Publishing type：Research paper (scientific journal)  

DOI： 10.1103/PHYSREVB.105.054515

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Publishing type：Research paper (scientific journal)  

High-sensitivity capacitive Faraday magnetometers were developed for static DC magnetization measurements in a sub-Kelvin region that can be used with 3He-4He dilution refrigerators (∼50 mK) and 3He refrigerators (∼0.28 K). For high-resolution magnetization measurements, the background magnetization of the force-sensing capacitor should be as small as possible, compared with the magnetization value of a measured specimen. In this study, we succeeded in reducing the background of the capacitor in both low- and high-field regions by compensating for the diamagnetic response of a thin quartz plate, making use of Pauli-paramagnetic alloys and Van Vleck paramagnets as a counter magnetization for a diamagnetic signal. Having established an ultra-high-sensitivity capacitor, we achieved a resolution of 10-5 (∼10-5-10-6) emu in the low- (high-) field region below (above) 1 T. In particular, the newly developed capacitors with a Van Vleck paramagnet Pr0.1La0.9Be13 and paramagnetic MgAl alloys are demonstrated to be very useful for high-resolution magnetization measurements at milli-Kelvin temperatures in low and high magnetic fields, respectively.

DOI： 10.1063/5.0067759

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

We present a model compound with a spin-1/2 frustrated square lattice, in which three ferromagnetic (F) interactions and one antiferromagnetic (AF) compet. Considering the effective spin-1 formed by the dominant F dimer, this square lattice can be mapped to a spin-1 spatially anisotropic triangular lattice. The magnetization curve exhibits gapped behavior indicative of a dominant one-dimensional (1D) AF correlation. In the field-induced gapless phase, the specific heat and magnetic susceptibility show a phase transition to an ordered state with 2D characteristics. These results indicate that the spin-1 Haldane state is extended to the 2D system. We demonstrate that the gapped ground state observed in the present spin-1/2 frustrated square lattice originates from the one-dimensionalization caused by frustration.

DOI： 10.1103/physrevb.104.l060411

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.104.L060411/fulltext

Publishing type：Research paper (scientific journal)  

We have employed a magnetic field angle as a tuning parameter in a comprehensive measurement of the specific heat, magnetocaloric effect, and magnetization for the quasi-kagome Kondo lattice CeRhSn, which is considered to exhibit zero-field quantum criticality driven by geometrical frustration. By constructing the field-angle-resolved landscape of the entropy, we unexpectedly revealed that the non-Fermi-liquid nature survives up to a metamagnetic crossover field of roughly 3 T in the very narrow field-orientation range, close to the direction parallel to the quasi-kagome plane. We propose that spin fluctuations along the hexagonal c axis are the dominant driving force for the non-Fermi-liquid behavior because it is strongly suppressed by a magnetic-field component along the c axis. The multidimensional entropy landscape, which directly reflects the degeneracy of ground states, opens a new route for uncovering the nature of exotic phases in anisotropic systems.

DOI： 10.7566/JPSJ.90.064703

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In this study, we measure the magnetic field and temperature variations of the magnetization MðH;TÞ of the cubic chiral compound EuPtSi and extensively investigate the magnetic phase transitions in a wide temperature range down to 60mK. The incommensurate–commensurate transition at TN ’ 2:5K in the helical state, which has not been observed in transport or thermodynamic measurements, is exclusively detected in M(T) for H ∥ 〈111〉. TN shows a rapid decrease in H. For H ∥ [100] and [111], M(H) at T < 100mK showed that the helical q vectors are depinned from the preferred orientations at H ≳ 10kOe and gradually rotate toward the field directions as H increases. The skyrmion lattice phase (A-phase) field-induced in H ∥ 〈111〉 only above 0.4K in equilibrium conditions can be created at 60mK under field cooling, with a sharp tilted plateau structure developing in M(H) of the A-phase. A similar metastable state is also created below 100mK for the AA phase induced by H ∥ 〈100〉 at T ≳ 0:3K in equilibrium conditions. The magnetic phase diagrams established for the three directions [111], [110], and [100] are highly anisotropic and discussed herein on the basis of the q vectors rather strongly pinned to the lattice and transform under the lattice symmetry operations.

DOI： 10.7566/JPSJ.90.064701

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

We investigate the thermal-transport properties of the kagome antiferromagnet Cd-kapellasite (Cd-K). We find that a field-suppression effect on the longitudinal thermal conductivity K-xx, sets in below approximately 25 K. This field-suppression effect at 15 T becomes as large as 80% at low temperatures, suggesting a large spin contribution K-xx(sp) in K-xx. We also find clear thermal Hall signals in the spin liquid phase in all Cd-K samples. The magnitude of the thermal Hall conductivity K-xy shows a significant dependence on the sample's scattering time, as seen in the rise of the peak K-xy value in almost linear fashion with the magnitude of K-xx. On the other hand, the temperature dependence of K-xy is similar in all Cd-K samples; K-xy shows a peak at almost the same temperature of the peak of the phonon thermal conductivity ex which is estimated by K-xx, at 15 T. These results indicate the presence of a dominant phonon thermal Hall K-xy at 15 T. In addition to e y , we find that the field dependence of K-xy at low fields turns out to be nonlinear at low temperatures, concomitantly with the appearance of the field suppression of K-xx, indicating the presence of a spin thermal Hall K-xy(sp) at low fields. Remarkably, by assembling the K-xx dependence of ex K-xy(sp) data of other kagome antiferromagnets, we find that, whereas K-xy(sp) stays a constant in the low-K-xx region, K-xy(sp) starts to increase as K-xx does in the high-K-xx region. This K-xx dependence of K-xy(sp) indicates the presence of both intrinsic and extrinsic mechanisms in the spin thermal Hall effect in kagome antiferromagnets. Furthermore, both K-xy(ph) and K-xy(sp), disappear in the antiferromagnetic ordered phase at low fields, showing that phonons alone do not exhibit the thermal Hall effect. A high field above approximately 7 T induces K-xy(ph), concomitantly with a field-induced increase of K-xx and the specific heat, suggesting a coupling of the phonons to the field-induced spin excitations as the origin of K-xy(ph).

DOI： 10.1103/PhysRevX.10.041059

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

© 2020 The Physical Society of Japan. An osmium chloride with the chemical formula of OsxCl3 (x = 0.81) was synthesized and its crystal structure and thermodynamic properties were investigated. OsxCl3 crystallizes in a layered CdCl2-type structure with the triangular lattice partially occupied by Os ions on average. However, on microscopic length scales, the triangular lattice is composed of nano-domains with a honeycomb arrangement of Os ions, as observed by electron microscopy and Raman scattering experiments. Magnetization and heat capacity measurements revealed an absence of magnetic long-range order down to 0.08 K, while a broad peak in heat capacity at 0.15K may indicate a short-range order in the local honeycomb lattice. OsxCl3 may exhibit certain aspects of the Kitaev spin liquid that are expected for a perfect honeycomb lattice of osmium trichloride.

DOI： 10.7566/jpsj.89.114709

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Publishing type：Research paper (scientific journal)   Publisher：Elsevier BV  

DOI： 10.1016/j.scib.2020.04.039

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

The gap structure of a novel uranium-based superconductor UTe2, situated in the vicinity of ferromagnetic quantum criticality, has been investigated via specific-heat C(T, H, Omega) measurements in various field orientations. Its angular Omega(phi, theta) variation shows a characteristic shoulder anomaly with a local minimum in H parallel to a at moderate fields rotated within the ab and ac planes. Based on theoretical calculations, these features can be attributed to the presence of point nodes in the superconducting gap along the a direction. Under the field orientation along the easy-magnetization a axis, an unusual temperature dependence of the upper critical field at low fields together with a convex downward curvature in C(H) were observed. These anomalous behaviors can be explained on the basis of a nonunitary triplet state model with equal-spin pairing whose T-c is tuned by the magnetization along the a axis. From these results, the gap symmetry of UTe2 is most likely described by a vector order parameter of d(k) = (b + ic)(k(b) + ik(c)).

DOI： 10.1103/physrevresearch.2.032014

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevResearch.2.032014/fulltext

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

We investigate the superconducting gap function of the topological superconductor PbTaSe2. The temperature, magnetic field, and three-dimensional (3D) field-angle dependences of the specific heat prove that the superconductivity of PbTaSe2 is fully gapped, with two isotropic s-wave gaps. The pair-breaking effect is probed by systematically increasing nonmagnetic disorders through H+ irradiation. The superconducting transition temperature T-c is found to be robust against disorders, which suggests that the pairing should be sign preserved rather than sign reversed.

DOI： 10.1103/PhysRevB.102.024517

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.102.024517/fulltext

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

DOI： 10.1103/physrevb.101.174412

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Other Link： http://harvest.aps.org/v2/journals/articles/10.1103/PhysRevB.101.174412/fulltext

Publishing type：Research paper (scientific journal)   Publisher：The Magnetics Society of Japan  

DOI： 10.3379/msjmag.2005r004

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

Ferro-quadrupole (FQ) order in the non-Kramers Gamma(3) doublet system PrTi2Al20 has been investigated via angle-resolved measurements of the specific heat, rotational magnetocaloric effect, and entropy, under a rotating magnetic field within the (1 (1) over bar0) plane. The FQ transition occurring at 2K is robust when the magnetic field B is applied precisely along the [111] direction or in low fields below 1 T. By contrast, the magnetic field of larger than 1 T tilted away from the [111] direction sensitively changes the FQ transition to a crossover. The energy gap between the ground and first-excited states in the FQ order increases remarkably with the magnetic field in B parallel to [001], but hardly depends on the magnetic-field strength, at least up to 5 T, in the field orientation between the [111] and [110] axes. These features can be reproduced by using a phenomenological model for FQ order assuming an anisotropic field-dependent interaction between quadrupoles, which has been recently proposed to explain the field-induced first-order phase transition in PrTi2Al20. The present study demonstrates the great potential of the field-angle-resolved measurements for evaluating possible scenarios for multipole orders.

DOI： 10.7566/jpsj.89.043701

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

DOI： 10.7566/JPSJ.89.024705

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

DOI： 10.1103/PhysRevB.101.014437

DOI： 10.1103/physrevb.101.014437

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

We report the synthesis, electronic properties, and electronic structure of ullmannite-type PtSbS, which has a cubic crystal structure without space inversion symmetry. Electrical resistivity and magnetization measured at low temperatures suggested that this compound is a bulk superconductor with a superconducting transition temperature of T-c = 0.15 K. First principles calculations indicated that Fermi surfaces of PtSbS include strongly nested hole pockets, which can make this compound interesting if they contribute to the emergence of superconductivity.

DOI： 10.7566/JPSJ.88.093709

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

DOI： 10.1103/PhysRevB.100.054442

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Other Link： http://orcid.org/0000-0001-7613-3721

DOI： 10.7566/JPSJ.88.093701

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

In the electronic nematic state, an electronic system has a lower symmetry than the crystal structure of the same system. Electronic nematic states have been observed in various unconventional superconductors such as cuprate, iron-based, heavy-fermion, and topological superconductors. The relation between nematicity and superconductivity is a major unsolved problem in condensed matter physics. By angle-resolved specific heat measurements, we report bulk quasiparticle evidence of nematicity in the topological superconductor SrxBi2Se3. The specific heat exhibited a clear twofold symmetry despite the threefold symmetric lattice. Most importantly, the twofold symmetry appeared in the normal state above the superconducting transition temperature. This is explained by the angle-dependent Zeeman effect due to the anisotropic density of states in the nematic phase. Such results highlight the interrelation between nematicity and unconventional superconductivity.

DOI： 10.1103/PhysRevLett.123.027002

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

We present a model compound for the S = 1/2 ferromagnetic Heisenberg chain composed of the verdazyl-based complex [Zn(hfac)(2)][4-Cl-o-Py-V-(4-F)(2)]. Ab initio MO calculations indicate a predominant ferromagnetic interaction forming an S = 1/2 ferromagnetic chain. The magnetic susceptibility and specific heat indicate a phase transition to an antiferromagnetic (AF) order due to the finite interchain couplings. We explain the magnetic susceptibility and magnetization curve above the phase-transition temperature based on the S = 1/2 ferromagnetic Heisenberg chain. The magnetization curve in the ordered phase is described by a conventional AF two-sublattice model. Furthermore, the obtained magnetic specific heat reproduces the almost temperature-independent behavior of the S = 1/2 ferromagnetic Heisenberg chain. In the low-temperature region, the magnetic specific heat exhibits a root T dependence, which is attributed to the energy dispersion in the ferromagnetic chain.

DOI： 10.1103/PhysRevB.99.094418

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

DOI： 10.1103/physrevb.99.054506

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We investigate the low-temperature magnetic properties of the molecule-based chiral spin chain [Cu(pym)(H_{2}O)_{4}]SiF_{6}·H_{2}O (pym=pyrimidine). Electron-spin resonance, magnetometry and heat capacity measurements reveal the presence of staggered g tensors, a rich low-temperature excitation spectrum, a staggered susceptibility, and a spin gap that opens on the application of a magnetic field. These phenomena are reminiscent of those previously observed in nonchiral staggered chains, which are explicable within the sine-Gordon quantum-field theory. In the present case, however, although the sine-Gordon model accounts well for the form of the temperature dependence of the heat capacity, the size of the gap and its measured linear field dependence do not fit with the sine-Gordon theory as it stands. We propose that the differences arise due to additional terms in the Hamiltonian resulting from the chiral structure of [Cu(pym)(H_{2}O)_{4}]SiF_{6}·H_{2}O, particularly a uniform Dzyaloshinskii-Moriya coupling and a fourfold periodic staggered field.

DOI： 10.1103/PhysRevLett.122.057207

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.74.1.0_2313

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DOI： 10.1103/PhysRevB.98.224411

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

DOI： 10.7566/jpsj.87.114001

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DOI： 10.15669/pnst.5.123

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© 2018 American Physical Society. The Ce atoms in CeIrSn form a quasikagome lattice in the hexagonal c plane. Our single-crystal x-ray diffraction analysis has confirmed the absence of superstructure in the ZrNiAl-type structure. Motivated by the recent observation of quantum critical behaviors in the isostructural and isoelectronic compound CeRhSn, we have studied low-temperature properties of single-crystalline samples of CeIrSn by measuring the electrical resistivity, magnetic susceptibility χ, magnetization M, and specific heat C down to 0.05 K. Our study does not show evidence of a long-range magnetic ordering down to the lowest temperature. The χ(T) data show an Ising character, χc a, and power-law behaviors at low temperatures: χcT-0.8 and χaT-0.6. When external field B is applied along the a axis, both M(B) and C(B)/T exhibit metamagnetic anomalies at 5.5 T, which resemble those observed in CeRhSn at 3.5 T. We attribute the metamagnetic crossover under in-plane magnetic fields to the alleviation of magnetic frustration inherent in the quasikagome Ising Kondo lattice.

DOI： 10.1103/PhysRevB.98.155147

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DOI： 10.1063/1.5042119

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DOI： 10.7566/jpsj.87.093703

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DOI： 10.1103/PhysRevB.98.094402

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

DOI： 10.1103/physrevb.98.064505

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

DOI： 10.7566/jpsj.87.073601

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

Low-temperature thermodynamic investigation for UBe13 and UPd2Al3 were performed in order to gain insight into their unusual ground states of 5f electrons. Our heat-capacity data for the cubic UBe13 strongly suggest that nodal quasiparticles are absent and its superconducting (SC) gap is fully open over the Fermi surface. Moreover, two unusual thermodynamic anomalies are also observed in UBe13 at ∼ 3 T and ∼ 9 T
the lower-field anomaly is seen only in the SC mixed state by dc magnetization M(H) as well as heat-capacity C(H), while the higher-field anomaly appears for C(H) in the normal phase above the upper critical field. On the other hand, field-orientation dependence of the heat capacity in the hexagonal UPd2Al3 shows a significantly anisotropic behavior of C(H)∝H1/2, reflecting the nodal gap structure of this system. Our result strongly suggests the presence of a horizontal line node on the Fermi surface with heavy effective mass in UPd2Al3.

DOI： 10.1016/j.physb.2017.09.036

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

Quantum criticality of the spin-1/2 ferromagnetic-leg ladder 3-I-V [=3-(3-iodophenyl)-1,5-diphenylverdazyl] has been examined with respect to the antiferromagnetic to paramagnetic phase transition near the saturation field Hc. The phase boundary Tc(H) follows the power-law Tc(H) Hc-H for a wide temperature range. This characteristic behavior is discussed as a quasi-one-dimensional (quasi-1D) Bose-Einstein condensation, which is predicted theoretically for weakly coupled quasi-1D ferromagnets. Thus, 3-I-V provides the first promising candidate for this attractive prediction.

DOI： 10.1103/PhysRevB.97.100406

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DOI： 10.1103/physrevb.96.220505

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

We report the crystal structure, magnetic properties, and magnetic phase diagrams of single crystalline DyNi3Ga9 studied using X-ray diffraction, electrical resistivity, specific heat, and magnetization measurements. DyNi3Ga9 crystallizes in the chiral structure with space group R32. The dysprosium ions, which are responsible for the magnetism in this compound, form a two-dimensional honeycomb structure on a (0001) plane. We show that DyNi3Ga9 exhibits successive phase transitions at TN = 10K and TN0 ¼ 9K. The former suggests quadrupolar ordering, and the latter is attributed to the antiferromagnetic order. It is considered that DyNi3Ga9 forms the canted-antiferromagnetic structure below TN0 owing to a small hysteresis loop of the low-field magnetization curve. We observe the strong easy-plane anisotropy, and the multiple-metamagnetic transitions with magnetization-plateaus under the field applied along the honeycomb plane. For H k ½2110 , the plateau-region arises every 1=6 for saturation magnetization. The magnetic phase diagrams of DyNi3Ga9 are determined for the fields along principal-crystal axes.

DOI： 10.7566/JPSJ.86.124704

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

Quantum entanglement in magnetic materials is expected to yield a quantum spin liquid (QSL), in which strong quantum fluctuations prevent magnetic ordering even at zero temperature. This topic has been one of the primary focuses of condensed-matter science since Anderson first proposed the resonating valence bond state in a certain spin-1/2 frustrated magnet in 1973. Since then, several candidate materials featuring frustration, such as triangular and kagome lattices, have been reported to exhibit liquid-like behavior. However, the mechanisms that stabilize the liquid-like states have remained elusive. Here, we present a QSL state in a spin-1/2 honeycomb lattice with randomness in the exchange interaction. That is, we successfully introduce randomness into the organic radialbased complex and realize a random-singlet (RS) state (or valence bond glass). All magnetic and thermodynamic experimental results indicate the liquid-like behaviors, which are consistent with those expected in the RS state. Our results suggest that the randomness or inhomogeneity in the actual systems stabilize the RS state and yield liquid-like behavior.

DOI： 10.1038/s41598-017-16431-0

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DOI： 10.1103/physrevb.96.100505

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

High-precision angle-resolved dc magnetization and magnetic torque studies were performed on a singlecrystalline sample of URhGe, an orthorhombic Ising ferromagnet with the c axis being the magnetization easy axis, in order to investigate the phase diagram around the ferromagnetic (FM) reorientation transition in a magnetic field near the b axis. We have clearly detected a first-order transition in both the magnetization and the magnetic torque at low temperatures, and determined detailed profiles of the wing structure of the three-dimensional T-Hb-Hc phase diagram, where H-c and H-b denote the field components along the c and the b axes, respectively. The quantum wing critical points are located at mu H-0(c) similar to +/- 1.1 T and mu H-0(b) similar to 13.5 T. Two second-order transition lines at the boundaries of the wing planes rapidly tend to approach each other with increasing temperature up to similar to 3 K. Just at the zero conjugate field (H-c = 0), however, a signature of the first-order transition can still be seen in the field derivative of the magnetization at similar to 4 K, indicating that the tricritical point exists in a rather high temperature region above 4 K. This feature of the wing plane structure is consistent with the theoretical expectation that three second-order transition lines merge tangentially at the tricritical point.

DOI： 10.1103/PhysRevB.96.094411

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER ASSOC ADVANCEMENT SCIENCE  

In exotic superconductors, including high-T-c copper oxides, the interactions mediating electron Cooper pairing are widely considered to have a magnetic rather than a conventional electron-phonon origin. Interest in this exotic pairing was initiated by the 1979 discovery of heavy-fermion superconductivity in CeCu2Si2, which exhibits strong antiferromagnetic fluctuations. A hallmark of unconventional pairing by anisotropic repulsive interactions is that the superconducting energy gap changes sign as a function of the electron momentum, often leading to nodes where the gap goes to zero. We report low-temperature specific heat, thermal conductivity, and magnetic penetration depth measurements in CeCu2Si2, demonstrating the absence of gap nodes at any point on the Fermi surface. Moreover, electron irradiation experiments reveal that the superconductivity survives even when the electron mean free path becomes substantially shorter than the superconducting coherence length. This indicates that superconductivity is robust against impurities, implying that there is no sign change in the gap function. These results show that, contrary to long-standing belief, heavy electrons with extremely strong Coulomb repulsions can condense into a fully gapped s-wave superconducting state, which has an on-site attractive pairing interaction.

DOI： 10.1126/sciadv.1601667

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

The low-temperature thermodynamic properties of the frustrated pyrochlore Tb2+xTi2-xO7+y have been studied using the single crystal of x = 0.005 sitting in a long range ordered phase in the x-T phase diagram. We observed that the specific heat exhibits a minimum around 2 K and slightly increases on cooling, similar to a Schottky-like anomaly for canonical spin ices. A clear specific-heat peak observed at Tc = 0.53 K is ascribable to the phase transition to a quadrupolar state, which contributes to a relatively large change in entropy, S ≃ 2.7 J K-1mol-1. However, it is still smaller than R ln 2 for the ground state doublet of the Tb ions. The entropy release persists to higher temperatures, suggesting strong fluctuations associated with spin ice correlations above Tc. We discuss the field dependence of the entropy change for H ∥ [111] and H ∥ [001].

DOI： 10.1088/1742-6596/828/1/012007

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Language：English   Publishing type：Research paper (international conference proceedings)   Publisher：Institute of Physics Publishing  

URu2Si2 is a promising candidate for a chiral d-wave superconductor whose gap is composed of a horizontal line node at equator and point nodes at the north and south poles. However, previous measurements of the specific heat and the thermal conductivity did not detect quasiparticle excitations from a horizontal line node in heavy-mass bands. Here, we have provided strong evidence for the presence of a horizontal line node from field-angle-dependent specific-heat measurements by using a high-quality single crystal of URu2Si2. We observed the behavior of the specific heat at low fields which does not depend on inplane field angle φ but shows a shoulder-like anomaly in the polar-angle θ dependence around θ ∼ 45°. This feature is more clearly seen at lower temperatures, suggesting the detection of low-energy quasiparticle excitations reflecting the gap structure. From theoretical analyses based on microscopic calculations, we have demonstrated that this anomaly can be explained by the existence of a horizontal line node at kz = 0. Thus, the gap structure of URu2Si2 matches well with the Eg chiral kz(kx + iky) symmetry.

DOI： 10.1088/1742-6596/807/5/052001

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

The electrical resistivity, magnetization, and specific heat of the caged compounds ROs2Zn20 ( R = La, Ce, Pr, and Nd) have been measured to study their structural, magnetic, and superconducting properties. These measurements indicate that the compounds undergo structural transitions at T-s = 151, 109, 87, and 62 K, respectively. The decrease in Ts along with the lanthanoid contraction suggests that the high-temperature phase is more stable for a smaller lattice volume. Analysis of the lattice specific heat of LaOs2Zn20 reveals that the Zn atom at the 16c site encapsulated in the R2Zn12 cage vibrates at a low energy of 3meV. For CeOs2Zn20, the small magnetic susceptibility with a broad maximum indicates the valence-fluctuating state of the Ce ions. In PrOs2Zn20, the crystalline electric field ground state of the 4f(2) state of the Pr3+ ion remains in a non-Kramers doublet at T &gt; T-s, which is lifted by symmetry lowering of the Pr site at T &lt; T-s. Thereby, the quadrupolar degrees of freedom are quenched, avoiding the long-range quadrupole order. PrOs2Zn20 and LaOs2Zn20 exhibit superconducting transitions at 0.06 and 0.07 K, respectively.

DOI： 10.7566/JPSJ.86.034707

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

Low-temperature (T) thermodynamic properties of the quasi-Kagome antiferromagnet CePdAl ( T-N = 2.7 K) in magnetic fields (B) were studied by magnetization and specific-heat C(T, B) measurements. The unusual magnetic phase diagram, including three thermodynamic phases I-III and crossover anomalies, has been obtained. In the low-field phase I, the existence of the partial Kondo screening state on one-third of the Ce sites is supported from the following thermodynamic view points: (i) the appearance of a paramagnetic moment with one-third of the full moment around 3 T and (ii) the occurrence of an entropy release of similar to 0.3R ln 2 around the observed crossover. On the other hand, a strong enhancement of C/T was found around the point where T-N( B) meets the boundary of phase III, indicating the possible presence of a tricritical point. Furthermore, it is suggested that the three metamagnetic transitions at B-mi (i = 1, 2. 3) originate from successive increments of the Ising-type magnetic moment along the c-axis. In the high-field region above 20 T, the behavior of f electrons can be roughly described by the crystal-electric-field model.

DOI： 10.7566/JPSJ.86.034709

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Language：Japanese   Publisher：物性研究・電子版 編集委員会  

DOI： 10.14989/225165

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

We successfully synthesized single crystals of the verdazyl radical 2-Cl-3,6-F-2-V [=3-(2-chloro-3,6-difluorophenyl)-1,5-diphenylverdazyl], which is a rare model compound with an S = 1/2 Heisenberg antiferromagnetic (HAF) honeycomb lattice. Ab initio molecular orbital calculations indicate two dominant AF interactions, forming a slightly distorted honeycomb lattice. We explain the magnetic susceptibility and the magnetization curve up to the saturation field based on the expected spin model using the quantum Monte Carlo method. In the low-temperature regions, we found a phase transition to an AF ordered state at about 0.77 K for the zero field and obtained the magnetic field-temperature phase diagram from the magnetic susceptibility and the specific heat for various magnetic fields. Through the analysis considering the effect of lattice distortion on magnetic behavior, we confirm that the lattice distortion of the present model is small enough that it does not affect the intrinsic behavior of the uniform S = 1/2 HAF honeycomb lattice.

DOI： 10.1103/PhysRevB.95.075120

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Language：English   Publisher：IOP PUBLISHING LTD  

Owing to a strong Coulomb repulsion, heavy electron superconductors mostly have anisotropic gap functions which have nodes for certain directions in the momentum space. Since the nodal structure is closely related to the pairing mechanism, its experimental determination is of primary importance. This article discusses the experimental methods of the gap determination by bulk heat capacity measurements in a rotating magnetic field. The basic idea is based on the fact that the quasiparticle density of states in the vortex state of nodal superconductors is field and direction dependent. We present our recent experimental results of the field-orientation dependence of the heat capacity in heavy fermion superconductors CeTIn5 (T = Co, Ir), UPt3, CeCu2Si2, and UBe13 and discuss their gap structures.

DOI： 10.1088/0034-4885/79/9/094002

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

This paper presents the results of specific-heat and magnetization measurements, in particular their field-orientation dependence, on the first discovered heavy-fermion superconductor CeCu2Si2 (T-c similar to 0.6K). We discuss the superconducting gap structure and the origin of the anomalous pair-breaking phenomena, leading, e.g., to the suppression of the upper critical field H-c2, found in the high-field region. The data show that the anomalous pair breaking becomes prominent below about 0.15 K in any field direction, but occurs closer to H-c2 for H parallel to c. The presence of this anomaly is confirmed by the fact that the specific-heat and magnetization data satisfy standard thermodynamic relations. Concerning the gap structure, field-angle dependencies of the low-temperature specific heat within the ab and ac planes do not show any evidence for gap nodes. From microscopic calculations in the framework of a two-band full-gap model, the power-law-like temperature dependencies of C and 1/T-1, reminiscent of nodal superconductivity, have been reproduced reasonably. These facts further support multiband full-gap superconductivity in CeCu2Si2.

DOI： 10.1103/PhysRevB.94.054514

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

Orbital degrees of freedom in condensed matter could play important roles in forming a variety of exotic electronic states by interacting with conduction electrons. In 4f-electron systems, because of strong intra-atomic spin-orbit coupling, an orbitally degenerate state inherently carries quadrupolar degrees of freedom. The present work has focused on a purely quadrupole-active system PrIr2Zn20 showing superconductivity in the presence of an antiferroquadrupole order at T-Q = 0.11 K. We observed non-Fermi-liquid (NFL) behaviors emerging in the electrical resistivity rho and the 4f contribution to the specific heat, C-4f, in the paramagnetic state at T &gt; T-Q. Moreover, in magnetic fields B &lt;= 6 T, all data sets of rho(T) and C-4f (T) are well scaled with characteristic temperatures T-0's. This observation of the NFL state in the nonmagnetic quadrupole-active system has an origin intrinsically different from that observed in the vicinity of the conventional quantum critical point. It implies possible formation of a quadrupole Kondo lattice resulting from hybridization between the quadrupoles and the conduction electrons with an energy scale of k(B)T(0). At T &lt;= 0.13 K, rho(T) and C-4f (T) exhibit anomalies as B approaches 5 T. This is the manifestation of a field-induced crossover toward a Fermi-liquid ground state in the quadrupole Kondo lattice.

DOI： 10.1103/PhysRevB.94.075134

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：IEEE-INST ELECTRICAL ELECTRONICS ENGINEERS INC  

We have measured the magnetic and thermal properties below 2 K of a quasi-1-D frustrated spin-1/2 system Cs2Cu2Mo3O12 called CuO2 ribbon chain systems in order to investigate its magnetic ground state. In the temperature dependence of the specific heat divided by temperature, C/T-T, we found a sharp peak structure at T-N = 1.85 K, which indicates the existence of a phase transition. Considering the behavior of the magnetization and specific heat, it is found that Cs2Cu2Mo3O12 exhibits an antiferromagnetic transition at TN = 1.85 K, and its magnetic ground state is an antiferromagnetic long-range ordered one. The determined ground state of Cs2Cu2Mo3O12 significantly contrasts with a nonmagnetic ground state of isostructural compound Rb2Cu2Mo3O12. We discuss the origins of difference of the magnetic ground states between Rb2Cu2Mo3O12 and Cs2Cu2Mo3O12.

DOI： 10.1109/TMAG.2016.2517091

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Quasiparticle excitations in UPd2Al3 were studied by means of heat-capacity (C) measurements under rotating magnetic fields using a high-quality single crystal. The field dependence shows C(H) proportional to H-1/2-like behavior at low temperatures for both two hexagonal crystal axes, i. e., H parallel to[0001] (c axis) and H parallel to[1120] (a axis), suggesting the presence of nodal quasiparticle excitations from heavy bands. At low temperatures, the polar-angle (theta) dependence of C exhibits a maximum along H parallel to[0001] with a twofold symmetric oscillation below 0.5 T, and an unusual shoulder or hump anomaly has been found around 30 degrees-60 degrees from the c axis in C(theta) at intermediate fields (1 less than or similar to mu(0) less than or similar to 2 T). These behaviors in UPd2Al3 purely come from the superconducting nodal quasiparticle excitations, and can be successfully reproduced by theoretical calculations assuming the gap symmetry with a horizontal linear line node. We demonstrate the whole angle-resolved heat-capacity measurements done here as a novel spectroscopic method for nodal gap determination, which can be applied to other exotic superconductors.

DOI： 10.1103/PhysRevLett.117.037001

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A hidden order that emerges in the frustrated pyrochlore Tb2+xTi2-xO7+y with T-c = 0.53 K is studied using specific heat, magnetization, and neutron scattering experiments on a high-quality single crystal. Semiquantitative analyses based on a pseudospin-1/2 Hamiltonian for ionic non-Kramers magnetic doublets demonstrate that it is an ordered state of electric quadrupole moments. The elusive spin liquid state of the nominal Tb2Ti2O7 is most likely a U(1) quantum spin-liquid state.

DOI： 10.1103/PhysRevLett.116.217201

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Measurements of magnetization (M) and specific heat (C) under a [001] magnetic field were carried out on a single crystal of a quantum spin ice system Yb2Ti2O7 in order to investigate a feature of the transition occurred at T-C similar to 0.2 K. As a result of applying the magnetic field mu H-0 &lt; 0.1 T, the C/T-T curve structure and transition temperature barely changed. On the other hand, applying the more than 0.1 T magnetic field, the C/T-T curve structure drastically change from sharp peak structure to broad peak one, and the broad peak temperature of C/T-T curves linearly increases with increasing magnetic field (H). In the magnetic field mu H-0 &lt; 0.1 T, the magnetization drastically increases around T-C similar to 0.2 K with decreasing T, and a thermal hysteresis loop of the M-T curve is observed. With increasing H, the thermal hysteresis loop of the M-T curves disappears above mu H-0(C) = 0.1 T. We can understand these results, where Yb2Ti2O7 exhibits a first-order ferromagnetic transition associated with the latent heat corresponding to the energy of mu H-0(C) = 0.1 T. Basis of the H-T phase diagram along [001] magnetic field, the feature of the transition occurred at T-C similar to 0.2 K in quantum spin ice system Yb2Ti2O7 is discussed. (C) 2016 Author(s). All article content, except where otherwise noted, is licensed under a Creative Commons Attribution 3.0 Unported License.

DOI： 10.1063/1.4944337

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

We demonstrate an advanced spin-system design using a hybrid spin consisting of a strongly coupled metal ion and verdazyl radical. Ab initio calculation, magnetization, and ESR measurements evidenced the first realization of an S = 2 ferromagnetic-antiferromagnetic alternating chain with Ising anisotropy in a Mn-verdazyl complex [Mn(hfac)(2)]center dot(o-Py-V) [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; o-Py-V = 3-(2-pyridyl)-1,5-diphenylverdazyl]. Furthermore, we find an anomalous change in magnetization at 1/4 of the saturation value, which is probably a manifestation of the quantum nature of the system.

DOI： 10.1103/PhysRevB.93.115145

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

Low-energy quasiparticle (QP) excitations in the heavy-fermion superconductor URu2Si2 were investigated by specific-heat C(T, H, phi, theta) measurements of a high-quality single crystal. The occurrence of QP excitations due to the Doppler-shift effect was detected regardless of the field direction in C(H) of the present clean sample, which is in sharp contrast to a previous report. Furthermore, the polar-angle-dependent C(theta) measured under a rotating magnetic field within the ac plane exhibits a shoulder-like anomaly at theta similar to 45 degrees and a sharp dip at theta = 90 degrees (H || a) in the moderate-field region. These features are supported by theoretical analyses based on microscopic calculations assuming the gap symmetry of k(z)(k(x) + ik(y)), whose gap structure is characterized by a combination of a horizontal line node at the equator and point nodes at the poles. The present results have settled the previous controversy over the gap structure of URu2Si2 and have authenticated its chiral d-wave superconductivity.

DOI： 10.7566/JPSJ.85.033704

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

We performed electric polarization P, dielectric constant epsilon and magnetization M measurements on the perfect triangular-lattice antiferromagnet RbFe(MoO4)(2) in high magnetic fields B applied along the c-axis. Whereas M(B) at T=1.5 K monotonically increases with B up to the saturation at B-s = 21 T, P(B) disappears about 14 T, far below the saturation point. In a field range 13 &lt;= B &lt;= 14T, the temperature variation of e shows a double peak structure below 2.8 K, indicating two successive transitions. These results strongly suggest the presence of a new high-field phase; the magnetic structure changes from a low-field conical state having chirality to a high-field coplanar non-collinear structure without chirality. The obtained phase diagram is in agreement with a theoretical prediction on XY-like Heisenberg spin triangular lattice antiferromagnets. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2015.08.013

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

In order to gain insight into the superconducting (SC) gap of UBe13, we studied its quasiparticle excitations by means of heat-capacity measurements. Quite unexpectedly, we found the isotropic C(H) alpha H behavior in low fields at low temperatures, implying the absence of nodal quasiparticle excitations. This result indicates that the SC gap in UBe13 is fully open over the Fermi surfaces. Furthermore, we observed a characteristic oscillation of heat capacity both in the SC and non-Fermi-liquid normal states above similar to 2 T, and the angular variation of heat capacity possibly originates from anisotropic magnetic response of the heavy-electron state. Our result regarding the low-energy quasiparticle excitations in the SC and normal states will be a clue to understand the unusual nature of UBe13. (C) 2015 Elsevier BY. All rights reserved.

DOI： 10.1016/j.jmmm.2015.08.003

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

The low-energy elementary excitations in frustrated quantum magnets have fascinated researchers for decades. In frustrated Ising magnets on a pyrochlore lattice possessing macroscopically degenerate spin-ice ground states, the excitations have been discussed in terms of classical magnetic monopoles, which do not contain quantum fluctuations. Here we report unusual behaviours of magneto-thermal conductivity in the disordered spin-liquid regime of pyrochlore Yb2Ti2O7, which hosts frustrated spin-ice correlations with large quantum fluctuations owing to pseudospin-1/2 of Yb ions. The analysis of the temperature and magnetic field dependencies shows the presence of gapped elementary excitations. We find that the gap energy is largely suppressed from that expected in classical monopoles. Moreover, these excitations propagate a long distance without being scattered, in contrast to the diffusive nature of classical monopoles. These results suggests the emergence of highly itinerant quantum magnetic monopole, which is a heavy quasiparticle that propagates coherently in three-dimensional spin liquids.

DOI： 10.1038/ncomms10807

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The first-order superconducting transition, recently discovered in a long-standing candidate for a spin-triplet chiral-p-wave superconductor Sr2RuO4, is mysterious because it is not expected within the present scenario of spin-triplet superconductivity. To uncover its mechanism, we have measured the magnetization and the magnetic torque using an ultra-clean single crystal of Sr2RuO4 down to 0.1 K. In this study, the magnetic-field orientation has been controlled with a high precision within the accuracy of 0.01 degrees. We observed a sharp magnetization jump of as large as 0.01 emu/g with a field hysteresis of 100 Oe at 0.1 K. With increasing temperature, this magnetization jump decreases and diminishes above 0.8 K. The magnetic torque indicates that the first-order transition occurs only when the magnetic field is applied close to the ab plane within 2 degrees deviation. These features of the first-order transition provide the keystone issues whose resolution is crucial in uncovering a "hidden" pair-breaking mechanism within the triplet-pairing scenario, or might urge the reconsideration of the order parameter in Sr2RuO4. (C) 2015 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.jmmm.2015.07.004

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.71.2.0_2208

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DOI： 10.11316/jpsgaiyo.71.1.0_1229

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

DOI： 10.11316/jpsgaiyo.71.1.0_2128

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DOI： 10.11316/jpsgaiyo.71.1.0_2460

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DOI： 10.11316/jpsgaiyo.71.1.0_1049

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DOI： 10.11316/jpsgaiyo.71.1.0_1063

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DOI： 10.11316/jpsgaiyo.71.1.0_1963

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DOI： 10.11316/jpsgaiyo.71.1.0_1076

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DOI： 10.11316/jpsgaiyo.71.2.0_1754

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DOI： 10.11316/jpsgaiyo.71.2.0_2236

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DOI： 10.11316/jpsgaiyo.71.2.0_1759

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DOI： 10.11316/jpsgaiyo.71.2.0_1753

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DOI： 10.11316/jpsgaiyo.71.2.0_2006

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

We have studied the hidden long-range order (LRO) of the frustrated pyrochlore magnet Tb2Ti2O7 by means of specific-heat experiments and Monte-Carlo (MC) simulations, which has been discussed as the LRO of quadrupole moments inherent to the non-Kramers ion of Tb3+. We have found that the sharp specific-heat peak is collapsed into a broad hump by magnetic fields above 0.3 T for H//[001]. This result, qualitatively reproduced by MC simulations, suggests that a field-induced magnetic state overcomes the quadrupolar LRO state, as a similar case of a classical spin ice. The present results support the interpretation that Tb2+xTi2-xO7+y is a unique material in the boundary between the quadrupolar (x &gt;= x(c) = -0.0025) and spin-liquid (x &lt;= x(c)) states, where the magnetic field along the [001] axis is a tuning parameter which induces the magnetic ordered state.

DOI： 10.1088/1742-6596/683/1/012022

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

Geometric frustration, in which competing interactions give rise to degenerate ground states, potentially induces various exotic quantum phenomena in magnetic materials. Minimal models comprising triangular units, such as triangular and Kagome lattices, have been investigated for decades to realize novel quantum phases, such as quantum spin liquid. A pentagon is the second-minimal elementary unit for geometric frustration. The realization of such systems is expected to provide a distinct platform for studying frustrated magnetism. Here, we present a spin-1/2 quantum pentagonal lattice in the new organic radical crystal alpha-2,6-Cl-2-V [=alpha-3-(2,6-dichlorophenyl)-1,5-diphenylverdazyl]. Its unique molecular arrangement allows the formation of a partially corner-shared pentagonal lattice (PCPL). We find a clear 1/3 magnetization plateau and an anomalous change in magnetization in the vicinity of the saturation field, which originate from frustrated interactions in the PCPL.

DOI： 10.1038/srep15327

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

The neutron diffraction and measurements of specific heat and magnetization were carried out on single crystals of a quantum spin ice system Yb2Ti2O7. The thermal hysteresis loops of temperature dependence of the magnetization and neutron diffraction intensity indicate a first-order ferromagnetic transition, where the used single crystals are confirmed to have a sharp peak structure in the specific heat (C)-temperature (T) curve at T-C similar to 0.2 K. The magnetic structure analysis of neutron diffraction results revealed that the Yb3+ moments have a ferromagnetic collinear structure along [001] directions with ordered moment value 1.1 +/- 0.1 mu(B) at 0.03 K. The mechanism of ferromagnetic transition of Yb2Ti2O7 will be discussed.

DOI： 10.1142/S2010324715400020

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

Low-energy quasiparticle excitations in the superconducting (SC) state of UBe13 were studied by means of specific-heat (C) measurements in a rotating field. Quite unexpectedly, the magnetic-field dependence of C(H) is linear in H with no angular dependence at low fields in the SC state, implying that the gap is fully open over the Fermi surfaces, in stark contrast to previous expectations. In addition, a characteristic cubic anisotropy of C(H) was observed above 2 T with a maximum (minimum) for H parallel to[001] ([111]) within the (1 (1) over bar0) plane, in the normal as well as in the SC states. This oscillation possibly originates from the anisotropic response of the heavy quasiparticle bands, and might be a key to understand the unusual properties of UBe13

DOI： 10.1103/PhysRevLett.114.147002

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We present low-temperature specific heat and magnetization measurements of the S = 1/2 organic spin ladder 3-I-V [3-(3-iodophenyl)-1,5-diphenylverdazyl]. Ab initio molecular orbital calculations indicate the existence of an intraladder diagonal interaction J(diag) and three types of interladder interactions. The ferromagnetic J(diag) induces frustration in the spin ladder, which has an essential contribution to the ground state. The obtained phase diagram exhibits a low-field three-dimensional order and a nontrivial phase near the saturation field. The absence of successive phase transition with decreasing temperature and the anomalous behavior of the magnetization curve near the saturation field suggest that these two phases are longitudinal spin-density-wave and spin-multipolar orders, respectively.

DOI： 10.1103/PhysRevB.91.125104

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We successfully synthesized the zinc-verdazyl complex [Zn(hfac)(2)]center dot(o-Py-V) [hfac = 1,1,1,5,5,5-hexafluoroacetylacetonate; o-Py-V = 3-(2-pyridyl)-1,5-diphenylverdazyl], which is an ideal model compound with an S = 1/2 ferromagnetic-antiferromagnetic alternating Heisenberg chain (F-AF AHC). Ab initio molecular-orbital (MO) calculations indicate that two dominant interactions J(F) and J(AF) form the S = 1 2 F-AF AHC in this compound. The magnetic susceptibility and magnetic specific heat of the compound exhibit thermally activated behavior below approximately 1 K. Furthermore, its magnetization curve is observed up to the saturation field and directly indicates a zero-field excitation gap of 0.5 T. These experimental results provide evidence for the existence of a Haldane gap. We successfully explain the results in terms of the S = 1 2 F-AF AHC through quantum Monte Carlo calculations with vertical bar J(AF)/J(F)vertical bar = 0.22. The ab initio MO calculations also indicate a weak AF interchain interaction J' and that the coupled F-AF AHCs form a honeycomb lattice. The J' dependence of the Haldane gap is calculated, and the actual value of J' is determined to be less than 0.01 vertical bar J(F)vertical bar.

DOI： 10.1103/PhysRevB.91.085117

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DOI： 10.11316/jpsgaiyo.70.2.0_1668

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DOI： 10.11316/jpsgaiyo.70.2.0_756

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DOI： 10.11316/jpsgaiyo.70.2.0_1665

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DOI： 10.11316/jpsgaiyo.70.2.0_1771

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DOI： 10.11316/jpsgaiyo.70.1.0_1122

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DOI： 10.11316/jpsgaiyo.70.1.0_2207

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DOI： 10.11316/jpsgaiyo.70.1.0_2350

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DOI： 10.11316/jpsgaiyo.70.1.0_1947

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DOI： 10.11316/jpsgaiyo.70.1.0_1240

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DOI： 10.11316/jpsgaiyo.70.1.0_967

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

We examine low-temperature magnetic properties of a new verdazyl radical crystal alpha-2-Cl-4-FV. Molecular orbital calculations predict that this material is a spin-1/2 quasi-one-dimensional antiferromagnet; the exchange interaction is strongest in the chains along the b axis, whereas there are three interchain coupling parameters, the ratio of which are 0.06 similar to 0.14 to the intrachain. The magnetization curve at 0.1 K shows gapless behavior like conventional one-dimensional quantum spin systems and saturates at about 5 T. A peak is observed in the temperature dependence of the heat capacity at 0 T, indicating that a three-dimensional ordering occurs at about 0.2 K. On the other hand, the temperature dependence of the magnetization at various magnetic fields shows an anomaly similar to that observed in many of gapped spin systems. This behavior is attributed to the highly frustrated nature of the interchain interactions.

DOI： 10.1016/j.phpro.2015.12.088

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We examined the low temperature magnetic properties of a single crystal of TmTi2Al20, a hole analogue of PrTi2Al20, by means of specific heat C(T) and magnetization M(T, H) measurements. At low field, an anomaly that can be ascribed to an antiferromagnetic ordering is observed in M(T) at T-N = 0.7 K. C(T) shows a very sharp lambda-type anomaly at 0.7 K, confirming the phase transition. From the calculated magnetic entropy S(T), we found that the ground state of TmTi2Al20 is likely to be a magnetic Gamma(5) triplet in the J = 6 multiplet.

DOI： 10.1088/1742-6596/592/1/012052

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DOI： 10.11316/jpsgaiyo.70.2.0_750

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

The magnetization and magnetic torque of a high-quality single crystal of Sr2RuO4 have been measured down to 0.1 K under precise control of the magnetic-field orientation. When the magnetic field is applied exactly parallel to the ab plane, a sharp magnetization jump 4 pi delta M of (0.74 +/- 0.15) G at the upper critical field H-c2,H-ab similar to 15 kOe with a field hysteresis of 100 Oe is observed at low temperatures, evidencing a first-order superconducting-normal transition. A strong magnetic torque appearing when H is slightly tilted away from the ab plane confirms an intrinsic anisotropy Gamma = xi(a)/xi(c) of as large as 60 even at 100 mK, in contrast with the observed H-c2 anisotropy of similar to 20. The present results raise fundamental issues in both the existing spin-triplet and spin-singlet scenarios, providing, in turn, crucial hints toward the resolution of the superconducting nature of Sr2RuO4.

DOI： 10.1103/PhysRevB.90.220502

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Magnetic field (B) variation of the electrical polarization P-c (parallel to c) of the perfect triangular lattice antiferromagnet RbFe(MoO4)(2) is examined up to the saturation point of the magnetization for B perpendicular to c.P-c is observed only in phases for which chirality is predicted in the in-plane magnetic structures. No strong anomaly is observed in P-c at the field at which the spin modulation along the c axis, and hence the spin helicity, exhibits a discontinuity to the commensurate state. These results indicate that the ferroelectricity in this compound originates predominantly from the spin chirality, the explanation of which would require a new mechanism for magnetoferroelectricity. The obtained field-temperature phase diagram of ferroelectricity agree well with those theoretically predicted for the spin chirality of a Heisenberg spin triangular lattice antiferromagnet.

DOI： 10.1103/PhysRevLett.113.147202

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DOI： 10.7566/JPSCP.3.015042

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We report magnetization, specific heat, and NMR measurements of 3-Br-4-F-V [=3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl], a strong-rung S = 1/2 Heisenberg spin ladder with ferromagnetic leg interactions. We explain the magnetic and thermodynamic properties based on the strong-rung regime. Furthermore, we find a field-induced successive phase transition in the specific heat and the nuclear spin-lattice relaxation rate 1/T-1. F-19-NMR spectra for higher-and lower-temperature phases indicate partial magnetic order and incommensurate long-range order, respectively, evidencing the presence of frustration due to weak interladder couplings.

DOI： 10.1103/PhysRevB.89.220402

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We have succeeded in synthesizing two types of new organic radical crystals 3-I-V [= 3-(3-iodophenyl)-1,5-diphenylverdazyl] and 3-Br-4-F-V [= 3-(3-bromo-4-fluorophenyl)-1,5-diphenylverdazyl]. Their crystal strucutures are found to be isomorphous to that of previously reported spin ladder 3-Cl-4-F-V. Through the quantitative analysis of their molecular arrangements and magnetic properties, we confirm that these materials form ferromagnetic chain-based spin ladders with slightly modulated magnetic interactions. These results present the first quantitative demonstration of the fine-tuning of magnetic interactions in the molecular-based materials.

DOI： 10.7566/JPSJ.83.033707

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Spin-orbit coupling has been conjectured to play a key role in the low-energy electronic structure of Sr2RuO4. By using circularly polarized light combined with spin-and angle-resolved photoemission spectroscopy, we directly measure the value of the effective spin-orbit coupling to be 130 +/- 30 meV. This is even larger than theoretically predicted and comparable to the energy splitting of the d(xy) and d(xz,yz) orbitals around the Fermi surface, resulting in a strongly momentum-dependent entanglement of spin and orbital character in the electronic wavefunction. As demonstrated by the spin expectation value &lt;(S-k) over arrow.(S-k) over arrow &gt; calculated for a pair of electrons with zero total momentum, the classification of the Cooper pairs in terms of pure singlets or triplets fundamentally breaks down, necessitating a description of the unconventional superconducting state of Sr2RuO4 in terms of these newly found spin-orbital entangled eigenstates.

DOI： 10.1103/PhysRevLett.112.127002

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Superconductivity in the heavy-fermion compound CeCu2Si2 is a prototypical example of Cooper pairs formed by strongly correlated electrons. For more than 30 years, it has been believed to arise from nodal d-wave pairing mediated by a magnetic glue. Here, we report a detailed study of the specific heat and magnetization at low temperatures for a high-quality single crystal. Unexpectedly, the specific-heat measurements exhibit exponential decay with a two-gap feature in its temperature dependence, along with a linear dependence as a function of magnetic field and the absence of oscillations in the field angle, reminiscent of multiband full-gap superconductivity. In addition, we find anomalous behavior at high fields, attributed to a strong Pauli paramagnetic effect. A low quasiparticle density of states at low energies with a multiband Fermi-surface topology would open a new door into electron pairing in CeCu2Si2.

DOI： 10.1103/PhysRevLett.112.067002

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The temperature, field, and field-orientation dependences of the electronic specific heat C-e of the iron-pnictide superconductor KFe2As2 have been investigated. Thermodynamic evidence of the presence of line nodes is obtained from the T and root H linear dependences of C-e/T in the low-T and low-H region. Under a magnetic field rotated within the tetragonal ab-plane, a fourfold oscillation is observed in C-e with a sign change at 0.08T(c). On the basis of the Dopplershift analysis, the observed C-e minima in H parallel to [100] at low T indicate the presence of line nodes somewhere on the Fermi surface where the Fermi velocity is parallel to the [100] direction; this is consistent with the octet-line-node scenario proposed recently by a photoemission experiment. In addition, the low-T C-e/T exhibits an unusual upturn on cooling at moderate fields only for H parallel to ab, which is understood in terms of the strong Pauli paramagnetic effect on multiband superconductivity.

DOI： 10.7566/JPSJ.83.013704

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The field-angle-resolved specific heat C(T,H,Ø) of the f -electron superconductor CeRu2 (Tc=6.3 K) has been measured at low temperatures down to 90mK on two single crystals of slightly different qualities. We reveal that the C(Ø) oscillation in a rotating magnetic field, originating from the gap anisotropy, diminishes at low temperatures below the characteristic field H*, as expected for an anisotropic gap without nodes. We also observe the suppression of H* by decreasing the gap anisotropy ratio Δmin/Δmax, a behavior that has been predicted from a microscopic theory for anisotropic s-wave superconductors. The present technique is established as a powerful tool for investigating minimum-gap structures as well as nodal structures. © 2013 The Physical Society of Japan.

DOI： 10.7566/JPSJ.82.123706

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

We have succeeded in synthesizing single crystals of an organic radical p-Br-V [3-(4-bromophenyl)-1,5-diphenylverdazyl]. The dominant intermolecular interactions forming a one-dimensional spin chain are deduced from the molecular packing in the crystal, and the ab initio molecular orbital calculation also indicates such an exchange network. We analyzed the magnetic susceptibility, magnetization curve, and magnetic specific heat through the quantum Monte Carlo method and successfully explained them as being attributed to an S = 1/2 Heisenberg antiferromagnetic chain. Furthermore, we have found an unexpected successive phase transition in the specific heats, which indicates the formation of a noncollinear magnetic structure induced by frustration.

DOI： 10.1103/PhysRevB.88.184431

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We have succeeded in synthesizing single crystals of the verdazyl radical beta-3-(2,6-dichlorophenyl)-1,5-diphenylverdazyl. The ab initio molecular orbital calculation indicates the formation of an S = 1/2 Heisenberg antiferromagnetic chain with fourfold magnetic periodicity consisting of three types of exchange interactions. We successfully explain the magnetic and thermodynamic properties based on the expected spin model by using the quantum Monte Carlo method. Furthermore, we reveal that the alternating and unique Ising ferromagnetic chains become effective in the specific field regions and observe a cooperative phenomenon caused by the magnetic order and quantum fluctuations. These results demonstrate that the verdazyl radical could form an unconventional spin model with interesting quantum behavior and provide a way to study a variety of quantum spin systems.

DOI： 10.1103/PhysRevB.88.174410

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

Low-temperature magnetization (M) measurements down to 0.1 K have been performed in magnetic fields up to 14.5 T for a single piece of a tiny single-crystalline sample (similar to 0.2 mg weight) of the spin-gap system YbAl3C3. At the base temperature of 0.1 K, several metamagnetic transitions were clearly observed for H aEuro- c in the range 6 T &lt; A mu (0) H &lt; 9 T whereas only two transitions were observed, one at 4.8 T and the other at 6.6 T, for H aEuro- a. At fields above 9 T, the magnetization becomes almost saturated for both H aEuro- a and H aEuro- c. The present results indicate that a singlet-triplet crossover occurs in a relatively narrow field range, suggesting a rather weak interdimer interaction in spite of the nearly triangular lattice of Yb ions.

DOI： 10.3938/jkps.62.2088

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We have succeeded in synthesizing single crystals of a new organic radical 3-Cl-4-F-V [3-(3-chloro-4-fluorophenyl)-1,5-diphenylverdazyl]. Through the ab initio molecular orbital calculation and the analysis of the magnetic properties, this compound was confirmed to be the first experimental realization of an S=1/2 spin-ladder system with ferromagnetic leg interactions. The field-temperature phase diagram indicated that the ground state is situated very close to the quantum critical point. Furthermore, we found an unexpected field-induced successive phase transition, which possibly originates from the interplay of low dimensionality and frustration. © 2013 American Physical Society.

DOI： 10.1103/PhysRevLett.110.157205

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We revisit the normal-state electronic structure of Sr2RuO 4 by angle-resolved photoemission spectroscopy with improved data quality, as well as ab initio band structure calculations in the local-density approximation with the inclusion of spin-orbit coupling. We find that the current model of a single surface layer (√2×√2)R45 reconstruction does not explain all detected features. The observed depth-dependent signal degradation, together with the close quantitative agreement with the slab calculations based on the surface crystal structure as determined by low-energy electron diffraction, reveal that - at a minimum - the subsurface layer also undergoes a similar although weaker reconstruction. This model accounts for all features - a key step in understanding the electronic structure - and indicates a surface-to-bulk progression of the electronic states driven by structural instabilities. Finally, we find no evidence for other phases stemming from either topological bulk properties or, alternatively, the interplay between spin-orbit coupling and the broken symmetry of the surface. © 2013 American Physical Society.

DOI： 10.1103/PhysRevLett.110.097004

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We have investigated the field-angle variation of the specific heat C(H, φ, θ) of the heavy-fermion superconductor UPt3 at low temperatures T down to 50 mK, where φ and θ denote the azimuthal and polar angles of the magnetic field H, respectively. For T = 88 mK, C(H, θ = 90°) increases proportionally to √H up to nearly the upper critical field Hc2, indicating the presence of line nodes. By contrast, C(H, θ = 0°) deviates upward from the √H dependence for √H/Hc2 ≥ 0.5. This behavior can be related to the suppression of Hc2 along the c direction, whose origin has not been resolved yet. Our data show that the unusual Hc2 limit becomes marked only when θ is smaller than 30°. In order to explore the possible vertical line nodes in the gap structure, we measured the ′ dependence of C in wide T and H ranges. However, we did not observe any in-plane angular oscillation of C within the accuracy of ΔC/C ~ 0.5%. This result implies that field-induced excitations of the heavy quasiparticles occur isotropically with respect to ′, which is apparently contrary to the recent finding of a twofold thermal-conductivity κ(′) oscillation. Copyright © 2013 The Physical Society of Japan.

DOI： 10.7566/JPSJ.82.024707

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We report on the superconducting properties of the cage compound Sc5Rh6Sn18 with T-c = 5 K determined by magnetic susceptibility, electrical resistivity, and specific heat measurements. Measurements in magnetic fields indicate type-II superconductivity with the upper critical field mu H-0(c2)(0) = 7.24 T. H-c2( T) shows a clear enhancement over the Werthamer-Helfand-Hohenberg prediction, suggesting a strong electron-phonon coupling. Specific heat data reveal that Sc5Rh6Sn18 is indeed a strong-coupling BCS superconductor having an isotropic superconducting gap.

DOI： 10.1143/JPSJS.81SB.SB016

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We investigated temperature, field, and field-orientation dependences of the specific heat of CeIrIn5. The upper critical field H-c2 was determined down to 80 mK from the specific heat measurements and exhibits a slight suppression at low temperatures. We revealed that the specific heat at 80 mK increases proportional to the square root of H up to nearly H-c2 for both H parallel to ab and H parallel to c, implying the presence of line nodes and a weak contribution of the Pauli paramagnetic pair-breaking effect. Furthermore, we revealed that the specific heat shows a fourfold oscillation under a rotating magnetic field in the ab plane. This result indicates that CeIrIn5 has fourfold symmetry around the c axis in its gap amplitude.

DOI： 10.1143/JPSJS.81SB.SB014

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

Superconducting and antiferroquadrupolar (AFQ) transitions in a Pr-based compound PrRh2Zn20 have been found to occur simultaneously at T-c = T-Q = 0.06 K. The superconducting transition manifests itself by zero resistance and large diamagnetic susceptibility. The specific heat exhibits a Schottky anomaly peaking at 14 K, and magnetization curves measured at 2 K show anisotropic behaviors. The analysis of these data indicates that the crystalline electric field ground state of the trivalent Pr ion is the non-Kramers Gamma(3) doublet with the quadrupolar degrees of freedom. A sharp peak in the specific heat at 0.06 K has been attributed not to the superconducting transition but to the AFQ transition because the ordering temperature T-Q decreases in B parallel to [100] but increases in B parallel to [110] and B parallel to [111] with increasing B up to 6 T. This anisotropic behavior of T-Q(B) can be well explained by a two-sublattice mean-field calculation, which corroborates the AFQ ordered state below T-Q. The entropy release at TQ is only 10% of R ln 2 expected for the Gamma(3) doublet, suggesting possible interplay between the quadrupolar degrees of freedom and the superconductivity.

DOI： 10.1103/PhysRevB.86.184426

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We performed electrical resistivity rho, magnetic susceptibility chi, specific heat C and electron diffraction measurements on single-crystalline samples of PrT2Zn20 (T = Ru, Rh and Ir). The three compounds show the Van Vleck paramagnetic behavior, indicating the nonmagnetic crystalline electric field (CEF) ground states. A Schottky-type peak appears at around 14 K, irrespective of the T element, which can be moderately reproduced by a doublet-triplet model. For T = Ru, a structural transition occurs at T-s = 138 K, below which no phase transition appears down to 0.04 K. On the other hand, for T = Ir, antiferroquadrupole (AFQ) ordering arising from the nonmagnetic 03 doublet takes place at T-Q = 0.11 K. For T = Rh, despite a structural transition between 170 and 470 K, the CEF ground state is still the non-Kramers Gamma(3) doublet. However, no phase transition due to the Gamma(3) doublet was observed even down to 0.1 K.

DOI： 10.1088/0953-8984/24/29/294207

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

In order to identify the gap structure of CeIrIn5, we measured field-angle-resolved specific heat C(phi) by conically rotating the magnetic field H around the c axis at low temperatures down to 80 mK. We revealed that C(phi) exhibits a fourfold angular oscillation, whose amplitude decreases monotonically by tilting H out of the ab plane. Detailed microscopic calculations based on the quasiclassical Eilenberger equation confirm that the observed features are uniquely explained by assuming the d(x2-y2)-wave gap. These results strongly indicate that CeIrIn5 is a d(x2-y2)-wave superconductor and suggest the universal pairing mechanism in CeMIn5 (M = Co, Rh, and Ir).

DOI： 10.1103/PhysRevB.85.060505

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This review presents a summary and evaluations of the superconducting properties of the layered ruthenate Sr2RuO4 as they are known in the autumn of 2011. This paper appends the main progress that has been made since the preceding review by Mackenzie and Maeno was published in 2003. Here, special focus is placed on the critical evaluation of the spin-triplet, odd-parity pairing scenario applied to Sr2RuO4. After an introduction to superconductors with possible odd-parity pairing, accumulated evidence for the pairing symmetry of Sr2RuO4 is examined. Then, significant recent progress on the theoretical approaches to the superconducting pairing by Coulomb repulsion is reviewed. A section is devoted to some experimental properties of Sr2RuO4 that seem to defy simple explanations in terms of currently available spin-triplet scenario. The next section deals with some new developments using eutectic boundaries and micro-crystals, which reveals novel superconducting phenomena related to chiral edge states, odd-frequency pairing states, and half-fluxoid states. Some of these properties are intimately connected with the properties as a topological superconductor. The article concludes with a summary of knowledge emerged from the study of Sr2RuO4 that are now more widely applied to understand the physics of other unconventional superconductors, as well as with a brief discussion of relatively unexplored but promising areas of ongoing and future studies of Sr2RuO4.

DOI： 10.1143/JPSJ.81.011009

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

One of the interesting issues on the spin-triplet superconductor Sr2RuO4 (T-c = 1.5 K) is the origin of the T-c enhancement up to about 3 K in the eutectic system Sr2RuO4-Ru. We have recently revealed a striking fact that T-c of pure Sr2RuO4 is also enhanced up to 3.2 K by uniaxial pressure along the c axis (P-parallel to c). When P-parallel to c is applied to Sr2RuO4-Ru, there is a crossover at P-parallel to c* similar to 0.4 GPa from behavior attributable to interfacial superconductivity (SC) to behavior similar to that in pure Sr2RuO4 under P-parallel to c. We focus on the field-temperature phase diagrams of Sr2RuO4-Ru above and below P-parallel to c*. We revealed that the H-T curves of Sr2RuO4-Ru both above and below P-parallel to c* are concave-up. This fact suggests that the P-parallel to c-originated SC above P-parallel to c*c as well as the interfacial SC below P-parallel to c*c are granular-like. We also found that the H-T curves of these SCs do not scale with T-c's, which is possibly related to a difference in the spatial distribution of superconducting regions.

DOI： 10.1088/1742-6596/391/1/012108

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Ferroelectricity was studied in magnetic fields up to 14 T on the XY-like perfect-triangular-lattice antiferromagnet RbFe(MoO4)(2), which exhibits a magnetically induced ferroelectric polarization along the c axis in the low-field 120 degrees structure phases P1 and P2. We have experimentally confirmed that the high-field P4/P5 phases of RbFe(MoO4)(2) for B parallel to a are nonferroelectric. The results favor the scenario that the ferroelectricity in the low-field phases of this system is driven by the in-plane spin chirality.

DOI： 10.1088/1742-6596/391/1/012099

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In order to identify the gap structure of a spin-triplet superconductor UPt3, we performed field-angle-resolved specific heat (C-phi) measurements in a magnetic field rotated around the [0001] axis. Although various theories predict fourfold symmetry of the gap in the superconducting C phase, we observed no angular variation in C-phi in a temperature range down to 88 mK (similar to 0.17T(c)). This absence of the variation in C-phi implies that the field-angle-dependent quasiparticle excitations, reflecting the gap anisotropy, are averaged out in the bulk of UPt3.

DOI： 10.1088/1742-6596/391/1/012031

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

In the Sr3Ru2O7 region of the Sr3Ru2O7-Sr2RuO4 eutectic system, multiple superconducting (SC) transitions are observed with onset temperatures of 1.3 and 1 K. Based on various experimental results, the SC transition at 1.3 K is attributable to thin Sr2RuO4 layers embedded in the Sr3Ru2O7 region as stacking faults. In this study, in order to clarify the origin of the SC transition at 1 K we have investigated its uniaxial pressure effect along the c axis. We revealed that, although Sr3Ru2O7 exhibits ferromagnetism under the uniaxial pressure along the c axis above 0.3 GPa, the onset temperature of the SC transition at 1 K is almost invariant or increases slightly under the uniaxial pressure up to 0.4 GPa. This result suggests that this SC transition is not strongly related to the electronic state of Sr3Ru2O7 and it is consistent with the scenario that the 1-K transition also originates from the embedded thin Sr2RuO4 layers whose thickness is thinner than that of the 1.3-K transition. (C) 2009 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2009.11.038

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

We have investigated uniaxial pressure effect on superconductivity of pure Sr2RuO4, whose intrinsic superconducting transition temperature T-c is 1.5 K. It is revealed that a very low uniaxial pressure along the c axis, only 0.2 GPa, induces superconductivity with the onset T-c above 3 K. The present results indicate that pure Sr2RuO4 has two superconducting phases with T-c=1.5 K and with varying T-c up to 3.2 K. The latter phase exhibits unusual features and is attributable to anisotropic crystal distortions beyond the elastic limit.

DOI： 10.1103/PhysRevB.81.180510

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One of the remaining issues concerning the spin-triplet superconductivity of Sr2RuO4 is the strong limit of the in-plane upper critical field H-c2 at low temperatures. In this study, we clarified the dependence of H-c2 on the angle theta between the magnetic field and the ab plane at various temperatures, by precisely and accurately controlling the magnetic field direction. We revealed that, although the temperature dependence of H-c2 for vertical bar theta vertical bar &gt;= 5 degrees is well explained by the orbital pair-breaking effect, H-c2(T) for vertical bar theta vertical bar &lt; 5 degrees is clearly limited at low temperatures. We also revealed that the H-c2 limit for vertical bar theta vertical bar &lt; 5 degrees is present not only at low temperatures but also at temperatures close to T-c. These features may provide additional hints for clarifying the origin of the H-c2 limit. Interestingly, if the anisotropic ratio in Sr2RuO4 is assumed to depend on temperature, the observed angular dependence of H-c2 is reproduced better at lower temperature with an effective-mass model for an anisotropic three-dimensional superconductor. We discuss the observed behavior of H-c2 based on existing theories.

DOI： 10.1103/PhysRevB.80.174514

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

While the superconducting transition temperature T-c of Sr2RuO4 is 1.5 K. its onset T-c is enhanced as high as 3 K in the Sr2RuO4-Ru eutectic system, which is often referred to as the 3-K phase. We have investigated effects of uniaxial pressure on the non-bulk Superconductivity in the 3-K. phase. While T-c of pure Sr2RuO4 is known to be suppressed by hydrostatic pressure, a large enhancement of the superconducting volume fraction of the 3-K phase was observed for both out-of-plane and in-plane uniaxial pressures. Especially, under the in-plane pressure, the shielding fraction at 1.8 K of only less than 0.5% at 0 GPa exceeds 40% at 0.4 GPa. Such a large shielding fraction suggests that under the uniaxial pressure interfacial 3-K phase superconductivity penetrates deep into the bulk of Sr2RuO4. The present finding provides a significant implication to the unresolved origin of the enhancement of T-c to 3 K in the Sr2RuO4-Ru eutectic system.

DOI： 10.1143/JPSJ.78.103705

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：AMER INST PHYSICS  

Sr2RuO4/Sr3Ru2O7 eutectic system is investigated by transmission electron microscopy (TEM) and high angle annular dark field scanning TEM (STEM). The Sr2RuO4/Sr3Ru2O7 lamellar structure allows studying tunneling and proximity effects between spin-triplet superconductor Sr2RuO4 and metamagnetic normal metal Sr3Ru2O7. Our analyses reveal two typologies of interfaces within the eutectic: interfaces parallel to the growth direction are sharp and defect-free whereas interfaces perpendicular to the growth direction appear wavy and decorated with Ru precipitates. These results indicate that interfaces parallel to the growth direction are the best candidates for Sr2RuO4/Sr3Ru2O7 natural junctions to study the unusual superconductivity of Sr2RuO4. (C) 2009 American Institute of Physics. [doi: 10.1063/1.3243691]

DOI： 10.1063/1.3243691

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

Polymorphic materials are known for being prone to intergrowth. Remarkable examples are strontium ruthenates whose properties are dramatically tuned by impurities and disorder. In particular, Sr3Ru2O7 shows a strong variation in transport and magnetic properties depending on synthesis process. It is therefore crucial to correlate atomic structure and properties to identify the functionalities of individual nanostructural constituents. We report a comparative study between Sr3Ru2O7 crystals grown as single phase and in Sr3Ru2O7-Sr2RuO4 eutectics. Our analysis by transmission electron microscopy reveals that Sr3Ru2O7 domains of the eutectic have a significantly lower level of impurities compared to Sr3Ru2O7 single-phase crystals, where intergrowths of Sr4Ru3O10 and SrRuO3 phase are seen. This is confirmed by magnetic measurements. These results identify the eutectic solidification as a fruitful way to grow highly pure crystals of polymorphic materials which, in combination with recent technological developments allowing the extraction of embedded features of crystals, opens a pathway for understanding of their physical properties and applications.

DOI： 10.1103/PhysRevB.80.054110

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We have clarified how the enhanced superconductivity, often referred to as the 3-K phase superconductivity, develops in the Sr2RuO4-Ru eutectic system. From the detailed ac and dc Susceptibility measurements on well-characterized crystals, we revealed strongly anisotropic shielding, governed by the direction of the screening current dominated within the RuO2 plane rather than by the orientation of the Ru lamellae. The onset temperature of the 3-K phase superconductivity probed by diamagnetic screening is as high as 3.5 K. The temperature dependence of the diamagnetic shielding above around 2 K is well ascribed by (he interfacial screening around each Ru lamella. Below around 2 K, the rapid development of the shielding fraction as well as its peculiar response to ac and dc magnetic fields are explained by the formation of the Josephson network consisting of inter-lamellar supercurrents.

DOI： 10.1143/JPSJ.78.064703

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

We report on super conducting behavior of Sr3Ru2O7 macrodomain taken out from Sr2RuO4-Sr3Ru2O7 eutectic crystals. Transport measurements performed down to 300 mK provide evidence of a supercurrent owing through the whole sample. Structural and compositional analyses are used to estimate the percentage of possible Sr2RuO4 inclusions. On the base of a model that describes the system as a proximity network made of superconducting Sr2RuO4 grains dispersed in the normal Sr3Ru2O7 matrix we determine the conditions for having a percolating superflow through the system.

DOI： 10.1088/1742-6596/150/5/052056

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We have studied the upper critical field H(c2) of the spin-triplet superconductor Sr(2)RuO(4). For H parallel to c, H(c2) increases linearly with decreasing temperature down to well below T(c) = 1.5 K, consistent with the orbital depairing effect. However, for H parallel to ab, the increase of H(c2) is strongly suppressed below 1 K. The origin of this in-plane H(c2) limit has been actively discussed, but its mechanism is still unclear. In the present study, we performed ac susceptibility measurements in the applied fields with higher accuracy than in previous studies to obtain the field-temperature phase diagrams for H parallel to c and H parallel to ab. We revealed that H(c2 parallel to ab)(T) does not exhibit a linear temperature dependence even above 1 K, which indicates that the H(c2) suppression is operative even close to T(c). In addition, we found that the H(c2) anisotropy ratio H(c2 parallel to ab)/H(c2 parallel to c) decreases monotonically from 60 to 20 with decreasing temperature.

DOI： 10.1088/1742-6596/150/5/052112

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Although Sr3Ru2O7 has not been reported to exhibit superconductivity so far, ac susceptibility measurements revealed multiple superconducting transitions occurring in the Sr3Ru2O7 region cut from Sr3Ru2O7 - Sr2RuO4 eutectic crystals. Based on various experimental results, some of us proposed the scenario in which Sr2RuO4 thin slabs with a few layers of the RuO2 plane are embedded in the Sr3Ru2O7 region as stacking faults and multiple superconducting transitions arise from the distribution of the slab thickness. To examine this scenario, we measured the resistivity along the ab plane (rho(ab)) using a Sr3Ru2O7-region sample cut from the eutectic crystal, as well as along the c axis (rho(c)) using the same crystal. As a result, we detected resistance drops associated with superconductivity only in rho(ab), but not in rho(c). These results support the Sr2RuO4 thin-slab scenario. In addition, we measured the resistivity of a single crystal of pure Sr3Ru2O7 with very high quality and found that pure Sr3Ru2O7 does not exhibit superconductivity down to 15 mK.

DOI： 10.1088/1742-6596/150/5/052113

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We have investigated effects of uniaxial pressure on superconductivity in the 3-K phase in the eutectic system Sr(2)RuO(4)-Ru. We have found a large enhancement of 3-K phase superconductivity particularly in volume fraction for both in-plane pressure P parallel to ab and the c-axis pressure P parallel to c in contrast to expectations for the 1.5-K phase in pure Sr(2)RuO(4). Surprisingly, the effect of the in-plane pressure is even greater than that of the c-axis pressure. This could be related to the origin of the 3-K phase.

DOI： 10.1088/1742-6596/150/5/052285

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

We report the superconducting properties of Sr(3)Ru(2)O(7)-Sr(2)RuO(4) eutectic crystals, which consist of the spin-triplet superconductor Sr(2)RuO(4) with a monolayer stacking of RuO(2) planes and the metamagnetic normal metal Sr(3)Ru(2)O(7) with a bilayer stacking. Although Sr(3)Ru(2)O(7) so far has not been reported to exhibit superconductivity, our ac susceptibility measurements revealed multiple superconducting transitions that occur in the Sr(3)Ru(2)O(7) region of the eutectic crystals. The diamagnetic shielding essentially reached the full fraction at low ac fields parallel to the c axis. However, both the shielding fraction and the onset temperature are easily suppressed by ac fields larger than 0.1 mT rms and no anomaly was observed in the specific heat. Moreover, the critical field curves of these transitions have a positive curvature near zero fields, which is different from the upper critical field curve of the bulk Sr(2)RuO(4). These facts suggest that the superconductivity observed in the Sr(3)Ru(2)O(7) region is not a bulk property. To explain these experimental results, we propose the scenario that stacking RuO(2) planes, which are the building blocks of superconducting Sr(2)RuO(4), are contained in the Sr(3)Ru(2)O(7) region as stacking faults.

DOI： 10.1103/PhysRevB.77.214511

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Language：English   Publishing type：Research paper (scientific journal)   Publisher：EPL ASSOCIATION, EUROPEAN PHYSICAL SOCIETY  

Superconducting behavior has been observed in the Sr(2)RuO(4)-Sr(3)Ru(2)O(7) eutectic system as grown by the flux-feeding floating-zone technique. A supercurrent flows across a single interface between Sr(2)RuO(4) and Sr(3)Ru(2)O(7) areas at distances that are far beyond those expected in a conventional proximity effect. The current-voltage characteristics within the Sr(3)Ru(2)O(7) macrodomain, as extracted from the eutectic, exhibit signatures of superconductivity in the bilayered Sr(3)Ru(2)O(7) region. Detailed microstructural, morphological and compositional analyses address issues on the concentration and the size of Sr(2)RuO(4) inclusions within the Sr(3)Ru(2)O(7) matrix. We speculate on the possibility of inhomogeneous superconductivity in the eutectic Sr(3)Ru(2)O(7) and exotic pairing induced by the Sr(2)RuO(4) inclusions. Copyright (C) EPLA, 2008.

DOI： 10.1209/0295-5075/83/27007

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

We report a study of morphological, compositional and structural aspects of SrRuO4-Sr3Ru2O7 cutectic crystals to check the stoichiometry of the phases that compose the crystals and how the two phases match each other in the same crystal. This knowledge is also a necessary prerequisite for the understanding of the experimental results on the superconducting behaviour reported. (C) 2007 Elsevier B.V. All rights reserved.

DOI： 10.1016/j.physc.2007.03.178

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The low-field AC susceptibility of a eutectic crystal of the Sr2RuO4-Sr3Ru2O7 system exhibits superconducting transition features at three temperatures. We investigate the dependence on AC field strength from 0.021 to 2.1 Oe-rms and frequency from 17 to 3 kHz. The two lower-temperature peaks in the imaginary part originate in the Sr3Ru2O7 part. The lowest-temperature peak is exceedingly sensitive to the. eld amplitude and shifts to lower temperatures with increasing amplitude. By contrast, it slightly shifts to higher temperatures with increasing frequency. These results support that the sensitive features involve weak links forming the screening paths. (c) 2006 Published by Elsevier B.V.

DOI： 10.1016/j.jmmm.2006.10.103

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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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Publishing type：Rapid communication, short report, research note, etc. (scientific journal)  

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

DOI： 10.11316/jpsgaiyo.71.1.0_2061

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J-GLOBAL

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

DOI： 10.11316/jpsgaiyo.71.1.0_2158

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DOI： 10.11316/jpsgaiyo.71.1.0_2301

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DOI： 10.11316/jpsgaiyo.71.1.0_2130

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Language：Japanese   Publishing type：Article, review, commentary, editorial, etc. (scientific journal)  

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.70.2.0_1127

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.70.1.0_1038

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Language：Japanese   Publisher：The Physical Society of Japan  

DOI： 10.11316/jpsgaiyo.70.1.0_2243

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

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

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

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

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DOI： 10.11316/jpsgaiyo.69.2.3.0_460_3

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J-GLOBAL

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

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

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

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

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

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Language：Japanese   Publisher：The Physical Society of Japan  

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

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

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

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

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

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

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Language：English  

In order to identify the gap structure of CeIrIn 5, we measured field-angle-resolved specific heat C(φ) by conically rotating the magnetic field H around the c axis at low temperatures down to 80 mK. We revealed that C(φ) exhibits a fourfold angular oscillation, whose amplitude decreases monotonically by tilting H out of the ab plane. Detailed microscopic calculations based on the quasiclassical Eilenberger equation confirm that the observed features are uniquely explained by assuming the d x2-y2-wave gap. These results strongly indicate that CeIrIn 5 is a d x2-y2-wave superconductor and suggest the universal pairing mechanism in CeMIn 5 (M=Co, Rh, and Ir). © 2012 American Physical Society.

DOI： 10.1103/PhysRevB.85.060505

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Language：English   Publisher：Physical Society of Japan  

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Publishing type：Article, review, commentary, editorial, etc. (other)  

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

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