Language：English   Publishing type：Research paper (scientific journal)   Publisher：ACADEMIC PRESS INC ELSEVIER SCIENCE  

A novel family of cubic manganese oxides, AMg(4)Mn(6)O(15) (A=K, Rb, and Cs), were discovered. In this type of structure, the MnO6 octahedra share edges, constructing a highly symmetric framework where Mn atoms form a three-dimensional network of truncated octahedra. The crystallographic site of Mn is unique and the average oxidation state of Mn is 3.5, indicating a mixed-valence electronic state where Mn3+ and Mn4+ sites are completely indistinguishable. These compounds become ferromagnetic with fully polarized magnetic moments of Mn ions. The ferromagnetic transition temperature Tc is 170 K, that is considerably high for manganese oxides. The electric resistivity of KMg4Mn6O15 is about 10(5) Omega cm at 300 K and exhibits a non-metallic temperature dependence. It reveals a large negative magnetoresistance; about 40% of the resistivity is suppressed by 5 T of magnetic field at T-C.

DOI： 10.1016/j.jssc.2017.02.003

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The dielectric and structural properties of the S = 1 kagome antiferromagnet KV3Ge2O9 are examined. The low-temperature structure below 50 K is orthorhombic with a typical correlation length of 8 nm. While the high-temperature hexagonal phase can be considered C-centered orthorhombic, the C-centered symmetry is broken below 50 K. The low-temperature symmetry does not support the simplex solid state, which is the theoretically expected ground state for the S = 1 kagome lattice. Above 60 K, incommensurate lattice modulation is observed. The lock-in transition suggests that the origin of the orthorhombic deformation is the development of a short-range magnetic ordering.

DOI： 10.1103/PhysRevB.95.104416

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A single-crystal S = 3/2 perfect kagome lattice antiferromagnet, KCr3(OH)(6)(SO4)(2) (Cr-jarosite), has been studied by X-band and high-frequency electron spin resonance (ESR). The g-values perpendicular to the kagome plane (c-axis) and in the plane were determined to be g(c) = 1.9704 +/- 0.0002 and g(xi) = 1.9720 +/- 0.0003, respectively, by high-frequency ESR observed at 265 K. Antiferromagnetic resonances (AFMRs) with an antiferromagnetic gap of 120 GHz were observed at 1.9 K, which is below T-N = 4.5 K. The analysis of AFMR modes using the conventional molecular field theory gave d(p) = 0.27K and d(z) = 0.07 K, where d(p) and d(z) are in-plane and out-of-plane components of d vector of the Dzyaloshinsky-Moriya (DM) interaction, respectively. On the basis of these results and the exchange interaction of J = 6.15K estimated by Okuta et al., the ground state of Cr-jarosite was discussed in connection with the Monte Carlo simulation results with classical Heisenberg spins on the kagome lattice by Elhajal et al. Finally, the angular dependence of the linewidth and lineshape observed at 296K by X-band ESR showed the typical behavior of a two-dimensional Heisenberg antiferromagnet, suggesting the good two-dimensionality of Cr-jarosite.

DOI： 10.7566/JPSJ.86.024703

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Publishing type：Research paper (international conference proceedings)  

© 2014 IEEE. THz ESR measurement has been a powerful means to study magnetic systems due to the advantages of high spectral resolution, ESR measurement beyond the zero field splitting or the magnetic phase transition. Using these advantages we have studied perfect kagome lattice antiferromagnet Cr-jarosite, which has attracted much attention in the field of magnetism recently, by our THz ESR.

DOI： 10.1109/IRMMW-THz.2014.6956420

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

The structure and magnetism of Fe-2(OH)[B2O4(OH)] are reported. Powder x-ray diffraction reveals a characteristic structure containing two crystallographically independent zigzag-ladder chains of magnetic Fe2+ ions. Magnetization measurements reveal a phase transition at 85 K, below which a weak spontaneous magnetization (approximate to 0.15 mu B/Fe) appears. Below 85 K, magnetization increases with decreasing temperature down to 70 K, below which it decreases and approaches a constant value at low temperature. The Mossbauer spectrum at room temperature is composed of two paramagnetic doublets corresponding to the two crystallographic Fe2+ sites. Below 85 K, each doublet undergoes further splitting because of the magnetic hyperfine fields. The temperature dependence of the hyperfine field is qualitatively different for the two distinguishable Fe2+ sites. This is responsible for the anomalous temperature dependence of the magnetization.

DOI： 10.1088/0953-8984/26/26/266002

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本高圧力学会  

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本高圧力学会  

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

The spin-1/2 triple-chain magnet Cu-3(OH)(4)SO4 (mineral name: antlerite) is a candidate 'idlespin' system. In order to study the phase transitions in this compound in detail, we have performed specific-heat measurements under magnetic fields up to 10 T and in the temperature range down to 1.9 K and proton nuclear magnetic resonance (H-1-NMR) experiments with single crystals. We found at least three successive phase transitions at zero field around 5 K which strongly depend on the value and the direction of the applied field. Another transition around 3 K was also found. H-1-NMR spectra in a low-temperature and low-field ordered phase indicated an antiferromagnetic commensurate spin structure with at least four sublattices, which is discussed in connection with the idle-spin state. Phase diagrams show a richness of magnetic phases for this compound.

DOI： 10.3938/jkps.62.2054

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Language：Japanese   Publishing type：Research paper (scientific journal)   Publisher：日本セラミックス協会  

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：IEEE  

Multi-extreme THz ESR measurements have been performed on Cr-jarosite, which is an ideal model substance of highly frustrated kagome antiferromagnet. The analysis of antiferromagnetic resonances observed at 1.9K revealed the Dzyaloshinsky-Moriya interaction, which plays an important role on the ground state.

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

Single crystals of the novel vanadium oxide compound KV3Ge2O9 have been synthesized. This compound has a hexagonal unit cell with the space group P6(3)/mmc. Its lattice constants are a = 5.8624(4) angstrom and c = 13.7094(7) angstrom. KV3Ge2O9 contains layers of edge-shared VO6 octahedra, where V3+ ions with an S = 1 spin form kagome lattice layers separated by double layers of GeO4 tetrahedra. The magnetic susceptibility shows a broad maximum at approximately 70 K, suggesting that magnetic transition is suppressed in spite of the presence of strong antiferromagnetic interactions. Below approximately 20 K, the susceptibility under a magnetic field perpendicular to the kagome plane steeply drops toward zero, while the in-plane susceptibility diverges with reduction in temperature.

DOI： 10.1143/JPSJ.81.073707

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

Single crystals of the S = 3/2 kagome lattice antiferromagnet KCr3(OH)(6)(SO4)(2) (Cr-jarosite) have been synthesized. Magnetic susceptibility measurement reveals an antiferromagnetic transition at T-N = 4.5 K. A weak ferromagnetism with a spontaneous magnetization along the c-axis appears below T-N. High-field magnetization measurement reveals an anomaly at about 1/3 of the saturation magnetization under in-plane magnetic fields, however, the out-of-plane magnetization curve is almost linear. In contrast to ordinary field-induced transitions, the anomaly is less pronounced at lower temperatures. This observation is consistent with the prediction made using the order-by-disorder model on the 1/3 magnetization plateau.

DOI： 10.1143/JPSJ.80.063703

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Single crystals of Cu-3(OH)(4)SO4 containing triple chains of S = 1/2 spins were synthesized by a hydrothermal method. This compound is a candidate "idle-spin" system in which some of its spins seem to disappear, while the rest exhibit a long-range order. Its magnetic susceptibility indicates an antiferromagnetic transition at T-N = 5: 5 K, below which a large anisotropy appears. Unlike its magnetic structure predicted from neutron studies, its spin-easy axis slightly deviates from the c-axis. Two-step field-induced magnetic transitions occur in the magnetization curves below T-N under magnetic fields along the c-and a-axes. Furthermore, magnetic fields much below the saturation field, regardless of their direction, induce a transition into a paramagnetic phase.

DOI： 10.1143/JPSJ.80.043701

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The magnetic properties of a newly synthesized compound, Fe3B7O13(OH), are reported. The space group and lattice constants at room temperature are R3cH (trigonal, #161), a=8.590(3) angstrom and c=21.107(7) angstrom. In this compound, three Fe2+ ions form an equilateral triangular trimer sharing a hydroxide ion, and the trimers further construct a trigonal lattice. The magnetic susceptibility exhibits an antiferromagnetic phase transition accompanied by a steep drop in susceptibility at T-N approximate to 4.8K under small magnetic fields. Mossbauer spectroscopy indicates that Fe ions are divalent. It also suggests a simple spin structure in which all of the spins are equally canted from the principal axis of the electric-field gradient below T-N. This magnetic ground state is, however, easily destroyed by almost 1 T of magnetic field regardless of its direction. In addition to this transition, successive metamagnetic transitions are induced by larger magnetic fields. The additional transitions are strongly dependent on the direction of the applied magnetic field, resulting in a rich magnetic phase diagram.

DOI： 10.1143/JPSJ.80.014801

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：TRANS TECH PUBLICATIONS LTD  

TlFe2Se2-delta was synthesized by a self-flux method with excess amount of selenium. Elemental analysis using energy-dispersive X-ray spectroscopy estimates the deficiency 8 at about 0.5. The magnetic susceptibility reveals an antiferromagnetic transition at T-N approximate to 475 K. The resistivity exhibits a semiconducting behavior with an activation energy, E-A = 0.04 eV. There is a clear anomaly in the resistivity at T-N, indicating a strong interplay between magnetism and electric conduction.

DOI： 10.4028/www.scientific.net/SSP.170.47

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We have discovered a novel compound Na(0.12)CrO(2)Ge(0.18)O(x)center dot yH(2)O using a hydrothermal method. Its powder X-ray diffraction reveals a monoclinic unit cell with lattice parameters: a = 7.260 angstrom, b = 2.950 angstrom, c = 5.117 angstrom and beta = 103.7 degrees. A Rietveld analysis shows that this compound has a layered structure based on CrO(2) sheets consisting of edge-shared CrO(6) octahedra. Such a layered structure is commonly seen in many hexagonal Cr oxides such as NaCrO(2). A heat treatment at 100 degrees C modifies the crystal structure without destruction of a layered structure based on Cr triangular-lattice, because H(2)O molecules among the CrO(2) sheets are removed at high temperature. Magnetization measurements reveal a spin-glass behavior below T(g) = 3 K.

DOI： 10.4028/www.scientific.net/SSP.170.29

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We succeeded in synthesizing single crystals of a novel nickel sulfate Ni(2-delta)SO(5) by a hydrothermal method. A single-crystal X-ray diffraction revealed a tetragonal structure with lattice constants, a = 5.184 angstrom and c = 12.91 angstrom. In this structure, one-dimensional chains composed of face-shared NiO(6) octahedra elongate along the a axis and the b axis alternately, constructing a log-cabin' type framework where neighbor chains lying orthogonally to each other are connected by SO(4) tetrahedra. A detailed refinement of the structure suggested that the site occupancy of Ni is only 63%, corresponding to delta approximate to 2/3. Furthermore, X-ray oscillation photographs exhibit diffuse-scattering patterns probably arising from a short-range order of the Ni defects. Magnetic susceptibility revealed an antiferromagnetic transition accompanied by a weak ferromagnetism at 17 K.

DOI： 10.4028/www.scientific.net/SSP.170.25

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Cu-3(OH)(4)SO4, a parallel Cu (S = 1/2) triple chain system, is an interesting magnet because an idle-spin state is expected on the central chain. We have succeeded in growing high-quality single crystals of Cu3SO4(OH)(4) by a hydrothermal method from copper sulfate and copper hydroxide. We measured field dependence of the magnetization on a single crystal under various directions of magnetic field up to 7 T. We found a clear magnetic anisotropy and confirmed that the existence of the easy axis lies in the ac plane. Under magnetic fields parallel to the c-axis and the a-axis at 2.5 K, we observed field-induced successive magnetic transitions. In the case of B parallel to c, two anomalies of the magnetization are observed at 0.9 and 1.7 T, On the other hand, there are broad anomalies in the magnetization at 3.7 and 5.6 T in the case of B parallel to a.

DOI： 10.4028/www.scientific.net/SSP.170.194

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We report single-crystal neutron diffraction studies on a spinel antiferromagnet GeCo2O4, which exhibits magnetic order with a trigonal propagation vector and tetragonal lattice expansion (c/a similar or equal to 1.001) below T-N = 21 K. For this inconsistency between spin and lattice in symmetry, magnetic Bragg reflections with a tetragonal propagation vector were discovered below TN. We discuss spin and orbital states of Co2+ ion underlying the new magnetic component.

DOI： 10.1088/1742-6596/320/1/012038

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Other Link： http://orcid.org/0000-0003-2134-7786

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

The structural and magnetic properties of Li7RuO6 were investigated in single crystals. This ruthenate is composed of quasi-two-dimensional distorted triangular lattices of RuO6 octahedra isolated from each other. Because of the (t(2g))(3) electron configuration of Ru5+, a Heisenberg-type magnetic behavior of S = 3/2 spins is expected. The magnetic susceptibility reveals an antiferromagnetic transition at T-N = 6.5 K, much lower than the absolute value of the Weiss temperature Theta = -75 K, indicating a strong geometric frustration. The small and almost isotropic drop of the susceptibility below T-N suggests a helical magnetic structure. Specific heat revealed an additional phase transition at T'(N) = 12 K, suggesting an unconventional magnetic phase diagram.

DOI： 10.1143/JPSJ.79.064705

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

The hollandite chromium oxide K2Cr8O16 has been synthesized in both powder and single-crystal form under high pressure. Combining electrical resistivity, magnetic susceptibility, and x-ray diffraction, we found that K2Cr8O16 is a ferromagnetic metal (or half-metal) with T-C=180 K and shows a transition to an insulator at 95 K without any apparent structural change but retaining ferromagnetism. K2Cr8O16 is quite unique in three aspects: It has a rare mixed valence of Cr3+ and Cr4+; it has a metal (or half-metal)-to-insulator transition in a ferromagnetic state; and the resulting low-temperature phase is a rare case of a ferromagnetic insulator. This discovery could open a new frontier on the relation of magnetism and conducting properties in strongly correlated electron systems.

DOI： 10.1103/PhysRevLett.103.146403

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Co4B6O13 contains undistorted tetrahedral clusters of magnetic Co2+ ions. The high-field magnetization of this magnet exhibits a periodic undulation indicating quantization of the total spin number per cluster. Measurements of magnetic susceptibility and specific heat reveal that the ground state is composed of several different singlet states, reflecting the high symmetry of the spin tetrahedron. An exact diagonalization calculation taking account of single-ion-type anisotropies and Dzyaloshinsky-Moriya interactions reproduces the experimental results very well.

DOI： 10.1103/PhysRevB.80.014424

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Using a hydrothermal method, single crystals of Co-4(BO2)(6)O were synthesized. The DC susceptibility revealed a Curie-Weiss behavior at high temperature. The Weiss temperature was estimated at -190 K, showing very strong antiferromagnetic interactions. However, the susceptibility does not show any phase transitions down to 2 K, although there is a broad maximum in the vicinity of 14 K. This demonstrates an effect of the geometric frustration arising from the crystal structure based on isolated Co-4 tetrahedral clusters. The magnetization curve exhibits an onset of a transition to higher magnetization state at around 5 T. The result suggests that the Co-4 cluster has a nonmagnetic ground state with a spin gap and a magnetic field induces a transition into a magnetic state above the spin gap.

DOI： 10.1088/1742-6596/150/4/042046

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We report on 'Li NMR studies of single crystals of the triangular-lattice Heisenberg anti ferromagnet Li7RuO6. A slow critical divergence with a wide critical region of vertical bar T/T-N - 1 vertical bar <= 7 was observed in Li-7 nuclear spin-lattice relaxation rate. The slowing down of staggered spin fluctuations was analyzed ill it renormalized classical region of a two-dimensional triangular-lattice nonlinear sigma model. Spill stiffness constant was found to decrease to about 20% of its Value in spin-wave approximation. The effect of spill frustration, e.g., Z(2) vortex excitations on critical phenomena, is suggested,

DOI： 10.1143/JPSJ.78.023705

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Single crystals of KSbO3-type rhenium oxides, La4Re6O19, Pb6Re6O19, Sr2Re3O9, and Bi3Re3O11, were synthesized by a hydrothermal method. Their crystal structures can be regarded as a network of three-dimensional orthogonal-dimer lattice of edge-shared ReO6 octahedra. All of them exhibit small magnitude of Pauli paramagnetism, indicating metallic electronic states without strong electron correlations. The resistivity of these rhenates, except Bi3Re3O11, have a temperature dependence of rho(T) = rho(0) + AT(n) (n approximate to 1.6) in a wide temperature range between 5 and 300 K, which is extraordinary for three-dimensional metals without strong electron correlations. The resistivity of Bi3Re3O11 shows an anomaly around at 50K, where the magnetic susceptibility also detects a deviation from ordinary Pauli paramagnetism.

DOI： 10.1143/JPSJ.76.044805

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

A novel lithium ruthenate, cubic-LixRuO1+x, was discovered. In this compound, Ru and Li randomly occupy the cation sites of the NaG-type lattice. Magnetic susceptibility measurements revealed a spin-glass transition at 10 K. (c) 2006 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2006.10.658

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DOI： 10.1016/j.carbon.2006.09.017

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In order to control the dimensionality and the topology of the arrangement of magnetic Ru ions, NaCl-type lithium ruthenates have been investigated. Three types of lithium ruthenates, Li3RuO4, Li2RuO3, and cubic-LixRuO1+x, were synthesized using a hydrothermal method. All of them have NaCl-type structure, but the arrangements of the cations, Li+ and Ru5+ (or Ru4+), differ from each other. Li3RuO4 with one-dimensional zigzag chains of Ru ions undergoes an antiferromagnetic transition at 66 K and exhibits an irreversibile magnetism below 32K, where the zero-field-cooled susceptibility differs from the field-cooled susceptibility. Li2RuO3 with a two-dimensional honeycomb network of Ru ions demonstrates a paramagnetism almost independent of temperature. A novel ruthenate cubic-LixRuO1+x, in which Ru and Li randomly occupy the cation sites of NaCl lattice, undergoes a spin-glass transition at 10 K.

DOI： 10.1143/JPSJ.75.124802

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Single crystals of a novel ruthenate, Ca2Ru2O7, were obtained. An X-ray diffraction study on a single crystal revealed that this material crystallizes in a pyrochlore structure with a lattice parameter, a = 10.197 angstrom. The magnetic susceptibility above 30 K is the summation of a Curie-Weiss contribution and a constant term independent of temperature. The effective moment per Ru atom is only 0.36 mu(B), one order of magnitude smaller than that expected from a localized spin model with S = 3/2 for Ru5+. Below 23 K, the localized spins freeze in a spin-glass state. The resistivity at room temperature is 2 x 10(-3) Omega(.)cm, comparable to that in metallic, highly correlated oxides.

DOI： 10.1143/JPSJ.75.103801

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Structural, magnetic, and transport properties of a novel hollandite-type molybdenum oxide, Rb1.5Mo8O16 have been investigated. Whereas a typical hollandite structure is characterized by uniform zigzag ladder chains of edge-shared MO6 (M = transition metal) octahedra, the zigzag ladders in Rb1.5Mo8O16 are strongly distorted, and consequently, "Mo-4" clusters composed of four edge-shared MoO6 octahedra are formed. A semiquantitative band calculation based on an extended Huckel method shows that the band structure assumes the characteristics of the molecular orbitals of ail isolated Mo-4 cluster. The magnetic susceptibility reveals the existence of spins spread over each Mo-4 cluster rather than localized on each Mo atom. The electrical resistivity and the thermoelectric power indicate that the electric conduction is governed by cluster-to-cluster electron hopping. Rb1.5Mo8O16 undergoes a phase transtion at T-1 = 208 K, accompanied by a change in the number of spins and carriers. X-ray diffractions at low temperatures detected a slight breakdown of the tetragonal symmetry below T-1. The transport and the magnetic properties of related compounds, K2Mo8O16 and Ba1.14Mo8O16 are also reported.

DOI： 10.1143/JPSJ.75.014802

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Nanographite having open edges has a nonbonding pi-electron state of edge origin (edge state), which causes unconventional electronic and magnetic features. Bromine-adsorption effect on the electronic and magnetic properties of nanoporus carbon consisting of a flexible three-dimensional random nanographite network system is investigated by using activated carbon fibers (ACFs) as host material. The irreversible adsorption of bromine into ACFs at room temperature gives compositions up to Br/C=0.43. The interaction between nanographite and the adsorbed bromine is classified into three groups; charge transfer, covalent bonding, and physisorption interactions, where the latter two are the majorities. Although the charge transfer rate from carbon to bromine is considerably small (0.0004 per C atom at maximum), in comparison with that in bulk graphite-bromine intercalation compounds, the downshift of the Fermi energy results in the large reduction of the localized spin concentration of edge state and the orbital diamagnetism. This proves the important role of the edge state in the charge transfer process. The electron spin resonance results demonstrate the contribution of the orbital character of bromine to the edge state in nanographite through orbital mixing. The physisorbed bromine accommodated into the nanopores induces the dielectric and structural effects on the nanographite. The former causes the modification of the carrier conduction process due to the charging effect, while the latter results in the magnetic switching phenomenon induced by the effective pressure of physisorbed bromine species to the edge-state spins.

DOI： 10.1103/PhysRevB.73.035435

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Language：Japanese   Publishing type：Research paper (other academic)   Publisher：日本物理学会  

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

Crystal structure, magnetic and transport properties of new ternary mixed-valence iridium oxide Ba2Ir3O9 are reported. A hydrothermal reaction of ItO(2) and BaO2 under a pressure of 150 MPa at 650degreesC yields small black crystals with cubic shape. Energy dispersive X-ray analysis on an as-grown crystal gives the atomic ratio Ba:Ir = 2:3. X-ray powder diffraction shows a pattern of cubic lattice with a cell parameter a = 9.42 Angstrom and further Rietveld analysis reveals the KSbO3-related structure. Although the structure of Ba2Ir3O9 is almost identical to Ba0.5IrO3 [Mat. Res. Bull. 9 (1974) 1777], there is a significant difference in the composition between them. Because the crystallographic site for It is unique, Ba2Ir3O9 is classified into class III mixed-valence compound where the average valence of It is +4.67. The magnetic susceptibility shows almost temperature independent Pauli paramagnetism of 2.9 x 10(-4) emu/mol Ir at room temperature. The resistivity for a pellet sample is measured to be 1.5 x 10(-1) Omega cm at room temperature and shows metallic temperature dependence. At low temperature, the temperature dependent component of the resistivity is almost proportional to T-2. This indicates that Ba2Ir3O9 is a bad metal because of strong electron-electron scattering. The narrow 5d-band feature characteristic of the KSbO3-type structure is related to the strong electron correlation. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jallcom.2004.04.062

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We report on structures, magnetisms, and transport properties of two new barium ruthenates, 10H-type BaRuO3 and Ba2Ru7O18. The crystal structure of 10H-BaRuO3 consists of face-shared dimers and trimers; of RuO6 octahedra. The magnetic susceptibility of 10H-BaRuO3 is 7.7 x 10(-4) emu/mol at RT. We interpret its very weak temperature dependence as Pauli paramagnetism enhanced by electron correlations. The resistivity of 10H-BaRuO3 is 8.5 x 10(-2) Omega cm at 296 K. The slope of the R versus T curve is negative in the whole temperature. It indicates metallic or semi-metallic electronic states. The structure of BaRu7O18 is composed of edge-shared trimers and tetramers of RuO6 octahedra. The magnetic susceptibility of Ba2Ru7O18 is 3.7 x 10(-4) emu/mol Ru at RT. Its almost temperature-independent character is also explained in terms of Pauli paramagnetism enhanced by electron correlations. The resistivity of Ba2Ru7O18 is 6.7 x 10(-4) Omega cm at 296 K. In this case, the slope of the R versus T curve is positive, and the temperature dependence is linear to T-1.2 below 25 K. This shows a large deviation from a Fermi-liquid criterion. (C) 2004 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jallcom.2004.04.035

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

The magnetic and transport properties of a new cubic KSbO3-type ruithenate, (Ba1-xSrx)Ru3O9 (xapproximate to0.35), have been investigated. The crystal structure has a singular geometry in which ruthenium atoms form an ideal three-dimensional orthogonal dimer lattice. The magnetic susceptibility is Pauli-paramagnetic but exhibits an anomalous temperature dependence reminiscent of a gap-like behavior. The resistivity exhibits a metallic behavior, except for a rise at low temperature. (C) 2004 Elsevier Ltd. All rights reserved.

DOI： 10.1016/j.ssc.2004.03.021

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

Antiferromagnetism in stacked nanographite is investigated with using the Hubbard-type models. The A-B stacking or the stacking near to that of A-B type is favorable for the hexagonal nanographite with zigzag edges, in order that magnetism appears. We also find that the open shell electronic structure can be an origin of the decreasing magnetic moment with the decrease of the inter-graphene distance, as experiments on adsorption of molecules suggest. Next, superperiodic patterns with a long distance in a nanographene sheet observed by STM are discussed in terms of the interference of electronic wave functions. The period and the amplitude of the oscillations decrease spatially in one direction. We explain the superperiodic patterns with a static linear potential theoretically. In the k - p model, the oscillation period decreases, and agrees with experiments. The spatial difference of the static potential is estimated as 1.3 eV for 200 nm in distance, and this value seems to be reasonable in order that the potential difference remains against perturbations, for example, by phonon fluctuations and impurity scatterings. It turns out that the long-distance oscillations come from the electronic structure of the two-dimensional graphene sheet. (C) 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physe.2003.12.105

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

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

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Language：Japanese   Publishing type：Research paper (other academic)   Publisher：日本物理学会  

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

The heat-treatment effect on electronic properties is investigated in relation to structural change for pulsed-laser-deposited amorphous carbon thin films having sp(2)/sp(3) ratio approximate to9. The heat treatment at temperatures 200-400 degreesC increases conductivity and modifies the hopping conduction mechanism at low temperatures, resulting in the generation of a Coulomb gap at E-F. This is attributed to the heat-treatment-induced modification of the disorder nature of the structure from atomic-scale sp(2)/sp(3) disorder to a disordered graphitic sp(2)-domain network by the migration of sp(3) defects. In the heat-treatment temperature region above 600 degreesC, where the structure is featured with graphitic sp(2) domains, considerably small positive thermoelectric power is suggestive of carrier compensation by the competition of hole and electron carriers that originate from the inhomogeneous charge distribution caused by the difference of Fermi levels among graphitic sp(2) domains. In the high-heat-treatment-temperature region 800-1100 degreesC, the formation of an infinite percolation path network of the graphitic sp(2) domains induces an insulator-to-metal transition, where the electron transport in the sp(2)-rich metallic state is featured by weakly temperature-dependent conductivity with majority hole and minority electron carriers.

DOI： 10.1103/PhysRevB.67.214202

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

It has been theoretically expected that critical phenomena of helimagnetism is drastically different from those of collinear antiferromagnetism due to the existence of chiral degree of freedom. In order to verify this prediction, we carried out X-ray magnetic scattering measurements on a simplest helimagnet beta-MnO2 using a synchrotron radiation. The critical exponent beta was estimated at 0.25+/-0.05 from the temperature dependence of the scattering intensity. This value is consistent with the predicted value 0.253 for chiral system rather than with 0.346 for collinear system. (C) 2003 Elsevier Science B.V. All rights reserved.

DOI： 10.1016/S0921-4526(02)02501-2

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

Activated carbon fibers (ACFs), composed of nanometer-size fragments of graphite (nanographites), show pi-electron-based paramagnetism. Measurements of ESR and magnetic susceptibility revealed that adsorption of water molecules drastically suppresses the paramagnetism of ACFs. Considering also the structural change, this is interpreted in terms of a crossover from a paramagnetic state to a low-spin state of each nanographite due to a structural change of nanographite-network caused by internal pressure of adsorbed water molecules. This phenomenon serves us as a new method for in situ controlling of the condensed state of electrons in soft materials. (C) 2003 Elsevier Science Ltd. All rights reserved.

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Language：English   Publishing type：Part of collection (book)   Publisher：Elsevier Ltd.  

This chapter discusses the magnetism of the nano-sized graphite using-the activated carbon fibers (ACFs) and the heat-treated nano-diamond particles. The structure of ACFs can be described by a 3D disorder network of nanographites, where each nano-graphite domain consists of the stacked 3-4 graphene sheets-having a mean size of 3 nm. The prepared ACFs exhibit Coulomb gap variable-range hopping conduction, revealing the essential role of the charging effects of the nano-graphite domains-where the magnetism is associated with the edge-state spins. The spin-glass-like disordered magnetism appears near a heat-induced insulator-metal transition, and is related to the random strengths of the inter-nano-graphite anti-ferromagnetic interactions-mediated by the p-conduction carriers. Fluorination of the ACFs generates localized spins of the sp3-defects at the expense of the edge-state spins. The heat-treatment of the diamond nano-particles converts them to the carbon nano-particles, with complete graphitization taking place around 1600°C. The itinerant p-electron system of the graphite is gradually developed with the heat-treatment. © 2003 Elsevier B.V. All rights reserved.

DOI： 10.1016/B978-008044163-4/50023-1

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

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

The structure, magnetic susceptibility and conductivity of a newly synthesized manganese oxide (BaMn3O6) are reported. Single crystal structure analysis revealed a layered structure. Two types of layers, (I) double layers composed of edge-shared MnO6 octahedra and (II) perovskite-like layers composed of vertex-shared MnO6, are stacked alternatively. All MnO6 octahedra are strongly distorted due to the Jahn-Teller effect. Although the chemical formula BaMn3O6 indicates a mixed-valence state of Mn3+ and Mn4+, the electrical conductivity shows an activation-type temperature dependence, suggesting a semiconductive band structure or Mott-type insulating electronic states. Magnetic susceptibility measurement revealed an antiferromagnetic transition at TN=295 K. Above TN, the magnetic susceptibility exhibits almost temperature-independent behavior. This deviation from molecular-field behavior can be attributed to a large frustration effect originating from the topology of the magnetic-interaction network. © 2001 Elsevier Science B.V.

DOI： 10.1016/S0925-8388(00)01320-7

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Language：English   Publisher：Moscow State University  

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Language：English   Publisher：Moscow State University  

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Language：English   Publisher：Moscow State University  

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

An X-ray scattering experiment using synchrotron radiation has been performed on rutile-type beta -MnO2 single crystal. Below T-N, satellites due to magnetic scattering appear, which provides direct evidence of a helical magnetic structure. The wave number of the magnetic helix is approximately 0.297c*, incommensurate to the lattice and shows very weak temperature dependence. The extinction rule of the satellites is consistent with the magnetic structure proposed by Yoshimori [J. Phys. Sec. Jpn. 14 (1959) 807] in which the nearest helix chains are coupled in an antiferromagnetic way. The temperature dependence of the scattering intensity indicates that the critical exponent beta is anomalously small for an ordinary three dimensional antiferromagnet; this strongly suggests a large chiral degeneracy effect of helical magnetism.

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

The electronic structure of networked nano-graphites is investigated in relation to the effects of fluorine-introduced sp(3)-defects on the nano-sized pi -electron system. In the low fluorine concentration region, the shrinkage of the rr-conjugated system due to introduced sp3-defects reduces the orbital diamagnetism and modifies the electron hopping process between nano-graphite domains, and the edge-inherited non-bonding pi -electrons having localized spins, which give a unique feature to nano-sized pi -electron system, are seriously affected by the attack of fluorine atoms to edge carbon sites. In the intermediate fluorine concentration region, the spin density is enhanced by dangling bond spins generated by the local destruction of the pi -conjugated system at interior carbon sites. The hyperfine-broadening of the linewidth and the reduced spin-lattice relaxation rate in ESR spectra suggest the isolated nature of the fluorine-induced localized spins in contrast to the edge-inherited spins.

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

Structural and electronic properties of K, (formula presented) and (formula presented) intercalated nanographite compounds, prepared using standard intercalation techniques, were investigated. The staging phenomenon observed in bulk-graphite-intercalation compounds is absent in the case of intercalation compounds of nanographite, as expected for a finite-size host-guest system. K-intercalated samples contain a small fraction of potassium clusters apart from forming homogeneous mixtures of several stages. Many Raman features of K-intercalated nanographite were found to be similar to K-doped single-wall carbon nanotubes. The first evidence of charge transfer from nanographite to iodine, which is absent in bulk graphite, is revealed based on Raman scattering results. The charge transfer per carbon atom (formula presented) follows the order (formula presented) which is similar to the trends observed in bulk-graphite-intercalation compounds. Intercalation of strong donors such as potassium makes the contribution of edge inherited nonbonding π states, which uniquely characterize pristine nanographite, less important. This is explained considering the large charge transfer and the accompanying shift of the Fermi level away from the edge states. In case of weak acceptors such as (formula presented) however, there is an enhancement in density of states that indicates a smaller shift in the Fermi level, keeping it in the vicinity of edge states. © 2001 The American Physical Society.

DOI： 10.1103/PhysRevB.64.235407

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Graphite nanoparticles were prepared by the heat treatment of diamond nanoparticles in the range 900-1600 °C. X-ray diffraction, transmission electron microscopy (TEM) and Raman scattering studies indicate that the onset temperature of the diamond-graphite transition is around 1200 °C and the complete conversion of diamond to graphite occurs at 1600 °C. Based on the structural characteristics the samples are categorized into sp3-dominated (as-prepared and 900 °C), sp2:sp3 mixed-phase (1200 and 1400 °C), and sp2-dominated systems (1600 °C). The larger c-axis repeat distances and the high-resolution TEM images for the sp2:sp3 mixed-phase systems denote the presence of the remnant buckling feature of the diamond (111) planes in the graphene sheets. Magnetic susceptibility and ESR studies suggest the development of itinerant-π-electron system from the 1200 °C and higher-temperature heat-treated samples. The completely graphitized sample reveals the important role of edge-inherited nonbonding π-electron states in the electronic structure. The Raman G-peak position and the orbital diamagnetism show considerable deviation from the bulk-graphite values, which is explained on the basis of charge transfer from the graphite π band to the localized edge states and the resulting shifting of the Fermi level. The enhanced spin-lattice relaxation rates in the case of more graphitized samples heat-treated at 1400 and 1600 °C are expected to arise from the involvement of the localized edge-state electrons. In the less-graphitized 1200 °C heat-treated sample, however, the corrugated nature of the graphene planes is likely to hinder such fast-relaxation processes. © 2000 The American Physical Society.

DOI： 10.1103/PhysRevB.62.11209

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

Graphite nanoparticles were prepared by the heat treatment of diamond nanoparticles in the range 900-1600 degreesC. X-ray diffraction, transmission electron microscopy (TEM) and Raman scattering studies indicate that the onset temperature of the diamond-graphite transition is around 1200 degreesC and the complete conversion of diamond to graphite occurs at 1600 degreesC. Based on the structural characteristics the samples are categorized into sp(3)-dominated (as-prepared and 900 degreesC), sp(2):sp(3) mixed-phase (1200 and 1400 degreesC), and sp(2)-dominated systems (1600 degreesC). The larger c-axis repeat distances and the high-resolution TEM images for the sp2:sp3 mixed-phase systems denote the presence of the remnant buckling feature of the diamond (111) planes in the graphene sheets. Magnetic susceptibility and ESR studies suggest the development of itinerant-pi -electron system from the 1200 degreesC and higher-temperature heat-treated samples. The completely graphitized sample reveals the important role of edge-inherited nonbonding pi -electron states in the electronic structure. The Raman G-peak position and the orbital diamagnetism show considerable deviation from the bulk-graphite values, which is explained on the basis of charge transfer from the gaphite pi band to the localized edge states and the resulting shifting of the Fermi level. The enhanced spin-lattice relaxation rates in the case of more graphitized samples heat-treated at 1400 and 1600 degreesC are expected to arise from the involvement of the localized edge-state electrons. In the less-graphitized 1200 degreesC heat-treated sample, however, the corrugated nature of the graphene planes is likely to hinder such fast-relaxation processes.

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

In order to consider the dimensionality and the interaction between conduction electrons and localized magnetic moments, angular dependences of the magnetoresistance of magnetic GIC's, stage-1, and stage-a MCl2 GIC's (M=Cu and Co) were measured. The c-axis (perpendicular to the graphite layers) resistance of stage-1 CuCl2 GIC shows a typical angular dependent magnetoresistance oscillation (ADMRO) which is commonly observed in several quasi two-dimensional (2D) electron systems with a periodically-warped cylindrical Fermi surface. This proves a band conduction along the c-axis in spite of extremely large anisotropy in conductivity (rho(c)/rho(a) = 10(5)). On the other hand, stage-a CoCl2 GIC shows a larger MR for the magnetic field along tilt? c axis than that along the graphitic plane. In addition, ADMRO is absent in this compound. These suggest that the c-axis conduction is governed by incoherent interlayer hoppings. The angular dependence of MR in stage-2 CuCl2 and stage-1 CoCl2 GIC's show an intermediate behavior between that of stage-1 CuCl2 GIC and stage-2 CoCl2 GIC.-The above material-dependence is attributed to the difference in interlayer coupling and suggests a dimensional crossover From a highly anisotropic three-dimensional (3D) system (stage-l CuCl2 GIG) to an almost pure 2D system (stage-a CoCl2 GIG). The c-axis resistances of stage-1 and stage-a CoCl2 GIC's reflect also the magnetic states of Co spins. This phenomenon is explained in terms of a model based on spin-dependent tunneling effect.

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

Activated carbon fibers (ACFs) are microporous carbons consisting of a three-dimensional disordered network of nano-graphites with a mean in-plane size of about 30 Angstrom. We investigated the structure, electronic properties and iodine doping effects for ACF samples heat-treated up to 2800 degrees C. The samples heat-treated below 1000 degrees C exhibit Coulomb gap variable-range hopping conduction and the presence of localized spins, suggesting the importance of charging effects and the edge-inherited non-bonding states in nano-graphites, the latter being predicted theoretically. Iodine doping reduces the charging effect due to the dielectric constant enhanced by the iodine that is accommodated in the micropores. Heat treatment above 1300 degrees C changes ACFs from an Anderson insulator to a disordered metal by the development of an infinite inter-nano-graphite percolation path network for electron transport, accompanied by a change from localized-spin magnetism to itinerant electron magnetism. In the metallic regime, carrier scattering is subjected to nano-graphite boundaries in terms of a short range random potential. Iodine-doping introduces ionized impurity scattering, which is caused by the I-3(-) ions generated by the charge transfer from iodine to nano-graphite.

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

The magnetism of activated carbon fibers composed of a disorder network of nanographites was investigated, where each nanographite has about 1 edge-inherited localized spin. The susceptibility, for samples situated around the metal-insulator threshold, shows a cusp around 4-7 K in addition to the presence of a field-cooling effect. These behaviors are explained in terms of disordered magnetism caused by random strengths of inter-nano-graphite antiferromagnetic interactions mediated by pi-conduction carriers.

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

Transport properties and magnetic susceptibility have been investigated for single crystals of the magnetic conductor beta-MnO2. Resistivity, Hall coefficient, and thermoelectric power reveal the behavior of n-type metal with an anomalously small mobility. The transport properties are accompanied by a large anomaly around T-N(approximate to 92 K) below which a helical magnetic long-range order forms. This indicates that the e(g) conduction electrons, originating from a slight off-stiochiometry, are strongly coupled with localized magnetic moments in the t(2g) orbitals by means of Hund's rule. Below T-N, the magnetoresistance shows a large anisotropy reflecting the helical magnetic structure. Below about 50 K, the mobility gradually decreases, suggesting a carrier localization. The anisotropic measurement of magnetic susceptibility on a single crystal reveals the behavior of proper-type helical magnetism qualitatively, but shows a significant deviation from a molecular-field analysis based on a localized spin model.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GORDON BREACH SCI PUBL LTD  

Activated carbon fibers (ACFs) composed of a nano-graphite disordered network have huge specific surface areas ranging 1000-3000m(2)g(-1). The electronic states of iodine adsorbed in ACFs were investigated by I-129 Mossbauer spectroscopy with a focus on charge transfer from nano-graphite to iodine. The observed spectra consisting of five different quadrupole octets demonstrate the coexistence of I-3(-) ion and I-2 molecule in micropores. This fact implies that I-3(-) ions are produced as a result of charge transfer from nano-graphite to iodine. The estimated charge transfer rate on the basis of the ratio of generated I-3(-) is in good agreement with the previous results obtained by magnetic susceptibility.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GORDON BREACH SCI PUBL LTD  

Angular dependence of the c-axis magnetoresistance of stage-1 and stage-2 MCl2 GIC (M=Cu and Co) was measured. Stage-1 CuCl2 GIC shows an angular dependent magnetoresistance oscillation (ADMRO) characteristic in the systems having a periodically-warped cylindrical Fermi surface (FS), This means that the conduction along the c-axis is band-like in spite of extremely large anisotropy in conductivity. On the other hand, the angular dependence of the magnetoresistance of stage-2 CuCl2 GIC and stage-1 CoCl2 GIC can be interpreted as the mixture of ADMRO based on a band conduction and incoherent interlayer hopping. Stage-2 CoCl2 GIC shows only incoherent hopping. In low magnetic field region, the c-axis resistances of stage-1 and stage-2 CoCl2 GIC are affected by the magnetic phase transitions. Spin-dependent tunneling effect can explain this phenomenon.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GORDON BREACH SCI PUBL LTD  

CucCo1-cCl2-FeCl3 graphite bi-intercalation compounds (GBIC's) have a c-axis stacking sequence of -G-I-1-G-I-2-G-I-1-G-I-2-G- (G = graphite layer, I-1 = CucCo1- Cl-c(2) layer, and I-2 = FeCl3 layer). These compounds undergo magnetic phase transitions at T-h, T-cu, T-cl, T-SG, and T-RSG (T-h>T-cl>T(RSG)approximate to T-SG), depending on the Cu concentration. The phase transition at Th is related to a helical spin order. The phase transitions at T-cu and T-cl are associated with a spin order of CucCo1-cCl2 layers. The re-entrant spin glass phase below T-RSG for c less than or equal to 0.4 and the spin glass phase below T-SG far c greater than or equal to 0.5 are due to the spin frustration effect occurring in FeCl3 layers. The nature of these phases has been studied using SQUID DC magnetization and SQUID AC magnetic susceptibility.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GORDON BREACH SCI PUBL LTD  

Activated carbon fibers (ACFs) are microporous systems comprised of a random network of nano-sized graphites. We investigated the structure and electronic properties of fluorinated ACFs in order to clarify the effects of fluorine-introduced sp(3)-carbon sites on the electronic state of the nano-sized pi-electron system. The orbital diamagnetism is monotonically reduced with increasing F/C, suggesting that introduced sp(3) carbon sites make the extended n-conjugated system reduced. In the intermediate fluorine concentration region, the localized spin concentration is enhanced by fluorination, reaching a maximum at F/C similar to 0.8, whereas it tends to be lowered in the high concentration region. The enhancement in the spin concentration is explained in terms of the local destruction of the pi-conjugated system at interior carbon sites by fluorine-inherited sp(3)-carbon sites.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GORDON BREACH SCI PUBL LTD  

Graphite-nano particles were prepared by the heat-treatment of diamond-nano particles. Various heat-treatment temperatures in the range 900-1600 degrees C were used. X-ray diffraction studies indicate that the onset temperature of diamond-graphite transition is around 1000 degrees C and the complete conversion of diamond to graphite occurs at 1600 degrees C. X-ray diffraction studies on the potassium intercalated compounds of these graphite-nano particles, while indicating the absence of staging phenomenon, show that small neutral potassium clusters are included in these intercalated compounds. The presence of neutral potassium clusters could also explain the observed magnetic susceptibilities and the large deviations in the ESR g-values of these intercalated nano-graphitic systems.

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

Magnetism and transport properties are reported for hollandite-type manganese oxides K-1.5(H3O)(x)Mn8O16 (0<x<0.5) having a tunnel structure. Three phase transitions are observed at T-1 (varies from 180 to 250 K, very sensitive to the synthetic condition), at T-2 (52 K) and at T-3 (20 K). The conductivity and the thermoelectric power indicate that the electric conduction above T-1 is governed by the hopping of small polarons and that the polarons begin to be trapped at a part of Mn sites below T-1. The magnetic susceptibility suggests that the T-1 transition is a phase transition from a single-spin system (T>T-1) composed of only S approximate to 1.63 spins, to a multispin system composed of S=2 spins and S=3/2 spins. These observations strongly suggest that T-1 is a kind of a charge-localization transition of Mn4+ and Mn3+. Magnetic susceptibility also reveals that a weak ferromagnetism appears between T-2 and T-3 and disappears again below T-3. From the anisotropy of the magnetic susceptibility, the ground state is considered to be a helical magnetism. [S0163-1879(99)09219-X].

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

The potential use of 4,5-dicyanoimidazolate ion (DCI-) as a bridging ligand for magnetically interesting co-ordination polymers has been explored. New Cu-II-bipy complexes containing DCI- were prepared and characterised: crystal structure analysis revealed DCI- co-ordinated Cu-II in one of the complexes. The ESR and magnetic susceptibility studies showed that the compounds are nearly Curie paramagnetic or weakly antiferromagnetic. A polymeric structure is proposed for one of the complexes.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：MATERIALS RESEARCH SOCIETY  

Microporous activated carbon fibers (ACF) having huge specific surface areas are characterized as a three-dimensional network of nano-sized graphite domains. We investigate ACF-hosting host-guest systems aiming at novel phenomena produced by the host-guest interface interactions. An extraordinarily large amount of helium is condensed in the ACF micropores, suggesting the presence of ultra-micropores and the enhancement in the helium-nano-graphite interaction. Chemisorbed oxygen molecules are stabilized in the singlet states caused by the interaction with nano-graphite and it makes the micorpore volume swell. Iodine forms an intercalation system with nano-graphites through charge transfer process, in contrast with the behavior of bulk graphite that does not accept iodine as an acceptor-type intercalate. Fluorine reacting with nano-graphite generates dangling bond spins by breaking graphite pi-bonds, where the fluorine-induced defect is found to have a widely extended electronic structure.

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

Magnetism and transport properties are reported for hollandite-type manganese oxides (Formula presented) (Formula presented) having a tunnel structure. Three phase transitions are observed at (Formula presented) (varies from 180 to 250 K, very sensitive to the synthetic condition), at (Formula presented) (52 K) and at (Formula presented) (20 K). The conductivity and the thermoelectric power indicate that the electric conduction above (Formula presented) is governed by the hopping of small polarons and that the polarons begin to be trapped at a part of Mn sites below (Formula presented) The magnetic susceptibility suggests that the (Formula presented) transition is a phase transition from a single-spin system (Formula presented) composed of only (Formula presented) spins, to a multispin system composed of (Formula presented) spins and (Formula presented) spins. These observations strongly suggest that (Formula presented) is a kind of a charge-localization transition of (Formula presented) and (Formula presented) Magnetic susceptibility also reveals that a weak ferromagnetism appears between (Formula presented) and (Formula presented) and disappears again below (Formula presented) From the anisotropy of the magnetic susceptibility, the ground state is considered to be a helical magnetism. © 1999 The American Physical Society.

DOI： 10.1103/PhysRevB.59.12836

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

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Language：English   Publisher：The Carbon Society of Japan  

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Language：English   Publisher：The Carbon Society of Japan  

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

Superconductivity is discovered in new type of two-dimensional organic metal kappa-(BEDSe-TTF)(2)CuN(CN)(2)Br consisting of BEDT-TTF donor analogue having selenium-substituted six-member outer rings. This salt is isostructural to the corresponding BEDT-TTF salt and shows metallic behavior accompanied with a spin-density-wave transition at TMI-25K. The application of pressure up to 0.15 GPa generates superconducting state at T-c = 7.5 K, which is lower than T-c of the BEDT-TTF salt (T-c = 11.6 K). The lowering of T-c and the modification in the electronic structure are explained in terms of the important contribution of selenium electrons and the concomitant enhancement in donor dimerization. (C) 1998 Elsevier Science Ltd. All rights reserved.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GORDON BREACH PUBLISHING, TAYLOR & FRANCIS GROUP  

CuCl(2) graphite intercalation compounds (GICs) having CuCl(2) magnetic intercalate layers are expected to give novel magnetic layer effect on their transport phenomena, through the pi-d interaction. We investigate the transport properties of stage 1-4 CuCl(2) GIG by means of c-axis magnetoresistance (MR) and thermoelectric powers. In stage-1 compound, anomalously large MR is observed for the field perpendicular to the c-axis, indicating a remarkable difference in the magnetic field direction dependence horn that of in-plane MR governed by the cyclotron motion of the carriers. This implies the important role of the magnetic layers in the c-axis hopping process. The c-axis thermoelectric power shows an anomalous change around 70-100 K where a short range order hump appears in the magnetic susceptibility.

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GORDON BREACH PUBLISHING, TAYLOR & FRANCIS GROUP  

Activated carbon fibers (ACFs) are microporous carbons consisting of a three-dimensional disordered network of micrographites. In order to clarify the correlation between the disordered structure and the electronic properties, we investigated the magnetic susceptibility of heat-treated and iodine-doped ACFs. The heat treatment reduced the Fermi energy of ACFs, but the carrier density per micrographite estimated from the Fermi energy is nearly equal to the density of dangling bond spins of as-prepared ACFs, irrespective of heat treatment temperature. In less graphitized ACFs, a charge transfer arises from ACFs to iodine, suggesting less graphitized micrographite has the electronic features of condensed polycyclic aromatic hydrocarbons. On the other hand, well graphitized micrographites in ACFs, are more graphitic in their electronic state, resulting in the absence of charge transfer to iodine.

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

CucCo1-cCl2-FeCl3 GBICs (0 less than or equal to c less than or equal to 1) undergo magnetic phase transitions at T-c1, T-c2 and T-c3 (T-c3 < T-c2 < T-c1) depending on the Cu concentration. The phase transitions at T-c1 and T-c3 show spin glass behaviors. The phase transitions at T-c2 and T-c3 are related to the in-plane spin orders of CucCo1-cCl2 layers and FeCl3 layers, respectively. The spin orders in the CucCo1-cCl2 layers and the FeCl3 layers are coupled to each other through an interplanar exchange interaction, leading to the appearance of phase transition at T-c1 and the decrease of T-c2 with increasing Cu concentration. (C) 1997 Published by Elsevier Science Ltd.

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

Conductivity, thermoelectric power (TEP) and magnetic susceptibility of the tunnel-type manganese oxides K1.5Mn8O16 are reported. The conductivity and the TEP around room temperature suggest that the electrical conduction is governed by hopping of small-polarons. Three phase transitions are observed at T-1 (varies from 175 K to 215 K, batch-dependent), at T-2 (52 K) and at T-3 (20 K). The T-1 transition is considered to be a charge-ordering transition of Mn4+ and Mn3+ coupled with the ordering of the potassium ions in the tunnels. Weak ferromagnetism appears between T-2 and T-3 and disappears again below T-3. The anisotropic measurement of the susceptibility strongly suggests the occurrence of a helical magnetism below T-3. After the single crystals are soaked in cone. HNO3 for a few days, a part of the potassium ions in the tunnels are removed. Besides, the activation energy is drastically reduced and the T-1 transition is suppressed. This suggests that the charge-ordering of Mn4+ and Mn3+ is very sensitive to the state of the tunnel cations. (C) 1997 Elsevier Science S.A.

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

Superconductivity in a layered organic compound, alpha-(BEDT-TTF)(2)NH4Hg(SCN)(4), has been characterized by measurements of the in-plane and out-of-plane resistivities and susceptibilities. The anisotropy of the normal state resistivity amounts to 10(5)-10(6). The profiles of the in-plane and out-of-plane superconducting transitions are qualitatively different in both resistive and inductive measurements. The out-of-plane penetration depth is as long as 1.4 mm. All these results indicate that this material is a model system of Josephson-coupled two-dimensional layers with a high superconductive anisotropy,

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

The low-temperature specific heat of alpha-phase of (BEDT-TTF)(2)MHg(SCN)(4) (M=K, Rb, NH4), where BEDT-TTF is bis(etylenedithio) tetrathiafulvalene, is studied. The electronic specific heat coefficient (gamma) estimated from the data between 1K and 2.3K amounts to 5.3, 10.3, 28.8mJ/mol K-2 for M=K, Rb and NH4, respectively. The K-salt does not have distinct anomally around 8K within the precision of our experiments, implying that the transition occurs with a gradual change of electronic structure.

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The electronic state of kappa-(BMDT-TTF)(2)Au(CN)(2) was investigated by measurements of H-1-NMR and thermoelectric power. In H-1-NMR, the profile of the nuclear relaxation curve changed drastically at 75 K from exponential time-dependence to non-single exponential one reflecting inhomogeneous relaxation. This change evidences a transition in the electronic state at this temperature. However, the longest T-1 below the transition temperature behaves like a Korringa law, suggesting that the low-temperature state is another metallic state. The thermoelectric power shows a strong anomaly around 75 K,being consistent with the NMR results. Above that, the temperature dependence is similar to that found in the K phase of BEDT-TTF compounds, suggesting a similar Fermi-surface topology. These results suggest that the transition at 75 K is not a conventional metal-to-insulator transition but a metal-to-metal transition.

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

We have obtained the parallel penetration depth of two representative compounds of layered organic superconductors, kappa-(BEDT-TTF)(2)Cu[N(CN)(2)]Br and alpha-(BEDT-TTF)(2)NH4(SCN)(4) by ac susceptibility measurements. The former salt gives the depth of 190 mu m, while that for the latter is 1.4 mm. These values demonstrate that the NH4(SCN)(4) salt is more highly-two-dimensional than the Cu[N(CN)(2)]Br salt. The ac susceptibility under dc magnetic field probes peculiarities of the vortex dynamics.

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

The title compound was obtained during the crystal growth of two kappa-type superconductors, kappa-(BEDT-TTF)(2)Cu(CN)[N(CN)(2)] and kappa'-(BEDT-TTF)(2) Cu-2(CN)(3). This compound is a magnetic semiconductor, whose spin susceptibility is expressed by quadratic-layer Heisenberg antiferromagnet model (QLAF) between 32.6K and 220K. At 220K, a semiconductor-semiconductor transition into a high conductivity phase was observed by transport measurements.

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

A crystal-structure analysis and a band calculation, as well as conductivity, thermoelectric power, ESR, and static susceptibility measurements were performed on the new BEDT-TTF salt, theta-(BEDT-TTF)(2)Cu-2(CN)[N(CN)(2)](2). This salt has a unique polymerized anion comprising infinite Cu-N(CN)(2)-Cu double helices bridged by disordered CN groups. Although the two-dimensional arrangement of crystallographically equivalent BEDT-TTF+0.5 cations provides a two-dimensional closed Fermi surface within the tight-binding approximation, this salt is semiconductive even at room temperature, probably due to a strong electron correlation in this system. A semiconductor-semiconductor transition has been observed around 220 K. Below 220 K the electronic system is represented as a Mott insulator and a two-dimensional Heisenberg antiferromagnet. Rapid decreases in the ESR line width and spin susceptibility below 50 K indicate the existence of an additional magnetic transition.

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

A quasi two-dimensional metal K3Cu8S6 has two phase transitions at T1 (153 K) and at T2 (55 K), accompanied by an incommensurate (between T1 and T2) and a commensurate (below T2) superstructures, respectively. On the other hand, the resistivity of Rb3Cu8S6, which is isostructural with K3Cu8S6, suggests that Rb3Cu8S6 has only the T1 transition. In order to investigate the superstructures of Rb3Cu8S6, X-ray diffuse scattering studies were carried out. The results show that an incommensurate superstructure with a wave vector of (0 eta 0), eta congruent-to 0.45, appears between T1 and the measuring lowest temperature (about 15 K). The temperature dependence of the intensity of the superstructure is that of an ordinary charge-density wave and there is no evidence for the T2 transition in Rb3Cu8S6. The X-ray scattering results on the mixed crystals (K1-xRbx)3Cu8S6 are also reported.

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

A quasi two-dimensional metal K3Cu8S6 undergoes two kinds of phase transitions at T1 (153K) and at T2 (55K), attended by an incommensurate (IC) and a commensurate superstructures, respectively. In order to investigate these phase transitions, the mixed crystals (K1-xRbx)3Cu8S6, (Rb1-xCsx)3Cu8S6, K3Cu8(S1-xSex), and Rb3Cu8(S1-xSex) have been synthesized and their transport properties have been studied. Their resistivities show that the T1 transition of K3CU8S6, assigned as onset transition of an IC charge-density wave (CDW), is slightly affected by the substitution of alkali atoms but strongly suppressed by the substitution of chalcogen atoms. Their thermoelectric powers suggest that the existence of n-type carriers is essential for the CDW transition. On the other hand, the T2 transition of K3Cu8S6, whose origin is not clear, is suppressed by Rb doping and by Se doping.

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

K3Cu8S6 is a two-dimensional metal which has two phase transitions at T1 (153 K) and T2 (55 K), accompanied by an incommensurate and a commensurate superstructure, respectively. In order to investigate these phase transitions, the mixed crystals (K1-xRbx)3Cu8S6 have been synthesized and the effect of the alkali substitution and that of pressure have been studied by the measurements of their transport properties. With increasing x, T1 significantly decreases, reaching T1 of Rb3Cu8S6 (= 110 K). Pressure also reduces T1 of K3Cu8S6 and Rb3Cu8S6. On the other hand, the T2 transition becomes obscure with increasing x, and disappears thoroughly for x greater-than-or-equal-to 0.50. In Rb3Cu8S6, the T2 transition is absent at ambient pressure but induced by the pressure of at most 0.5 GPa. Around x = 0.06, a remarkable rise in the resistivity appears below T2. Its mechanism is discussed.

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：JAPANESE JOUR APPLIED PHYSICS  

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：GORDON BREACH SCI PUBL LTD  

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

We have recently shown that insulating mixed-valence complex Cs2Au2I6 transforms gradually to a metallic state between ambient pressure and 4.5 GPa at room temperature. Also shown that Cs2Au2I6 undergoes a metal-to-metal transition in a process of increasing temperature at 6.5 GPa. The present energy-dispersive X-ray diffraction study has revealed that Cs2Au2I6 undergoes a pressure-induced first-order tetragonal-to-tetragonal phase transition at about 5.5 GPa and room temperature. At the transition pressure, the Au-Au distance elongates along the c-axis, which is due to the 'band Jahn-Teller' transition. The Au(II) valence state is considered to be realized at this pressure. This phase transition would relate to the minimum in the pressure-resistance curve near 5.5 GPa, previously reported. It was also found that Cs2Au2I6 undergoes a first-order tetragonal-to-cubic phase transition at 6.8 GPa and T approximately 350 K.

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

The present energy-dispersive X-ray diffraction study has revealed that Cs(z)Au2I6 undergoes a pressure-induced first-order tetragonal-tetragonal phase transition at about 5.5 GPa and room temperature. At the transition pressure, the Au-Au distance elongates along the c-axis. This phase transition would relate to the resistance minimum at 5.4 GPa, previously reported [1]. It was also found that Cs(z)Au(z)I6 undergoes a first-order tetragonal-cubic phase transition at 6.5 GPa and T approximately 350 K.

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

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (other)   Publisher：小学館  

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