously and cooperatively till the whole system is converted In contrast, interchain couplings slightly enhance nonlinearity of the otherwise uncooperative neutral-to-ionic transition dynamics.

DOI： 10.1007/s10909-006-9150-7

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Theories of photoinduced phase transitions have developed along with the progress in experimental studies, especially concerning their nonlinear characters and transition dynamics. At an early stage, paths from photoinduced local structural distortions to global ones are explained in classical statistical models. Their dynamics are governed by transition probabilities and inevitably stochastic, but they were sufficient to describe coarse-grained time evolutions. Recently, however, a variety of dynamics including ultrafast ones are observed in different electronic states. They are explained in relevant electronic models. In particular, a coherent lattice vibration and coherent motion of a macroscopic domain boundary need appropriate interactions among electrons and lattice displacements. Furthermore, some transitions proceed almost in one direction, which can be explained by considering relevant electronic processes. We describe the history of theories of photoinduced phase transitions and discuss a future perspective.

DOI： 10.1143/JPSJ.75.011008

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Dynamical properties of photoexcited states are theoretically studied in a one-dimensional Mott insulator dimerized by the spin-Peierls instability. Numerical calculations combined with a perturbative analysis have revealed that the lowest photoexcited state without nearest-neighbor interaction corresponds to an interdimer charge transfer excitation that belongs to dispersive excitations. This excited state destabilizes the dimerized phase, leading to a photoinduced inverse spin-Peierls transition. We discuss the purely electronic origin of midgap states that are observed in a latest photoexcitation experiment of an organic spin-Peierls compound, K-TCNQ (potassium- tetracyanoquinodimethane). © 2006 The American Physical Society.

DOI： 10.1103/PhysRevB.74.155105

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We study photoinduced optical responses of one-dimensional strongly correlated electron systems. Optical conductivity spectra are calculated for the around and photoexcited states in a one-dimensional Hubbard model at half filling by the exact diagonalization method. It is found that, in the Mott insulator phase, the photoexcited state has large spectral weights including the Drude weight below the optical gap. As a consequence, the spectral weight above the optical gap is markedly reduced. These results imply that a metallic state is induced by photoexcitation. A comparison between the photoexcited and hole-doped states shows that photoexcitation is similar to chemical doping.

DOI： 10.1143/JPSJ.74.2671

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We discuss the neutral-to-ionic phase transition and emergence of the multi-criticality in the quasi-one-dimensional charge-transfer salt TTF-CA under pressure. We stress that subtle interplay of Coulomb and lattice processes may be quite sensitive to pressure. Emergence or disappearance of the multi-critical points in a series of charge-transfer salts are understood through this interplay. What's behind is coexistence and coupling of the non-symmetry-breaking and symmetry-breaking order parameters.

DOI： 10.1016/j.synthmet.2005.07.066

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To clarify the mechanism of recently reported, ambipolar carrier injections into quasi-one-dimensional Mott insulators on which field-effect transistors are fabricated, we employ the one-dimensional Hubbard model attached to a tight-binding model for source and drain electrodes. To take account of the formation of Schottky barriers, we add scalar and vector potentials, which satisfy the Poisson equation with boundary values depending on the drain voltage, the gate bias, and the work-function difference. The current-voltage characteristics are obtained by solving the time-dependent Schrodinger equation in the unrestricted Hartree-Fock approximation. Its validity is discussed with the help of the Lanczos method applied to small systems. We find generally ambipolar carrier injections in Mott insulators even if the work function of the crystal is quite different from that of the electrodes. They result from balancing the correlation effect with the barrier effect. For the gate-bias polarity with higher Schottky barriers, the correlation effect is weakened accordingly, owing to collective transport in the one-dimensional correlated electron systems.

DOI： 10.1143/JPSJ.74.2544

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In the quasi-one-dimensional mixed-stack organic charge-transfer complex, TTF-CA, photoirradiation is known to trigger transitions between the neutral and ionic phases. Here we use a one-dimensional extended Peierls-Hubbard model with alternating potentials and calculate the mean-field dynamics of charge density coupled with that of lattice displacements. We show qualitative differences between the photoinduced ionic-to-neutral and neutral-to-ionic transitions, which are consistent with recent experimental findings. That is, the ionic-to-neutral transition proceeds cooperatively, characterized by threshold behavior, while the neutral-to-ionic transition proceeds uncooperatively, characterized by linear behavior. The threshold absorption in the former depends on the strength or the duration of the pulse. The coherence during the transition dynamics is also different between the two transitions. In the ionic-to-neutral transition, a clear interference effect is observed as a function of the interval when the pulse is spilt into two. (c) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jlumin.2004.09.047

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Electron-electron interactions play an important role in nonequilibrium properties of molecular materials. First, we show differences between photoinduced ionic-to-neutral and neutral-to-ionic transitions in quasi-one-dimensional extended Peierls Hubbard models with alternating potentials. Cooperative dynamics lead to nonlinear ionicity in the former, while uncooperative dynamics lead to quite linear ionicity in the latter, as a function of the energy supplied from the oscillating electric field. Interchain electron-electron interactions bring about initial competition among metastable and stable domains in neighboring chains, slowing down the phase transition. Interchain elastic couplings are necessary to form a ferroelectric long-range order. Second, we show differences between field-effect characteristics of Mott insulators and those of band insulators in one-dimensional Hubbard models, to which tight-binding models are attached for metallic electrodes and scalar potentials are added for interfacial barriers. Ambipolar characteristics are found in the former, while unipolar characteristics generally appear in the latter. In the former, charge transport is cooperative so that the drain current is insensitive to the difference between the work function of the channel and that of the electrodes, and thus insensitive to the polarity of the gate bias.

DOI： 10.1088/1742-6596/21/1/005

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Photoinduced optical responses are studied in the one-dimensional ionic Hubbard model with the nearest-neighbor repulsion V. For V=0, carriers introduced by photoirradiation move freely both in the Mott insulator phase and in the band insulator phase, giving rise to a Drude peak in the optical conductivity spectrum. The carriers in the Mott insulator phase remain conducting for 0 < V < 2t because their kinetic energy overcomes the binding energy. By contrast, the electrons and the holes in the band insulator phase are bound to form excitons for V > 0, which do not contribute to the charge transport unless the excitation energy allows them to be separated. The dependence of the Drude weight in the lowest-energy photoexcited state on V and the system size is investigated for both phases. Implications to experimental studies of halogen-bridged metal-complex chains are discussed.

DOI： 10.1088/1742-6596/21/1/030

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The effect of photo-doping on the quantum paraelectric SrTiO3 is studied by using the one-dimensional quantum Ising model, where the Ising spin describes the effective lattice polarization of an optical phonon. Two types of electron-phonon couplings are introduced through the modulation of transfer integral via lattice deformations. After the exact diagonalization and the perturbation studies, we find that photo-induced low-density carriers can drastically alter quantum fluctuations when the system locates near the quantum critical point between the quantum para- and ferro-electric phases.

DOI： 10.1088/1742-6596/21/1/039

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We study field-induced phase transitions and long-range orders in the S = 1/2 spin bond-alternating chain with frustrating interaction. By using the inter-chain mean field theory combined with the finite temperature density matrix renormalization group, we investigate properties of field-induced long-range orders and phase transitions. We find that the inter-chain interaction strongly affects the realized phases of this system.

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Photoinduced dynamics of charge density and lattice displacements is calculated by solving the time-dependent Schrodinger equation for a one-dimensional extended Peierls-Hubbard model with alternating potentials for the mixed-stack organic charge-transfer complex, TTF-CA. A pulse of oscillating electric field is incorporated into the Peierls phase of the transfer integral. The frequency, the amplitude, and the duration of the pulse are varied to study the nonlinear and cooperative character of the photoinduced transition. When the dimerized ionic phase is photoexcited, the threshold behavior is clearly observed by plotting the final ionicity as a function of the increment of the total energy. Above the threshold photoexcitation, the electronic state reaches the neutral one with equidistant molecules after the electric field is turned off. The transition is initiated by nucleation of a metastable neutral domain, for which an electric field with frequency below the linear absorption peak is more effective than that at the peak. When the pulse is strong and short, the charge transfer takes place on the same time scale with the disappearance of dimerization. As the pulse becomes weak and long, the dimerization-induced polarization is disordered to restore the inversion symmetry on average before the charge transfer takes place to bring the system neutral. Thus, a paraelectric ionic phase is transiently realized by a weak electric field. It is shown that infrared light also induces the ionic-to-neutral transition, which is characterized by the threshold behavior.

DOI： 10.1143/JPSJ.73.2868

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Dynamics of charge density and lattice displacements after the neutral phase is photoexcited is studied by solving the time-dependent Schrodinger equation for a one-dimensional extended Peierls-Hubbard model with alternating potentials. In contrast to the ionic-to-neutral transition studied previously, the neutral-to-ionic transition proceeds in an uncooperative manner as far as the one-dimensional system is concerned. The final ionicity is a linear function of the increment of the total energy. After the electric field is turned off, the electronic state does not significantly change, roughly keeping the ionicity, even if the transition is not completed, because the ionic domains never proliferate. As a consequence, an electric field with frequency just at the linear absorption peak causes the neutral-to-ionic transition the most efficiently. These findings are consistent with the recent experiments on the mixed-stack organic charge-transfer complex, TTF-CA. We artificially modify or remove the electron-lattice coupling to discuss the origin of such differences between the two transitions.

DOI： 10.1143/JPSJ.73.2879

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In order to Study consequences of the differences between the ionic-to-neutral and neutral-to-ionic transitions in the one-dimensional extended Peierls-Hubbard model with alternating potentials for the TTF-CA complex, we introduce a double pulse of oscillating electric field in the time-dependent Schrodinger equation and vary the interval between the two pulses as well as their strengths. When the dimerized ionic phase is photoexcited, the interference effect is clearly observed owing to the coherence of charge density and lattice displacements. Namely, the two pulses constructively interfere with each other if the interval is a multiple of the period of the optical lattice vibration, while they destructively interfere if the interval is a half-odd integer times the period, in the processes toward the neutral phase. The interference is strong especially when the pulse is strong and short because the coherence is also strong. Meanwhile, when the neutral phase is photoexcited, the interference effect is almost invisible or weakly observed when the pulse is weak. The photoinduced lattice oscillations are incoherent due to random phases. The strength of the interference caused by a double pulse is a key quantity to distinguish the two transitions and to evaluate the coherence of charge density and lattice displacements.

DOI： 10.1143/JPSJ.73.2887

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We study energetics of a cooperative low-spin (L) to high-spin (H) transformation in a crystal with two sublattices, where thermo- and photo-induced symmetry-broken phases have been recently observed. The model takes into account an intra-dimer antiferromatic coupling and inter-dimer couplings. Cooperative activation processes for switching between LH and HL states and that from LL to HH states of a dimer are analyzed. It is shown that locally (within a dimer) preferred symmetry-broken, LH and HL, states compete with inter-dimer couplings, which stimulates dynamical disorder via a decrease in the energy barrier for a flip between asymmetric configurations. On the other hand, a locally non-preferred symmetric state (HH) is supported by inter-dimer interaction via a cooperative activation mechanism. We conclude that symmetry-broken states can form an intermediate thermo-induced phase or can be observed as a photo-induced one at low temperatures as a result of local equilibrium. The global equilibrium follows from cooperative interaction and governs the thermo-induced re-entrant phase transition.

DOI： 10.1143/JPSJ.73.1237

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As an effective model for the two-step spin transition, we study a two-sublattice spin model that takes into account intra-site anti ferromagnetic and inter-site ferromagnetic Couplings. Due to the competition between these couplings, the temperature dependence of the high spin fraction shows a two-step transition. We perform Monte Carlo (MC) simulations including a photoexcitation term. The MC dynamics of the high spin fraction shows non-linear characteristics such as step-like changes, threshold-like behavior, and phase separation.

DOI： 10.1051/jp4:2004114152

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To elucidate a pressure-temperature phase diagram of the quasi-one-dimensional mixed-stack charge-transfer complex tetrathiafulvalene-P-chloranil (TTF-CA), we study the quasi-one-dimensional spin-1 Blume-Emery-Griffiths model. In addition to the local charge-transfer energy (Delta) and the inter-stack polar (dipole-dipole) interaction (J(perpendicular to)), we take account of the interstack electrostriction (Coulomb-lattice coupling). Using the self-consistent chain-mean-field theory, where the intra-stack degrees of freedom are exactly treated by the transfer-matrix method, we reproduce the gas-liquid-solid like phase diagram corresponding to the neutral (N), paraelectric ionic (I-para), and ferroelectric ionic (I-ferro) phases, respectively. Our classical model describes an essential point of the multicritical behavior of TTF-CA, i.e., the interchain electrostriction exclusively enhances the charge concentration (ionicity condensation), but does not affect the interchain ferroelectric coupling. This effect leads to appearance of the intermediate I-para phase in between the N and I-ferro phases on the Delta-T phase diagram.

DOI： 10.1103/PhysRevB.69.075115

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Real-time dynamics of charge density and lattice displacements is studied during photoinduced ionic-to-neutral phase transitions by using a one-dimensional extended Peierls-Hubbard model with alternating potentials for the one-dimensional mixed-stack charge-transfer complex, TTF-CA. The time-dependent Schrodinger equation and the classical equation of motion are solved for the electronic and lattice parts, respectively. We show how neutral domains grow in the ionic background. As the photoexcitation becomes intense, more neutral domains are created. Above threshold intensity, the neutral phase is finally achieved. After the photoexcitation, ionic domains with wrong polarization also appear. They quickly reduce the averaged staggered lattice displacement, compared with the averaged ionicity. As the degree of initial lattice disorder increases, more solitons appear between these ionic domains with different polarizations, which obstruct the growth of neutral domains and slow down the transition.

DOI： 10.1143/JPSJ.72.2282

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A photoinduced transition from a charge-density-wave (CDW) phase to a charge-polarization (CP) phase has been recently found in a one-dimensional halogen-bridged binuclear platinum complex R-4[Pt-2(pop)(4)I]nH(2)O [pop=P2O5H22-, R=(C2H5)(2)NH2]. Its mechanism is theoretically studied by solving the time-dependent Hartree-Fock equation for a one-dimensional two-band three-quarter-filled Peierls-Hubbard model. Above a threshold in the photoexcitation intensity, a transition takes place from the CDW to CP phases. The threshold intensity depends on the relative stability of these phases, which can be explained qualitatively by their diabatic potentials. However, the transition from the CP to CDW phases is hardly realized for two reasons: (i) low-energy charge-transfer processes occur only within a binuclear unit in the CP phase; (ii) it is difficult for the CDW order to become long ranged owing to its weak coherence. The effective transfer integrals required for the coherence are evaluated.

DOI： 10.1103/PhysRevB.68.075113

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Near the neutral-ionic phase transition in the one-dimensional extended Hubbard model with alternating potentials at half filling, the effects of alternating transfer integrals and a staggered magnetic field on the local spin excitation spectrum are studied by using the finite-temperature density-matrix renormalization-group method. In the neutral phase, the alternation increases the ionicity and lowers the spin excitation energies toward the ionic phase, while the staggered magnetic field does not modify the spectrum up to a critical field above which the system becomes ionic. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(02)02142-7

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Dynamics of the ionicity after photoexcitations in mixed-stack charge-transfer complexes is numerically studied by using a one-dimensional extended Peierls-Hubbard model with alternating potentials at half filling. The time-dependent Schrodinger equation and the Newton equation are solved for the electronic and lattice parts, respectively. Fourier analysis is performed for the ionicity. During the photoinduced phase transition, slow components are dominant and very broad reflecting complex motion of domain walls and lattice displacements. After the transition, fast and slow oscillations are clearly seen, which correspond to the electronic and lattice motion, respectively.

DOI： 10.1016/S0379-6779(02)00759-2

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In the one-dimensional two-band three-quarter-filled Peierls-Hubbard model for halogen-bridged binuclear metal complexes R-4[Pt-2(pop)(4)I]nH(2)O with counter ion R and pop=P2O5H22-, a transition from the charge-density-wave phase to the charge-polarization phase is photoinduced, but the opposite process is not. Its origin is explained by considering differences in low-energy photoexcitations in the two phases and coherence of respective order parameters. The different dynamics is demonstrated by solving the time-dependent Schrodinger equation.

DOI： 10.1016/S0379-6779(02)00712-9

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The commensurate state with threefold lattice distortion in the insulating phase of (DCNQI)(2)Cu is studied based on a two-band Peierls-Hubbard model by using the density-matrix renormalization-group method. With strong electron correlation among the d electrons, self-consistent lattice modulation strongly blocks the charge transfer between the pi and d orbitals in order to keep the commensurability condition even when the pi-d level difference is widely varied. A transition to an incommensurate phase requires a large deviation of the pi-d level difference from the optimal case. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0379-6779(02)00315-6

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Applying the finite-temperature density-matrix renormalization-group method to the spinless fermion model on a two-leg ladder, we have calculated the dynamical structure factors for the local charge transfer processes along and across the chains. The intra-chain excitation spectra are sensitive to the inter-chain transfer integral and the chemical potential, while the inter-chain excitation spectra are sensitive to the intra-chain electron correlation. These dynamical properties are due to the collective motions of fermions along the chains and the energy-dependent confinement of fermions in the chains. At low but finite energies, the effects of the increasing inter-chain transfer integral on the local charge-transfer spectra are similar to those of the chemical potential deviating from zero. (C) 2002 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0379-6779(02)00422-8

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Real-time dynamics of domain walls between the neutral and ionic phases just after photoexcitations is studied by fully solving the time-dependent Schrodinger equation for a one-dimensional extended Peierls-Hubbard (PH) model, not by relying on the adiabatic approximation. The unrestricted Hartree-Fock (HF) approximation is used for electrons, and the lattice displacements are treated classically. Three characteristic time scales are observed: rapid oscillation of ionicity owing to the local charge transfer; slow oscillation of lattice displacements; and even slower and collective motion of domain walls. Steady growth of a metastable domain is achieved after complicated competition of micro domains. The relevance to recently measured, time-resolved photoreflectance spectra in TTF-CA is discussed.

DOI： 10.1080/01411590290034029

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Local excitation spectra in different spin and charge channels are calculated in the one-dimensional extended Hubbard model with alternating energy levels at half filling for mixed-stack charge-transfer complexes. Near the boundary between the neutral and ionic phases, the electronic system is easily distorted by an additional term that reduces the symmetry and opens a gap. Alternating transfer integrals produce a nonmagnetic spin-Peierls phase; while staggered magnetic fields produce an antiferromagnetic phase. Both of them enhance the ionicity when they are introduced into the neutral phase near the boundary. Accordingly, these additional terms enhance low-energy spin excitations, although these excitations are suppressed when compared with those in the regular ionic phase. The regular ionic phase has a larger spectral weight in the local current channel than the neutral phase. This would imply that, in one dimension and if the lattice effect is negligible, the ionic phase has smaller activation energy in the electric conductivity near the boundary than the neutral phase.

DOI： 10.1080/01411590290033877

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Intrachain and interchain local charge-transfer excitation spectra and the single-particle density of states are calculated in the spinless fermion model on a ladder with varying intrachain nearest-neighbor repulsion and interchain transfer integral at and near half filling by using the finite-temperature density-matrix renormalization-group method. Collective excitations are found to govern the low-energy intrachain spectra, while only individual local excitations are present in the interchain spectra. For strong intrachain repulsion, the low-energy motion of fermions is confined within a chain. The interchain motion of fermions is not bandlike but incoherent. As a consequence, the low-energy intrachain spectra are sensitive at half filling to the interchain transfer integral that weakens the density-density correlation along the chains. Similarly, the low-energy intrachain spectra are sensitive near half filling to the chemical potential that reduces the effect of the umklapp process. Similarities are pointed out between these findings and the experimentally observed, optical conductivity spectra in the quasi-one-dimensional organic conductors (TMTSF)(2)X.

DOI： 10.1103/PhysRevB.66.035121

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The mixed-stack organic charge-transfer complex, (BEDO-TTF)(Cl(2)TCNQ), has shown a phase transition accompanied with a sharp drop in the magnetic susceptibility and with an increase in the intermolecular overlap, as the temperature is lowered. Here, we theoretically consider the possibility for a phase transition from the ionic phase to a neutral phase, which is driven by charge-transfer fluctuations. Using finite-temperature density-matrix renormalization-group calculations for the one-dimensional extended Hubbard model with alternating potentials, we show that, with increasing transfer integral t, a transition from the neutral phase to the ionic phase is induced by spin fluctuations for small t, while another transition from the ionic phase to the neutral phase is induced by charge-transfer fluctuations for large t. Near the phase boundary, the free energy is easily lowered by dimerization of transfer integrals or by staggered magnetic field. Thus, a further transition is also possible to a dimerized nonmagnetic phase or to an antiferromagnetic phase at a lower temperature. In two dimensions, we expect that the effect of charge-transfer fluctuations is larger and that the transition to the dimerized nonmagnetic phase is suppressed. (C) 2002 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(02)00043-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

The effects of dimerization causing the alternation of transfer integrals on the neutral-ionic phase transition are studied in the one-dimensional extended Hubbard model with alternating potentials at half filling. The finite-temperature density-matrix renormalization-group method is used to treat the thermodynamic limit. In the ionic phase, the free-energy gain is proportional to delta(4/3) with delta being the degree of dimerization even near the phase boundary at low temperatures. In the neutral phase, the free-energy gain is quadratic for small transfer integral far from the phase boundary, but it is nonlinearly enhanced near the boundary. Large transfer integrals make the gain faster than delta(2), so that they facilitate dimerization. The dimerization in the neutral phase increases the ionicity and lowers the spin excitation energies. These lattice effects are in contrast to the effects of a staggered magnetic field. The relevance of these results to recently observed dimerization in the neutral phase of ClMePD-DMeDCNQI is discussed.

DOI： 10.1103/PhysRevB.65.205105

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：WORLD SCIENTIFIC PUBL CO PTE LTD  

Based on the perturbative renormalization-group (PRO) approach, we have examined interplay or competition between one-particle (1P) and two-particle (2P) processes in strongly correlated low-dimensional electron systems. Throughout the article, we use the Grassmann functional integral approach, since it has an advantage that the 1P degrees of freedom are incorporated in an explicit manner. We mainly discuss an array of chains weakly-coupled via interchain one-particle hopping, where the constituent chains, if isolated, have a strong-coupling fixed point, such as the Mott-insulator, spin-gap-metal, or Anderson-insulator fixed point. In such cases, quantum fluctuations evolving toward the low-energy limit strongly suppress the interchain 1P coherence, and consequently a phase transition from the incoherent metallic (ICM) phase becomes possible. This kind of competition plays a key role to elucidate the interplay of correlation and dimensionality effects in real quasi-one-dimensional (Q1D) materials in nature. As examples, we take up spin-density-wave (SDW) phase transitions in dimerized quarter-filled Hubbard chains to elucidate the nature of the magnetic phase transitions in the Q1D organic conductors, (TMTTF)(2)X and (TMTSF)(2)X. Dimensional crossover problems in Q1D Hubbard ladders are also discussed to describe the pressure-induced superconductivity in the doped ladder systems. Interplay of randomness, electron correlation, and dimensionality effects in weakly-coupled half-filled Hubbard chains with weak quenched random potentials is also studied. We also discuss some 2D electron systems where the two-loop renormalization-group procedure is well defined and works.

DOI： 10.1142/S0217979202009962

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMERICAN PHYSICAL SOC  

Effects of quantum and thermal fluctuations on transitions between the ionic phase and the neutral phase are studied by applying the finite-temperature density-matrix renormalization-group method to, the one-dimensional extended Hubbard model with alternating potentials at half filling. Charge-transfer fluctuations lower the energy of the neutral phase more than that of the ionic phase, which is in contrast to the spin fluctuations favoring, the ionic phase. As a consequence, with increasing intermolecular overlap between the neighboring donor and acceptor sites, we expect a spin-fluctuation-induced transition from the neutral phase to the ionic phase, and a charge-transfer-fluctuation-induced transition from the ionic phase to the neutral phase, near the phase boundary. Relevance to a recently observed phase transition in a, new class of mixed-stack, charge-transfer complexes is discussed.

DOI： 10.1103/PhysRevB.65.085105

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We calculate optical conductivity spectra for various charge ordering (CO) states in the two-dimensional extended Hubbard model at quarter filling with the random phase approximation. The results are examined from a viewpoint of the atomic limit. We compare our results with the measured spectra for the theta-(ET)(2)X salts and discuss the CO patterns and their charge excitations realized in these compounds.

DOI： 10.1080/10587250290101595

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We have studied the optical conductivity for possible ground states of Pd(dmit)(2) salts, exactly diagonalizing a two-band Hubbard model for a dimer. Our results reproduce dominant peaks of the optical spectra measured by Tajima et al. and confirm their argument from the strong-coupling viewpoint. Interaction strengths are estimated and used to study the stability of magnetic orders.

DOI： 10.1080/10587250210769

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We theoretically study optical excitations in K-4[Pt-2(pop)(4)X-2] monomers (X=Cl, Br, I), using an extended Hubbard model. We show that long-range transfer integrals and long-range Coulomb interactions are substantially large to reproduce the experimental results, and discuss effects of long-range Coulomb interactions on the electronic phases and the optical conductivity spectra of the MMX chains.

DOI： 10.1080/10587250210802

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We study spin solitons in the alternate charge polarization background of the MMX chains, using the unrestricted Hartree-Fock approximation to a one-dimensional Peierls-Hubbard model. The effects of the electron-lattice coupling and the electron-electron interaction on the shape of the soliton are discussed.

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We study intra- and inter-chain excitation spectra near a quantum phase transition in a spinless fermion model on a two-leg ladder at half filling by the finite-temperature density-matrix renormalization-group method. Above a critical strength of intrachain nearest-neighbor repulsion, a gap appears, accompanied with a long-range order of density modulation in the checkerboard pattern at zero temperature. The intrachain excitation spectra do not change so much in the gapless phase below the critical strength, while the interchain excitation spectra are considerably altered by the intrachain repulsion even in the gapless phase. Relevance to the optical conductivity in quasi-one-dimensional electron systems is suggested.

DOI： 10.1080/10587250210787

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Stability of three-fold lattice distortion in the insulating state of (DCNQI)(2)Cu is studied by exactly diagonalizing a two-band Peierls Hubbard model on the 6 x 2 lattice. Self-doping is essentially important and caused by strong commensurability pinning, which are a consequence of moderate coupling of DCNQI pi electrons with lattice and strong correlation of Cu d electrons.

DOI： 10.1080/10587250290101973

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We study i) the ground-state and optical-excitation properties of halogen-bridged binuclear metal complexes, which are known as MMX chain compounds, by the strong-coupling expansion for a one-dimensional two-orbital extended Peierls-Hubbard model, and ii) the dynamics of domain walls between the neutral and ionic phases in mixed-stack charge-transfer complexes, by solving the time-dependent Schrodinger equation for a one-dimensional extended Peierls-Hubbard model with alternating energy levels. We find in i) that competition between electron-electron and electron-lattice interactions and competition between short- and long-range interactions are both important for the electronic phase variation, and in ii) that charge and lattice dynamics immediately after photoexcitations is complex and not explained simply within the domino picture.

DOI： 10.1080/10587250290090903

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Numerical simulation has shown that a polymeric molecule can possess a photoinduced phenomenon-photoinduced polarization reversion-in which the electric dipole of the polymeric molecule is reversed by absorbing one photon. This paper provides an analytic theory by means of a response function to prove that a polymeric molecule with a bipolaron has a negative static polarizability and explains the physical origin of this photoinduced phenomenon in detail. This paper also presents a dynamical calculation for the photoinduced polarization reversion, from which the relaxation time for the dipole reversion can be determined.

DOI： 10.1103/PhysRevA.64.032504

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We apply the renormalization-group (RG) approach to two model systems where the two-dimensional Fermi surface has portions which give rise to the logarithmically singular two-loop self-energy process.

DOI： 10.1142/S0217979201005878

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We investigate the optical conductivity of a halogen-bridged binuclear metal complex in an alternate-charge-polarization phase, using a one-dimensional two-band extended Peierls-Hubbard model and the Lanczos technique, to study the dependence of three main peaks ascribed to intradimer, interdimer and halogen-to-metal charge excitations on model parameters.

DOI： 10.1016/S0379-6779(00)01196-6

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Intra- and inter-chain excitation spectra are studied in a spinless fermion model on a two-leg ladder at half filling. Numerical results are obtained by application of the density matrix renormalization group technique to the quantum transfer matrix for infinite systems. Near the metal-insulator transition, the intra-chain Coulomb repulsion is found to affect local current correlation more sensitively in the rung direction than in the leg direction.

DOI： 10.1016/S0379-6779(00)00681-0

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The cation dependence of the magnetic phase transitions in the Pd(dmit)(2) salts is studied by calculating effective exchange interactions between spins on dimers in a two-band Hubbard model. For Et2Me2Sb, the antiferromagnetic order is shown to be destroyed by magnetic frustration within the linear spin wave theory.

DOI： 10.1016/S0379-6779(00)01476-4

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PERGAMON-ELSEVIER SCIENCE LTD  

Ground state phase diagrams for the MMX chains are studied in a one-dimensional two-band model by exactly diagonalizing 18-site clusters. Experimentally observed phases are reproduced by changing relative strengths of an electron-lattice coupling the interdimer transfer integral, and an elastic constant, whose variation is roughly estimated from the interatomic spacing, the species of the halogen ion, and the presence/absence of counter ions. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00183-9

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Quasi-one-dimensional quarter-filled organic conductors, (TMTTF)(2)X and (TMTSF)(2)X, show a dimensional crossover, and their electronic properties change accordingly. We describe this using the two-loop perturbative and density-matrix renormalization-group methods, the epsilon expansion around one dimension, and the random phase approximation, The effect uf dimerization on the charge gap and interchain one-particle coherence is clarified. The effect of random potentials is also studied to describe the Mott insulator, Anderson localization, and metallic phases of (DCNQI)(2)Ag(1-x)Cu(x) mixed crystals. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00108-6

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We calculate absorption spectra for various charge ordering (CO) states in the two-dimensional extended Hubbard model at quarter filling with the random phase approximation. Each CO state has: its own characteristic and anisotropic excitations. In principle, these features should be observed in the optical measurements and tell which CO state is realized in the theta-(ET)(2)X salts. (C) 2000 Elsevier Science Ltd. All rights reserved.

DOI： 10.1016/S0022-3697(00)00177-3

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：ROYAL SOC CHEMISTRY  

Mechanisms Of a variety of charge and lattice ordered phases observed in halogen-bridged binuclear metal complexes are theoretically studied by applying the exact diagonalization and strong-coupling expansion methods to one- and two-band extended Peierls-Hubbard models. In R-4[Pt-2(pop)(4)I]nH(2)O [R=Na, K, NH4, (CH3(CH2)(7))(2)NH2, etc., pop=P2O5H22-] containing charged MMX chains, three electronic phases are suggested by experiments, We find that the variation of the electronic phases originates not only from competition between site-diagonal electron-lattice and electron-electron interactions but also from competition between short-range and long-range electron-electron interactions. On the other hand, in Pt-2(RCS2)(4)I (R = CH3, n-C4H9) containing neutral MMX chains, a site-off-diagonal electron-lattice interaction and the absence of counter ions are found to be crucial to produce the recently found, ordered phase. The optical conductivity spectra are also studied, which directly reflect the electronic phases. Their dependence on the electronic phase and on model parameters is clarified from the strong-coupling viewpoint.

DOI： 10.1039/b101545l

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We study the optical conductivity of the halogen-bridged binuclear metal complexes, using a one-dimensional dimerized 3/4-filled-band model and the Lanczos method. The spectra are quite sensitive to the charge ordering pattern and the long-range Coulomb interaction.

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We study the interplay of randomness, electron correlation, and dimensionality effects in weakly coupled half-filled Hubbard chains with weak quenched random potentials, based on the renormalization-group (RG) approach. We perform a two-loop RG analysis of an effective action derived by using the replica trick, and examine the following crossovers and phase transitions from an incoherent metal regime: (1) a crossover to the Anderson localization regime, (2) an antiferromagnetic phase transition, and (3) a crossover to the quasi-one-dimensional weak-localization regime. The case of d = 1 + ε (ε≪ 1) dimensions is also mentioned.

DOI： 10.1103/PhysRevB.62.13323

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We study dimensionality effects on the charge gap in the dimerized Hubbard model at quarter filling, with two approaches. First, we examine three chains coupled via the interchain one-particle hopping integral t(b), by the density-matrix renormalization-group (DMRG) method. Next, we consider the d = 1 + epsilon dimensional model, using the perturbative renormalization-group (PRG) method. The dimensionality is controlled through t(b) and epsilon, respectively. Both approaches lead to the conclusion that, for a finite dimerization ratio, the charge gap decreases as the dimensionality increases.

DOI： 10.1143/JPSJ.69.2107

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We study ground state phase diagrams for the MMX chains, using a one-dimensional dimerized 3/4-filled extended Hubbard-Peierls model with site diagonal and off-diagonal electron-lattice interactions. The ground states are obtained both in the Hartree-Fock approximation and by the exact diagonalization. We discuss magnetic properties of the MMX chains, (C) 2000 Elsevier Science B.V. All rights reserved.

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We discuss interplay of randomness, electron correlation and dimensionality effects in half-filled random Hubbard chains. Low-temperature phases are given based on the two-loop renormalization group (RG) analysis. Feedback effects of interchain processes on the umklapp process are examined by the 1 + epsilon expansion. We comment on the relevance of the present result to a phase diagram of a doped organic compound, (DI-DCNQI)(2)Ag1 - xCux. (C) 2000 Published by Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(99)03065-3

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In order to see the effects of dimerization, interchain hopping, and nearest-neighbor repulsion on various one- and two-body correlation functions in quasi-one-dimensional organic conductors, (TMTTF)(2)X and (TMTSF)(2)X, we apply the density matrix renormalization group method to a three-chain extended Hubbard model at quarter filling. The hopping correlation function shows that the interchain one-particle coherence is strongly suppressed when a charge gap due to the dimerization is larger than the interchain hopping integral. Meanwhile, the spin-spin and charge-charge correlation functions show that the interchain particle-hole coherence is rather insensitive. The charge-charge correlation function is more sensitive to the nearest-neighbor repulsion than the charge gap. (C) 2000 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(99)02114-6

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We have studied possible ground-state phases of Pd(dmit)(2) salts using the two-band Hubbard model. From a strong coupling expansion, an effective model is derived and used to describe low-energy excitations. It is found that mutual interactions and dimensionality of effective bands cooperatively determine the transport and magnetic properties. (C) 2000 Elsevier Science B.V. All rights reserved.

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The polymeric molecule can exhibit a new photoinduced phenomenon where the electric dipole of the molecule with a bipolaron is reversed by absorbing one photon. This photoinduced polarization inversion occurred in a single molecule is an ultrafast process with a relaxation time of 200 fs.

DOI： 10.1103/PhysRevLett.84.2830

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We have studied the ground state properties of Pd(dmit)(2) salts using an effective dimer model. This model describes low-energy excitations of the two-band Hubbard model and is derived by a strong coupling expansion. Dimensionality of the Fermi surface, density-of-states singularity, and magnetic frustration in the dimer model are simultaneously controlled by substituting the cation.

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We study ground state phase diagrams for the MMX chains, using a one-dimensional dimerized 3/4-filled extended Hubbard-Peierls model with sire diagonal and off-diagonal electron-lattice interactions. The ground states no obtained mainly in the Hartree-Fock approximation, and their accuracy is checked by the exact diagonalization of small clusters. We find a new phase in addition to Frequently considered phases and compare our results with experimental results.

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We study the relation between the charge gap and the interchain one-body correlation function in quasi-one-dimensional dimerized organic conductors at quarter filling, by applying the density matrix renormalization group method to a three-chain extended Hubbard model. The charge gap increases with the degree of dimerization in the intrachain hopping integrals. When the charge gap is larger than the interchain hopping integral, the interchain hopping correlation is strongly suppressed, as observed in the (TMTTF)(2)X salts.

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We study interplay of electronic correlation, randomness and dimensionality effects in half-filled random Hubbard chains weakly coupled via an interchain one-particle hopping. Based on the two-loop renormalization-group approach, phase diagrams are given in terms of temperature vs. strengths of the intrachain electron-electron umklapp scattering, the random scattering and the interchain one-particle hopping.

DOI： 10.1080/10587250008026198

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Ground state phase diagrams of the MMX chains ale studied in a one-dimensional dimerized 3/4-filled model by exactly diagonalizing 12-site clusters. Experimentally observed phases are reproduced by changing relative strengths of an electron-lattice coupling, the inter-dimer transfer integral, and an elastic constant whose variation is roughly estimated from the interatomic spacing, the species of the halogen ions and the presence/absence of counter ions.

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We study qualitative differences of excited states between a charge ordered state and a charge uniform state. The excited states are calculated in the random phase approximation for the one-dimensional extended Hubbard model at quarter Filling. In the charge ordered state, a dominant excitation in the current-current correlation function has no charge density modulation. A dominant excitation in the charge uniform state originates from crossing the dimerization gap and modulates the charge density.

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We study effects of a preformed Mott yap and dimensionality on interchain one-particle coherence and spin-density-wave phase transitions in weakly-coupled dimerized quarter-filled Hubbard chains. A phase diagram is given, based on the two-loop perturbative renormalization-group (RG) approach together with the random phase approximation. Feedback effects of interchain processes on the umklapp process are examined by the 1 + epsilon expansion. We discuss relevance of the present result to the SDW phase transitions in the quasi-one-dimensional dimerized quarter-filled organic conductors, (TMTTF)(2)X and (TMTSF)(2)X.

DOI： 10.1023/A:1022504925213

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We study how magnetic and pairing correlation evolves with an increasing interchain hopping integral and decreasing dimerization of intrachain hopping integrals, by applying the density matrix renormalization group (DMRG) method to a three-chain extended Hubbard model at quarter filling for quasi-one-dimensional organic conductors, (TMTTF)(2)X and (TMTSF)(2)X. Magnetic correlation changes from weakly coupled chains of large-amplitude spin density waves to an interchain-coherence-developed spin density wave. Pairing correlation increases, though it Still decays exponentially owing to a charge gap for parameters considered here.

DOI： 10.1023/A:1022513127030

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We studied one of possible grounded states for Pd(dmit)(2) salts in a dimerized two-band Hubbard model. By using a, strong-coupling expansion, we obtained an effective dimer model which has a quasi-one-dimensional Fermi surface. In the insulating phase, it leeds to the Heisenberg model on an anisotropic triangular lattice. We calculated effective exchange coupling strengths within. the second order perturbation theory and found that both ferromagnetic and antiferromagnetic couplings are possible. Similarities to and differences from Ni(dmit)(2) salts are discussed.

DOI： 10.1023/A:1022548707466

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Electronic and geometrical structures of bis(ethylenedithio)tetrathiafulvalene (BEDT-TTF) molecules are studied using ab initio molecular orbital methods. The optimized structure of a BEDT-TTF monomer is close to the experimental one within errors of 0.02 Angstrom and 0.5 deg in bond length and angle, respectively, except the ethylene group. Ab initio parameters such as transfer integrals and Coulomb interactions are determined from the BEDT-TTF dimer and tetramer calculations. Using model Hamiltonians with the ab initio parameters, we investigate the electronic states based on the exact diagonalization method. The results show that the ground state has antiferromagnetic correlation which is consistent with experimental results. We study the effects of long-range Coulomb interactions employing the 2-D extended Hubbard model with the Hartree-Fock approximation. It is found that the ground state shows various phases; antiferromagnetic, charge ordering, and paramagnetic ones, controlled by the long-range interactions. (C) 1999 American Institute of Physics. [S0021- 9606(99)30437-2].

DOI： 10.1063/1.479894

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We have studied spin density wave (SDW) phase transitions in dimerized quarter-filled Hubbard chains weakly coupled via interchain one-particle hopping, t(bO). It is shown that there exists a critical value of t(bO), t(b)*, between the incoherent metal regime (t(bO) < t(b)*) and the Fermi liquid regime (t(bO) > t(b)*) in the metallic phase above the SDW transition temperature. By using the 2-loop perturbative renormalization-group (PRG) approach together with the random-phase-approximation, we propose a SDW phase diagram covering both of the regimes. The SDW phase transition from the incoherent metal phase for t(bO) < t(b)* is caused by growth of the intrachain electron-electron umklapp scattering toward low temperatures, which is regarded as preformation of the Mott gap. We discuss relevance of the present result to the SDW phase transitions in the quasi-one-dimensional dimerized quarter-filled organic conductors, (TMTTF)(2)X and (TMTSF)(2)X.

DOI： 10.1143/JPSJ.68.2790

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In electroluminescent polymers, the exciton is polarized by a weak electric field E and dissociated by strong E. These effects can quantitatively elucidate the observed dependence of the luminous intensity of the luminescent polymer on the electric field and the field-induced charge generation in conjugated polymers.

DOI： 10.1002/(SICI)1521-3951(199908)214:2<337::AID-PSSB337>3.0.CO;2-H

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Effects of the intrachain dimerization and the interchain one-particle hopping, t(b), in a quasi-one-dimensional dimerized chain system at quarter filling have been studied, based on the perturbative renormalization group (PRG) approach. Based on the results, we discuss difference in the low-energy properties between TMTTF and TMTSF compounds.

DOI： 10.1016/S0379-6779(98)00486-X

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We study crossover from the doped two-leg ladder system to a quasi-one-dimensional system by an inter-ladder one-particle process and stability of the superconducting phase, using a renormalization-group method and differentiating the contributions from the particle-particle channel and those from the particle-hole channel. Effects of electron-2k(F) phonon and electron-4k(F) phonon couplings are also investigated.

DOI： 10.1016/S0379-6779(98)00935-7

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Recently an X-ray experiment reported a coexisting phase of purely-electronic 2k(F) charge density wave (CDW) and the conventional 2k(F) spin density wave (SDW) in (TMTSF)(2)PF6. In order to understand such coexistence, we have performed a mean-field calculation for a Hubbard-type model of a quarter-filled one-dimensional chain. It is found that the coexistence of 2k(F) SDW and purely-electronic 2k(F) CDW is realized due to the effect of next-nearest-neighbor Coulomb repulsion. Phase diagrams are obtained as a function of nearest- and next-nearest-neighbor Coulomb interaction energies. Competition between the two coexistent states with 2k(F) CDW and with 4k(F) CDW is revealed, which gives a first order transition. It turns out that the internal degrees of freedom inside a dimer (consisting of two TMTSF molecules) play an important role. Our result supports the X-ray result of a coexisting phase in (TMTSF)(2)PF6.

DOI： 10.1016/S0379-6779(98)00567-0

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The ground states of the molecular conductors (Me(4)Z)[Pd(dmit)(2)](2), which have strong dimerization and two bands with different dimensionality, have been studied based on the Hartree-Fock approximation. The results indicate that two different magnetic orders appear depending on both the energy difference between the two bands and the strength of the Coulomb interaction. To clarify the origin of the magnetic orders, exchange interactions between dimers have been calculated within the second-order perturbation theory with respect to the inter-dimer transfer integrals. We have found that the different magnetic orders are due to the different symmetry of the molecular orbitals.

DOI： 10.1016/S0379-6779(98)00585-2

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Using the one-dimensional quarter-filled Heisenberg-Kondo lattice model for a pi-d electron system, CoPc(AsF6)(0.5), we study the effects of long-range Coulomb interaction among pi electrons on the conductivity. The density-matrix renormalization-group method is employed. As a first, step, we regard the localized spins of d electrons as classical ones, and then, as quantum S=1/2 spins, which are coupled antiferromagnetically with each other. In the former case, d electrons produce a static staggered magnetic field for pi electrons. It is pointed out that the next-nearest-neighbor repulsion among a electrons and the magnetic coupling between pi and d electrons are essential to open a charge gap.

DOI： 10.1016/S0379-6779(99)80004-6

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER PHYSICAL SOC  

Two-loop renormalization-group (RG) analysis for a two-dimensional electron system with a partially Bat Fermi surface has been carried out. We found that the RG flow of the quasiparticle weight depends on the location of the electron wave number k on the Fermi surface due to the kinematical restriction to the logarithmically divergent processes in the flat regions of the Fermi surface.

DOI： 10.1103/PhysRevB.59.14823

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We study spin-density wave (SDW) in polymers, using screened Coulomb potential to describe full electron interaction in the broad-band systems and developing a spin-dependent correlated-basis-function method to handle the correlation effect. The SDW is shown to be sensitively enhanced with shortening interaction range and suppressed by the correlation effect.

DOI： 10.1016/S0379-6779(98)00654-7

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We discuss dimensional crossover phenomena in weakly coupled one-dimensional conductors, in terms of the universality classes of the corresponding isolated one-dimensional systems.

DOI： 10.1016/S0379-6779(98)00665-1

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We study the tunneling dynamics of dopant-induced hole polarons that are self-localized by electron-phonon coupling in a two-dimensional antiferromagnet. Our treatment is based on a path-integral formulation of the adiabatic (Born-Oppenheimer) approximation, combined with many-body tight-binding, instanton, constrained lattice dynamics, and many-body exact diagonalization techniques. The applicability and limitations of the adiabatic approximation in polaron tunneling problems are discussed in detail and adiabatic results are compared to exact numerical results for a two-site polaron problem. Our results are mainly based on the Holstein-tJ and? for comparison, on the Holstein-Hubbard model. We also study the effects of second-neighbor hopping and long-range electron-electron Coulomb repulsion. The polaron tunneling dynamics is mapped onto an effective low-energy Hamiltonian that takes the form of a fermion tight-binding model with occupancy-dependent, predominantly second- and third-neighbor tunneling matrix elements, excluded double occupancy, and effective intersite charge interactions. Antiferromagnetic spin correlations in the original many-electron Hamiltonian are reflected by an attractive contribution to the first-neighbor charge interaction and by Berry phase factors that determine the signs of effective polaron tunneling matrix elements. In the two-polaron case, these phase factors lead to polaron-pair wave functions of either d(x2-y2)-wave symmetry or p-wave symmetry with zero and nonzero total pair momentum, respectively. Implications for the doping-dependent isotope effect, pseudogap, and T-c of a superconducting polaron-pair condensate are discussed and compared to observed properties of the cuprate high-T-c materials. [S0163-1829(99)04602-0].

DOI： 10.1103/PhysRevB.59.1444

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The localized vibrational modes of exciton and polaron are investigated, and the results show that the frequencies of the three even parity modes of exciton are very different from those of polaron. For the exciton, three modes are distinctly separated; but, for the polaron, two modes with higher frequencies are close to each other, Then, it is possible to distinguish exciton from polaron by watching their Roman spectra. Therefore, the localized mode may be used to specify the exciton and polaron in the electroluminescent polymers.

DOI： 10.1088/0256-307X/16/11/021

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We study electronic states of the (DCNQI)(2)M (M = Li and Ag) salts based on the full configuration interaction (FCI) method using effective Hamiltonians derived from ab initio molecular orbital theory. FCI results of the DCNQI tetramer and octamer models indicate that the ground state has antiferromagnetic and charge ordering correlations. It corresponds to the 2k(F) spin density wave and 4k(F) charge density wave states (SDW and CDW, respectively). In the octamer model, it is also found that some low-lying excited states have similar spin-flipped CDW correlations and the antiferromagnetic correlation is weakened. (C) 1998 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0009-2614(98)01212-3

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In order to clarify the effects of electron interactions on the competition between SDW (spin-density wave) and BOW (bond-order wave). the existing theories are improved in two aspects: (1) The electron interaction in polymers. which possess a broadband. is described by a screened Coulomb potential rather than the extended Hubbard model, which is a good approximation only for narrow-band systems. (2) A spin-dependent correlated-basis-function method is developed to calculate the correlation effect, which goes beyond the Hartree-Fock approximation, Our study shows that the off-diagonal interaction substantially influences the competition so that shortening of the interaction range greatly enhances the SDW: meanwhile the correlation effect promotes the BOW against the SDW. [S0163-1829(98)00537-2].

DOI： 10.1103/PhysRevB.58.9039

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Effects of the intrachain electron-electron umklapp process and the interchain one-particle hopping, II, in a weakly coupled half-filled chain system have been studied, based on the perturbative renormalization-group approach. By solving the scaling equations, we found that the intrachain umklapp process strongly suppresses an interchain one-particle process and causes a finite critical value for t(perpendicular to), t(perpendicular to c). For t(perpendicular to) &lt; t(perpendicular to c), as the temperature decreases, the system undergoes a phase transition into an antiferromagnetic long-range-ordered phase at a transition temperature T-N which increases with the increasing t(perpendicular to). On the other hand, for t(perpendicular to) &gt; t(perpendicular to c), the system undergoes a crossover to the Fermi liquid phase where the interchain coherent propagation of quasi-particles occurs.

DOI： 10.1143/JPSJ.67.2590

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Based on the perturbative renormalization group (PRG) approach, we have studied dimensional crossovers in Hubbard ladders coupled via weak interladder one-particle hopping, t(perpendicular to). found that the one-particle crossover is strongly suppressed through growth of the intraladder scattering processes which lead the isolated Hubbard ladder system toward the spin gap metal (SGM) phase. Consequently when t(perpendicular to) sets in, there exists, for any finite intraladder Hubbard repulsion, U &gt; 0, the region where the two-particle crossover dominates the one-particle crossover and consequently the d-wave superconducting transition, which is regarded as a bipolaron condensation, occurs. By solving the scaling equations for the interladder one-particle and two-particle hopping amplitudes, we give phase diagrams of the system with respect to U, t(perpendicular to 0) (initial value of t(perpendicular to)) and the temperature, T. We compared the above dimensional crossovers with those in a weakly coupled chain system: clarifying the difference between them.

DOI： 10.1143/JPSJ.67.1714

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We have performed a mean field calculation in order to determine the ground state of a modified Hubbard model of a quarter-filled one-dimensional chain. It is found that the coexistence of 2k(F) spin density wave (SDW) and purely-electronic 2k(F) charge density wave (CDW) is realized due to the effect of next-nearest-neighbor Coulomb repulsion. Phase diagrams are obtained as the values of nearest-and next-nearest-neighbor Coulomb interaction energies are varied. Competition between the two coexistent states with 2k(F) CDW and 4k(F) CDW is revealed, which gives a first order transition. Our result supports the X-ray result confirming the presence of a coexisting phase of purely-electronic 2k(F) CDW and 2k(F) SDW in (TMTSF)(2)PF6, a quarter-filled one-dimensional chain in terms of holes.

DOI： 10.1143/JPSJ.67.1098

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Because pi electrons in polymers possess a broad band, the off-diagonal part of the electron interaction, which is omitted in the extended Hubbard model, plays an essential role in the competition between bond-order waves (BOWs) and spin-density waves (SDWs). Our study, which goes beyond the extended Hubbard model and the Hartree-Fock approximation, shows that this competition depends on the range of the electron interaction; a short interaction range, where the off-diagonal part is more effective, is in favor of SDW. It is also shown that the correlation effect works against the exchange effect in building up the SDW. (C) 1998 Published by Elsevier Science B.V.

DOI： 10.1016/S0375-9601(97)00928-6

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We study crossovers from one to quasi-one dimensions in doped two-leg ladder systems, using a renormalization group method. When quasi-one dimensionality is regarded as controlled by incomplete interference between the Cooper and Peierls channels, we obtain spin-density wave, d-wave superconductor, and metallic phases with decreasing dominance of the Peierls channel. When transverse single-particle hopping is treated perturbationally, we obtain a one-particle crossover from incoherent to coherent transverse motion for weak interaction and a two-particle crossover from short- to long-range ordering of a d-wave superconductor for strong interaction. We also study a magnetic property in a doped Heisenberg-Kondo chain, using the exact diagonalization method. A fully saturated ferromagnetism in the ferromagnetic Kondo chain is destroyed by rather weak antiferromagnetic Heisenberg coupling between localized 5=1/2 spins. [doped ladder, pair tunneling, dimensional crossover, Heisenberg-Kondo lattice, ferromagnetic Kondo model]. © 1998, The Japan Society of High Pressure Science and Technology. All rights reserved.

DOI： 10.4131/jshpreview.7.490

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Effects of the interladder one-particle hopping, t(perpendicular to), on the low-energy asymptotics of a weakly coupled Hubbard ladder system have been studied, based on the perturbative renormalization-group approach. We found that for finite intraladder Hubbard repulsion, U, there exists a crossover value of the interladder one-particle hopping, t(perpendicular to c). For 0 &lt; t(perpendicular to) &lt; t(perpendicular to c), the spin gap metal (SGM) phase of the isolated ladder transits at a finite transition temperature, T-c, to the d-wave superconducting (SCd) phase via a two-particle crossover. In the temperature region, T &lt; T-c, interladder coherent Josephson tunneling of the Cooper pairs occurs, while the interladder coherent one-particle process is strongly suppressed. For tie &lt; tl, around a crossover temperature, T-cross, the system crosses over to the two-dimensional (2D) phase via a one-particle crossover. In the temperature region, T &lt; T-cross, the interladder coherent band motion occurs.

DOI： 10.1143/JPSJ.66.3725

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Metal-insulator transitions and different ground-state phases in the quasi-one-dimensional materials, (R1R2-DCNQI)(2)M (R-1 = R-2 = CH3, I and M = Ag, Cu), are studied with a renormalization-group method. We use one-dimensional continuum models with backward scatterings, umklapp processes, and couplings with 2k(F) and 4k(F) phonons (not static lattice distortion). We take a quarter-filled band for M = Ag and a sixth-filled band coupled with a third-filled band for M = Cu. Depending on electron-electron and electron-phonon coupling strengths, the ground-state phase becomes a Tomonaga-Luttinger liquid or a state with a gap(s). For M = Ag, there appears a spin-gap state with a dominant 2k(F) charge-density-wave correlation, a Mott insulator with a dominant 4k(F) charge-density-wave correlation, or a spin-Peierls state with different magnitudes of spin and charge gaps. Three dimensionality is taken into account by cutting off the logarithmic singularity in either the particle-particle channel or the particle-hole channel. The difference between the ground-state phase of the R-1 = R-2 = CH3 salt (spin-Peierls state) and that of the R-1 = R-2 = I salt (antiferromagnetic state) is qualitatively I explained by a difference in the cutoff energy in the particle-particle channel. For M = Cu, there appears a Mott insulator with a charge-density wave of period 3 and a Peierls insulator with a charge-ensity wave of period 6. The conditions for the experimentally observed, Mott insulator phase are strong correlation in the sixth-filled band, moderate electron-phonon couplings, and finite electron-4k(F) phonon coupling. Resistance is calculated as a function of temperature with a memory-function approximation in both cases above. It qualitatively reproduces the differences among the M = Ag and M = Cu cases as well as the R-1 = R-2 = CH3 and R-1 = R-2 = I cases.

DOI： 10.1103/PhysRevB.56.7262

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The scaling shows that superconductivity in a two-leg ladder is easily broken by couplings with 2k(F) and 4k(F) phonons at quarter filling. Three-dimensionality makes the system an antiferromagnet if the nesting is still perfect.

DOI： 10.1016/S0921-4534(97)90663-8

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The quasi-particle spectrum for s-wave superconductors in the Vortex-lattice state remains gapless in magnetic fields much below the upper critical field H-c2 where the cyclotron frequency is smaller than the amplitude of the pair potential. This is shown through self-consistent solutions of the Bogoliubov-de Gennes equations for the attractive Hubbard model on the square lattice. It has not been predicted in earlier theoretical works based on a continuous model because it is due to the dispersion of Landau subbands in the lattice model.

DOI： 10.1143/JPSJ.66.953

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DOI： 10.1016/S0379-6779(97)80379-7

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Renormalization-group equations are solved to study phase diagrams of nearly-half-filled one-dimensional conductors interacting with phonons. A spin-gap charge density wave phase exists between a gapless density wave phase and a spin-gap superconductor phase. On-site and intersite electron-phonon couplings show large differences near half-filling in the presence of on-site electron repulsion.

DOI： 10.1016/0921-4534(95)00715-6

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The exponents of spin and charge correlations of one-dimensional electron systems are calculated by a renormalization-group method. We study combined effects of electron-phonon interaction and the Umklapp process on renormalization flows. As scattering processes are successively integrated out, these two processes are interfered with each other depending on the type of electron-phonon coupling, phonon frequency and filling. We demonstrate how these factors control a spin gap formation and density-wave/superconductor correlations.

DOI： 10.1080/10587259608030805

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Asymptotic properties of nearly half-filled one-dimensional conductors coupled with phonons are studied through a renormalization-group method. Due to spin-charge coupling via an electron-phonon interaction, the spin correlation varies with filling as well as the charge correlation. Depending on the relation between cutoff energy scales of the umklapp process and of the electron-phonon interaction, various phases appear. We find a metallic phase with a spin gap and a dominant charge-density-wave correlation near half filling between a gapless density-wave phase (like in the doped repulsive Hubbard model) and a superconductor phase with a spin gap. The spin gap is produced by phonon-assisted backward scatterings that interfere with the umklapp process constructively or destructively depending on the character of electron-phonon coupling. © 1996 The American Physical Society.

DOI： 10.1103/PhysRevB.54.2410

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We use path integral and exact diagonalization techniques to study the tunneling of dopant-induced hole polarons which are self-localized by electron-phonon coupling in a two-dimensional anti-ferromagnet, Their low-energy dynamics is found to be dominated by 2nd and 3rd neighbor hopping processes which carry a (-1) Berry phase factor and lead to non-s-wave hole pairing symmetries, including possible nodeless d(x2-y2) pair wavefunctions. The nodeless The polarons obey effective spin-1/2-fermion quasi-particle statistics. d(x2-y2)-wave pairing state can exhibit a transition from gapless to fully gapped behavior in its quasi-particle excitation spectrum, without change of pairing symmetry.

DOI： 10.1007/BF00727424

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DOI： 10.1103/PhysRevA.50.5324

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DOI： 10.1103/PhysRevB.50.2899

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The Hubbard model on a two-dimensional square lattice is investigated within an unrestricted, inhomogeneous Hartree-Fock approximation. Vortexlike states discussed by Verges et al. [Phys. Rev. B 43, 6099 (1991)] are studied in detail. They are found to be unstable against transverse spin fluctuations. Doping of holes does not suppress these fluctuations strongly enough to stabilize the vortex states.

DOI： 10.1103/PhysRevB.48.6680

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Optical responses in a one-dimensional two-band Peierls-Hubbard model near 5/8 filling (i.e., near 1/4 filling in the effective one-band model) are studied within inhomogeneous Hartree-Fock and random-phase approximations. Low-energy collective excitations are produced by a sliding motion of a spin-density wave. In the commensurate case (i.e., at 5/8 filling) or near this filling with moderate strength of intersite electron-lattice coupling, the spin-density wave (with polaronic defects in the latter case) is pinned by commensurability with the lattice, and the lowest-frequency optical response lies in the infrared regime. Otherwise, the spin-density wave is incommensurate and moves freely unless it is pinned by extrinsic impurities. Implications for midinfrared absorptions observed in YBa2Cu3O7 and PrBa2Cu3O7 compounds are discussed. With sufficiently weak hopping between the Cu and O sites, a ferromagnetic phase is found.

DOI： 10.1103/PhysRevB.48.3349

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The ground-state energy and static and dynamic correlation functions are investigated in the inhomogeneous-Hartree-Fock (IHF) plus random-phase-approximation (RPA) approach applied to a one-dimensional spinless-fermion model showing self-trapped doping states at the mean-field level. Results are compared with those obtained using homogeneous HF and exact diagonalization. RPA fluctuations added to the generally inhomogeneous-HF ground state allow the computation of dynamical correlation functions that compare well with exact diagonalization results. The RPA correction to the ground-state energy agrees well with the exact results at strong- and weak-coupling limits. We also compare it with a related quasiboson approach. The instability towards self-trapped behavior is signaled by a RPA mode with frequency approaching zero.

DOI： 10.1103/PhysRevB.47.13156

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Doping states in a two-dimensional three-band Peierls-Hubbard model for the copper oxides are investigated with inhomogeneous Hartree-Fock (HF) and random-phase approximations. The doping states are sensitive to small changes of interaction parameters because they easily change local energy balance between different interactions around added holes. For the parameter values derived from constrained-density-functional methods for the copper oxides, added holes form isolated small ferromagnetic polarons. When the parameters are varied around these values, different types of doping states are obtained: For stronger on-site repulsion at Cu sites, larger ferromagnetic polarons are formed, which are qualitatively different from the small polarons; for stronger nearest-neighbor Cu-O repulsion, polarons are clumped or there occurs phase separation into Cu- and O-hole-rich regions; intersite electron-lattice coupling rapidly changes the small polarons by quenching a Cu magnetic moment and locally distorting the lattice in an otherwise undistorted antiferromagnetic background. This is regarded as a rapid crossover from a Zhang-Rice singlet to a covalent molecular singlet, and occurs substantially below a critical strength for destruction of the stoichiometric antiferromagnetic state. However intrasite electron-lattice coupling, in contrast to the intersite coupling, does not dramatically affect the hole-doping states. Doping induces modes in magnetic, optical, and vibronic response functions. Local infrared-active phonon modes are induced in infrared absorption spectra for finite electron-lattice coupling. They are correlated with doping-induced particle-hole excitations observed in optical absorption spectra and in magnetic excitation spectra. These doping-induced particle-hole excitations are associated with the local HF eigenstates in the charge-transfer gap. Each doping state has distinctive excitation spectra in the magnetic, optical, and vibronic channels. In particular, the hole-doping states with small polarons have doping-induced, infrared absorption peaks on the low-frequency side of the stoichiometric peak, while the electron-doping states have them on the high-frequency side.

DOI： 10.1103/PhysRevB.47.12059

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We discuss the consequences of both electron-phonon and electron-electron couplings in 1D an 2D multi-band (Peierls-Hubbard) models. After briefly discussing various analytic limits, we focus on (Hartree-Fock and exact) numerical studies in the intermediate regime for both couplings, where unusual spin-Peierls as well as long-period, frustrated ground states are found. Doping into such phases or near the phase boundaries can lead to further interesting phenomena such as separation of spin and charge, a dopant-induced phase transition of the global (parent) phase, or real-space (''bipolaronic'') pairing. We discuss possible experimentally observable consequences of this rich phase diagram for halogen-bridged, transition metal, linear chain complexes (MX chains) in 1D and the oxide superconductors in 2D.

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Stoichiometric phases and doping states are studied in a two-dimensional, three-band extended Peierls-Hubbard model, using an inhomogeneous Hartree-Fock approximation. Magnetism and covalency are investigated in a parameter space of strong intersite electron-lattice coupling and varying on-site electron-electron repulsion strength. In a crossover region between a charge-density-wave phase driven by intersite electron-lattice coupling and an antiferromagnetic phase driven by on-site electron-electron repulsion, a bond-order-wave state, a spin-Peierls state, and a mixed state of spin-Peierls bonds and antiferromagnetic spins axe found to be stable and caused by competing magnetism and covalency of different relative strengths. Doping states depend qualitatively upon the nature of these global phases. In particular, separation of spin and charge is observed in the background of the bond-order-wave state as a consequence of local competition between magnetism and covalency.

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DOI： 10.1142/S0217984993000369

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Nonlinear doping states in a charge-density-wave background and those in a spin-density-wave background are investigated through their response functions in vibronic, optical and magnetic channels in a one-dimensional two-band Peierls-Hubbard model. A random phase approximation is used for their analysis on the basis of totally unrestricted Hartree-Fock solutions. Doping-induced changes of excitation spectra in these different channels are distinctive to the nature of individual doping states.

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Doping states in a two-dimensional three-band Peierls-Hubbard model are investigated with inhomogeneous Hartree-Fock and random phase approximations. They are sensitive to small changes of electron-lattice and electron-electron interactions. For parameters relevant to the insulating copper oxides a small ferromagnetic polaron is found. Moderate intersite electron-lattice coupling triggers a nonlinear feedback mechanism resulting in a rapid crossover from a Zhang-Rice regime to a covalent molecular singlet state in which the local magnetic moment is quenched and local lattice distortion is large. Various states are characterized by distinct optical and infrared absorption spectra.

DOI： 10.1103/PhysRevLett.69.965

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As a convenient qualitative approach to strongly correlated electronic systems, an inhomogeneous Hartree-Fock plus random-phase approximation is applied to response functions for the two-dimensional multiband Hubbard model for cuprate superconductors. A comparison of the results with those obtained by exact diagonalization by Wagner, Hanke, and Scalapino [Phys. Rev. B 43, 10 517 (1991)] shows that overall structures in optical and magnetic particle-hole excitation spectra are well reproduced by this method. This approach is computationally simple, retains conceptual clarity, and can be calibrated by comparison with exact results on small systems. Most importantly, it is easily extended to larger systems and straightforward to incorporate additional terms in the Hamiltonian, such as electron-phonon interactions, which may play a crucial role in high-temperature superconductivity.

DOI： 10.1103/PhysRevB.45.5530

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All the linear excitations are calculated in the random-phase approximation around a doped inhomogeneous Hartree-Fock state as well as the undoped uniform antiferromagnetic state in the one-dimensional Hubbard model. Upon doping, shape modes appear, localized around an introduced hole, as well as some "shake-off" branches related to combinations of local and extended Hartree-Fock orbitals. These modes are in addition to spin wave and ultragap branches in the pure antiferromagnetic state. Some characteristics of the shape modes are similar to bare processes in the t-t'-J model derived from the Hubbard model at strong coupling. Spectral weights in different susceptibility channels are calculated and shown to have peaks in low-energy regions, arising from specific shape modes.

DOI： 10.1103/PhysRevB.44.2652

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOCIETY JAPAN KIKAI-SHINKO BUILDING  

DOI： 10.1143/JPSJ.59.2183

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DOI： 10.1143/JPSJ.58.4576

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DOI： 10.1143/JPSJ.58.3304

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DOI： 10.1143/JPSJ.57.3875

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：PHYSICAL SOC JAPAN  

DOI： 10.1143/JPSJ.56.4400

Web of Science

J-GLOBAL

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J-GLOBAL

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J-GLOBAL

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J-GLOBAL

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J-GLOBAL

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J-GLOBAL

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J-GLOBAL

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Language：Japanese   Publisher：The Physical Society of Japan  

J-GLOBAL

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J-GLOBAL

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J-GLOBAL

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Language：Japanese  

J-GLOBAL

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Language：Japanese   Publisher：The Physical Society of Japan  

&lt;p&gt;三角格子構造の二次元有機伝導体alpha-(ET)2I3の光強電場誘起の電荷局在を7 fs近赤外光によって明らかにしてきた、今回は、7 fsプローブパルスの偏光を変え、局在電荷状態の異方性を明らかにする。また、ポンプ光による印加電場の偏光方向に対して、電荷局在の効率がどのように変わるのかを調べ、厳密対角化に基ずく理論計算との比較から、光誘起電荷局在の機構を議論する。&lt;/p&gt;

J-GLOBAL

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J-GLOBAL

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J-GLOBAL

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.70.2.0_1351

CiNii Books

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J-GLOBAL

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J-GLOBAL

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.70.2.0_1354

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.70.1.0_1522

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan (JPS)  

CiNii Books

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Language：Japanese   Publisher：丸善  

CiNii Books

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Publisher：OSA  

Photoinduced metal to insulator (M-I) change were realized by strong electric filed (10 MV/cm) of 1.5-cycle (7 fs) near infrared pulse. Threshold for the M-I change decrease near the M-I transition temperature. The M-I change is driven by coherent charge oscillation with a period of 20 fs. © 2014 OSA.

DOI： 10.1364/up.2014.09.wed.p3.40

Scopus

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Language：Japanese   Publisher：The Physical Society of Japan  

CiNii Books

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Language：Japanese   Publisher：The Physical Society of Japan  

CiNii Books

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