町田 洋

Abstract The origin of phonon thermal Hall Effect (THE) observed in a variety of insulators is yet to be identified. Here, we report on the observation of a thermal Hall conductivity in a non-magnetic elemental insulator, with an amplitude exceeding what has been previously observed. In black phosphorus (BP), the longitudinal (κ_ii), and the transverse, κ_ij, thermal conductivities peak at the same temperature and at this peak temperature, the κ_ij/κ_jj/B is ≈ 10⁻⁴−10⁻³ T⁻¹. Both these features are shared by other insulators displaying THE, despite an absolute amplitude spreading over three orders of magnitude. The absence of correlation between the thermal Hall angle and the phonon mean-free-path imposes a severe constraint for theoretical scenarios of THE. We show that in BP a longitudinal and a transverse acoustic phonon mode anti-cross, facilitating wave-like transport across modes. The anisotropic charge distribution surrounding atomic bonds can pave the way for coupling between phonons and the magnetic field.

Abstract It has become common knowledge that phonons can generate thermal Hall effect in a wide variety of materials, although the underlying mechanism is still controversial. We study longitudinal κ_xx and transverse κ_xy thermal conductivity in Pr₂Ir₂O₇, which is a metallic analog of spin ice. Despite the presence of mobile charge carriers, we find that both κ_xx and κ_xy are dominated by phonons. A T/H scaling of κ_xx unambiguously reveals that longitudinal heat current is substantially impeded by resonant scattering of phonons on paramagnetic spins. Upon cooling, the resonant scattering is strongly affected by a development of spin ice correlation and κ_xx deviates from the scaling in an anisotropic way with respect to field directions. Strikingly, a set of the κ_xx and κ_xy data clearly shows that κ_xy correlates with κ_xx in its response to magnetic field including a success of the T/H scaling and its failure at low temperature. This remarkable correlation provides solid evidence that an indispensable role is played by spin-phonon scattering not only for hindering the longitudinal heat conduction, but also for generating the transverse response.

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 > T-Q. Moreover, in magnetic fields B <= 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 <= 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.

We report on a study of the Seebeck coefficient and resistivity in the quasi-one-dimensional conductor (TMTSF)(2) PF6 extended deep into the spin-density-wave state. The metal-insulator transition at T-SDW = 12 K leads to a reduction in carrier concentration by 7 orders of magnitude. Below 1 K, charge transport displays the behavior known as variable range hopping. Until now, the Seebeck response of electrons in this regime has barely been explored and is even less understood. We find that, in this system, residual carriers, hopping from one trap to another, generate a Seebeck coefficient as large as 400 k(B)/e. The results provide the first solid evidence for a long-standing prediction according to which hopping electrons in the presence of the Coulomb interaction can generate a sizable Seebeck coefficient in the zero-temperature limit.

Specific heat, elastic neutron scattering, and muon spin rotation (mu SR) experiments have been carried out on a well-characterized sample of "stuffed" (Pr-rich) Pr2+xIr2-xO7-delta. Elastic neutron scattering shows the onset of long-range spin-ice "2-in/2-out" magnetic order at T-M = 0.93 K, with an ordered moment of 1.7(1)mu(B)/Pr ion at low temperatures. Approximate lower bounds on the correlation length and correlation time in the ordered state are 170 angstrom and 0.7 ns, respectively. mu SR experiments yield an upper bound 2.6(7) mT on the local field B-loc(4f) at the muon site, which is nearly two orders of magnitude smaller than the expected dipolar field for long-range spin-ice ordering of 1.7 mu(B) moments (120-270 mT, depending on muon site). This shortfall is due in part to splitting of the non-Kramers crystal-field ground-state doublets of near-neighbor Pr3+ ions by the mu(+)-induced lattice distortion. For this to be the only effect, however, similar to 160 Pr moments out to a distance of similar to 14 angstrom must be suppressed. An alternative scenario, which is consistent with the observed reduced nuclear hyperfine Schottky anomaly in the specific heat, invokes slow correlated Pr-moment fluctuations in the ordered state that average B-loc(4f) on the mu SR time scale (similar to 10(-7) s), but are static on the time scale of the elastic neutron scattering experiments (similar to 10(-9) s). In this picture, the dynamic muon relaxation suggests a Pr3+ 4f correlation time of a few nanoseconds, which should be observable in a neutron spin echo experiment.

The thermal conductivity of YbRh2Si2 has been measured down to very low temperatures under field in the basal plane. An additional channel for heat transport appears below 30 mK, both in the antiferromagnetic and paramagnetic states, respectively, below and above the critical field suppressing the magnetic order. This excludes antiferromagnetic magnons as the origin of this additional contribution to thermal conductivity. Moreover, this low temperature contribution prevails a definite conclusion on the validity or violation of the Wiedemann-Franz law at the field-induced quantum critical point.

We report measurements of in-plane electrical and thermal transport properties in the limit T -> 0 near the unconventional quantum critical point in the heavy-fermion metal beta-YbAlB4. The high Kondo temperature T-K similar or equal to 200 K in this material allows us to probe transport extremely close to the critical point, at unusually small values of T/T-K < 5 x 10(-4). Here we find that the Wiedemann-Franz law is obeyed at the lowest temperatures, implying that the Landau quasiparticles remain intact in the critical region. At finite temperatures we observe a non-Fermi-liquid T-linear dependence of inelastic-scattering processes to energies lower than those previously accessed. These processes have a weaker temperature dependence than in comparable heavy fermion quantum critical systems, revealing a temperature scale of T similar to 0.3 K which signals a sudden change in the character of the inelastic scattering.

The de Haas-van Alphen effect, which is a powerful method to explore Fermi surface properties, has been observed in cerium, uranium, and nowadays even in neptunium and plutonium compounds. Here, we present the results of several studies concerning the Fermi surface properties of the heavy fermion superconductors UPt3 and NpPd5Al2, and of the ferromagnetic pressure-induced superconductor UGe2, together with those of some related compounds for which fascinating anisotropic superconductivity, magnetism, and heavy fermion behavior has been observed. (C) 2014 Academie des sciences. Published by Elsevier Masson SAS. All rights reserved.

We represent our recent advances on the study of the gap symmetry in UPt3 by thermal conductivity tensors kappa(ij) (i, j = x, y, z). The field-angle-resolved thermal conductivity kappa(zz) shows spontaneous twofold symmetry breaking in the gap function for the high-field C-phase, indicating that the pairing symmetry of UPt3 belongs to an E-1u representation in the f-wave category. We also demonstrate that the proposed pairing symmetry is compatible with most of the experimental results reported until now.

We develop a physical understanding for how anhamonicity of lattice vibration affects transition temperature, T-c, of superconductivity mediated by phonons, via specific heat measurements on beta-pyrochlore oxide superconductors under high pressure and in magnetic fields. We find that T-c demonstrates unimodal dependence on pressure, where T-c increases more than three times by pressure. On the other hand, an electron-phonon coupling constant, lambda, is monotonically enhanced as pressure is applied, leading to the enhancement of effective mass of conduction electrons. From the analysis based on the McMillan equation, moreover, an average phonon frequency, omega(log), is found to reflect a downward trend on all the pressure regions, which is consistent with the results of the previous inelastic neutron scattering and specific heat measurements. The downward trend of omega(log) means that the application of pressure enhances a magnitude of anhamonicity, contrary to a common sense of pressure effects. On the basis of all the results, we indicate that the unimodal pressure variation in T-c is attributed to two competing effects, that is, the enhancement of lambda and the simultaneous suppression of omega(log) caused by the pressure evolution of anhamonicity.

The thermal conductivity measurements are performed on the heavy-fermion compound YbRh2Si2 down to 0.04 K and under magnetic fields through a quantum critical point (QCP) at Bc=0.66 Tâ̂\c axis. In the limit as T→0, we find that the Wiedemann-Franz law is satisfied within experimental error at the QCP despite the destruction of the standard signature of Fermi liquid. Our results place strong constraints on models that attempt to describe the nature of the unconventional quantum criticality of YbRh2Si2. © 2013 American Physical Society.

Thermoelectric power measurements on the heavy fermion antiferromagnet Ce(Ru0.92Rh0.08)(2)Si-2 under magnetic field (H) show clearly that the antiferromagnetic state below the critical field H-c similar to 2.8 T can be fully decoupled from the pseudo-metamagnetic crossover at H-m similar to 5.8 T which occurs when the magnetization reaches a critical value. By contrast to the weak field variation of the Sommerfeld coefficient of the specific heat in the field window from H-c to H-m, two electronic singularities with opposite signs are detected in the thermoelectric power. The interplay between the magnetic instability and the topological change of the Fermi surface is discussed and we argue similarities to other field instabilities in various heavy fermion compounds.

The thermoelectric coefficients have been measured down to a very low temperature for the Yb-based heavy-fermion compounds beta-YbAlB4 and YbRh2Si2, often considered as model systems for the local quantum criticality case. We observe a striking difference in the behavior of the Seebeck coefficient S in the vicinity of their respective quantum critical point (QCP). Approaching the critical field, S=T is enhanced in beta-YbAlB4, but drastically reduced in YbRh2Si2. The ratio of thermopower to specific heat remains constant for beta-YbAlB4, but it is significantly reduced near the QCP in YbRh2Si2. In both systems, on the other hand, the Nernst coefficient shows a diverging behavior near the QCP. The interplay between valence and magnetic quantum criticality and the additional possibility of a Lifshitz transition crossing the critical field under magnetic field are discussed as the origin of the different behaviors of these compounds.

The field-orientation dependent thermal conductivity of the heavy-fermion superconductor UPt3 was measured down to very low temperatures and under magnetic fields throughout the distinct superconducting phases: B and C phases. In the C phase, a striking twofold oscillation of the thermal conductivity within the basal plane is resolved reflecting the superconducting gap structure with a line of node along the a axis. Moreover, we find an abrupt vanishing of the oscillation across a transition to the B phase, as a clear indication of a change of gap symmetries. We also identify extra two line nodes below and above the equator in both B and C phases. From these results together with the symmetry consideration, the gap function of UPt3 is determined as a E-1u representation characterized by a combination of two line nodes at the tropics and point nodes at the poles.

We uncover a strong anisotropy in both the anomalous Hall effect (AHE) and the magnetoresistance of the chiral spin states of Pr2Ir2O7. The AHE appearing below 1.5 K at a zero magnetic field shows hysteresis which is most pronounced for fields cycled along the [111] direction. This hysteresis is compatible with the field-induced growth of domains composed by the 3-in 1-out spin states which remain coexisting with the 2-in 2-out spin ice manifold once the field is removed. Only for fields applied along the [111] direction, we observe a large positive magnetoresistance and Shubnikov-de Haas oscillations above a metamagnetic critical field. These observations suggest the reconstruction of the electronic structure of the conduction electrons by the field-induced spin texture.

Thermal transport measurements have been made on the Fe-based superconductor Lu2Fe3Si5 (T-c similar to 6 K) down to a very low temperature T-c/120. The field and temperature dependences of the thermal conductivity confirm the multigap superconductivity with fully opened gaps on the whole Fermi surfaces. In comparison to MgB2, Lu2Fe3Si5 reveals a remarkably enhanced quasiparticle heat conduction in the mixed state. The results can be interpreted as a consequence of the unequal weight of the Fe 3d-electron character among the distinct bands.

Single crystals of the quantum low-dimensional antiferromagnet Rb4Cu(MoO4)(3) and the nonmagnetic analogue Ru4Zn(MoO4)(3) have been synthesized by a flux-growth method. Detailed structural studies indicate that the Cu(II)-O network separated by a Moat layer has a strongly anisotropic hybridization along the a-axis, forming a quasi-one-dimensional (1-d) chain of Cu(II) S = 1/2 spins. Furthermore, our low-temperature thermodynamic measurements have revealed that a quantum paramagnetic state with Wilson ratio similar to 2 remains stable down to at least 0.1 K, 100 times lower than the intrachain antiferromagnetic coupling scale. The low-temperature magnetic and thermal properties are found to be consistent with theoretical predictions made for a 1-d network of S = 1/2 spins.

Spin liquids are magnetically frustrated systems, in which spins are prevented from ordering or freezing, owing to quantum or thermal fluctuations among degenerate states induced by the frustration. Chiral spin liquids are a hypothetical class of spin liquids in which the time-reversal symmetry is macroscopically broken in the absence of an applied magnetic field or any magnetic dipole long-range order. Even though such chiral spin-liquid states were proposed more than two decades ago(1-3), an experimental realization and observation of such states has remained a challenge. One of the characteristic order parameters in such systems is a macroscopic average of the scalar spin chirality, a solid angle subtended by three nearby spins. In previous experimental reports, however, the spin chirality was only parasitic to the non-coplanar spin structure associated with a magnetic dipole long-range order or induced by the applied magnetic field(4-10), and thus the chiral spin-liquid state has never been found. Here, we report empirical evidence that the time-reversal symmetry can be broken spontaneously on a macroscopic scale in the absence of magnetic dipole long-range order. In particular, we employ the anomalous Hall effect(4,11) to directly probe the broken time-reversal symmetry for the metallic frustrated magnet Pr(2)Ir(2)O(7). An onset of the Hall effect is observed at zero field in the absence of uniform magnetization, within the experimental accuracy, suggesting an emergence of a chiral spin liquid. The origin of this spontaneous Hall effect is ascribed to chiral spin textures(4,5,12,13), which are inferred from the magnetic measurements indicating the spin ice-rule formation(14,15).

Thermal transport measurements have been performed on single-crystalline Co-doped BaFe2As2 down to 0.1 K and under magnetic fields up to 7 T. Significant peak anomalies are observed in both thermal conductivity and thermal Hall conductivity below T-c as an indication of the enhancement of the quasiparticle mean-free path. Moreover, we find a sizable residual T-linear term in thermal conductivity, possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair breaking. Our findings support a pairing symmetry compatible with the theoretically predicted sign-reversing s-wave state.

beta-YbAlB4 is the first Yb-based heavy fermion superconductor with T-c = 80 mK. Our study using high-purity single crystals reveals that strongly type-II heavy fermion superconductivity emerges from the non-Fermi-liquid state with enhanced ferromagnetic fluctuations. High sensitivity of Tc to sample purity indicates strong pair-breaking effects due to impurities, probably of nonmagnetic type, suggesting an unconventional character of the superconductivity.

A long-standing question in the field of superconductivity is whether pairing of electrons can arise in some cases as a result of magnetic interactions instead of electron-phonon-induced interactions as in the conventional Bardeen-Cooper-Schrie V er theory(1). A major challenge to the idea of magnetically mediated superconductivity has been the dramatically different behaviour of the cerium and ytterbium heavy-fermion compounds. The cerium-based systems are often found to be superconducting(1-6), in keeping with a magnetic pairing scenario, but corresponding ytterbium systems, or hole analogues of the cerium systems, are not. Despite searches over two decades there has been no evidence of heavy-fermion superconductivity in an ytterbium system, casting doubt on our understanding of the electron-hole parallelism between the cerium and the ytterbium compounds. Here we present the first empirical evidence that superconductivity is indeed possible in an ytterbium-based heavy-fermion system. In particular, we observe a superconducting transition at T(c) = 80mK in high-purity single crystals of YbAlB(4) in the new structural beta phase(7). We also observe a novel type of non-Fermi-liquid state above T c that arises without chemical doping, in zero applied magnetic field and at ambient pressure, establishing beta-YbAlB(4) as a unique system showing quantum criticality without external tuning.

Single crystals of R2Ir2O7 (R = Pr, Eu) have been synthesized using molten KF at 1373 K. The pyrochlore compounds crystallize in a cubic space group Fd(3) over bar m (No. 227, origin choice 2), with Z = 8. At room temperature, the lattice parameters are a = 10.3940(4) angstrom, V = 1122.92(7) angstrom(3) and a = 10.274(3) angstrom, V = 1084.5(6) angstrom(3) for Pr2Ir2O7 and Eu2Ir2O7, respectively. In this paper, we report the crystal growth of R2Ir2O7 (R. = Pr, Eu) and their structure determinations from single crystal X-ray diffraction experiments at temperatures of 110, 115, and 298 K. (c) 2006 Elsevier Ltd. All rights reserved.

Single crystals of YbAlB4 were grown in excess Al flux. Plate- and needle-shaped crystals were found. The plates are found to be beta-YbAlB4, which crystallizes with the ThMoB4 structure type in space group Cmmm (No. 65), Z = 4, with lattice parameters of a = 7.3080(4), b = 9.3150(5), and c = 3.4980(2) angstrom. The needle-shaped crystals were identified as the first form of YbAlB4 which crystallizes with the YCrB4 structure type in space group Pbam (No. 55), Z = 4, with lattice parameters of a = 5.9220(2), b = 11.4730(3), and c = 3.5060(5) angstrom. While both compounds have heavy fermion ground states with Ising-like magnetic anisotropy, the electronic specific heat coefficients (gamma) differ. The beta-phase has a gamma value near 300 mJ mol(-1) K-2, more than twice that of the alpha-phase, gamma = 130 mJ mol(-1) K-2. A comparison of the structures and physical properties of both polymorphs is presented.

We have investigated the Hall effect in the geometrically frustrated Kondo lattice Pr2Ir2O7, In its spin-liquid-like paramagnetic regime, the Hall resistivity rho(xy) is found to increase logarithmically on cooling. Moreover, in this low temperature region, the field dependence of the Hall conductivity sigma(xy) shows a large enhancement up to 30 Omega(-1) cm(-1) as well as a nonmonotonic change with the magnetization. Our results are far different from the anomalous Hall effect due to the spin-orbit coupling observed in ordinary magnetic conductors. We discuss the possible spin-chirality effect in the Ir 5d conduction band due to the noncoplanar texture of Pr < 111 > Ising-like moments.

Strongly frustrated magnetism of the metallic pyrochlore oxide Pr2Ir2O7 has been revealed by single crystal study. While Pr 4f moments have an antiferromagnetic RKKY interaction energy scale of parallel to T-*parallel to=20 K mediated by Ir 5d-conduction electrons, no magnetic long-range order is found except for partial spin freezing at 120 mK. Instead, the Kondo effect, including a lnT dependence in the resistivity, emerges and leads to a partial screening of the moments below parallel to T-*parallel to. Our results indicate that the underscreened moments show spin-liquid behavior below a renormalized correlation scale of 1.7 K.

The thermal diffuse scattering of synchrotron x-rays was measured for a single crystal of 1T-TaS2 along high symmetry direction in reciprocal space within the temperature range of 300-460 K. Least-squares analysis in terms of a lattice dynamics yielded phonon dispersion curves. The dispersion curves revealed Kohn anomalies both in a longitudinal acoustic (LA) mode and transverse acoustic (TA(parallel to)) mode polarized along b* direction. The results show that the softened TA(parallel to) modes trigger the rotation of the ordering vector in the nearly commensurate phase at low temperature.

Magnesium diboride, MgB2, has the highest transition temperature (T-c = 39 K) of the known metallic superconductors(1). Whether the anomalously high T-c can be described within the conventional BCS (Bardeen-Cooper-Schrieffer) framework 2 has been debated. The key to understanding superconductivity lies with the 'superconducting energy gap' associated with the formation of the superconducting pairs. Recently, the existence of two kinds of superconducting gaps in MgB2 has been suggested by several experiments(3-9); this is in contrast to both conventional and high-T-c superconductors. A clear demonstration of two gaps has not yet been made because the previous experiments lacked the ability to resolve the momentum of the superconducting electrons. Here we report direct experimental evidence for the two-band superconductivity in MgB2, by separately observing the superconducting gaps of the sigma and pi bands (as well as a surface band). The gaps have distinctly different sizes, which unambiguously establishes MgB2 as a two-gap superconductor(10,11).

We synthesized single crystalline (formula presented) under ambient pressure by using conventional materials and equipment. The single crystals of (formula presented) were of good quality, where the crystal structure refinements were successfully converged with (formula presented) The measurements of the magnetic properties yielded a sharp superconducting transition at 38 K with transition width (formula presented) The upper critical field for applied field parallel to the ab plane (formula presented) reveals a positive curvature, while (formula presented) parallel to the c axis (formula presented) increases linearly in temperature dependence, which yields a temperature dependence of the superconducting anisotropy ratio of (formula presented) with (formula presented) near (formula presented) and 4.0 at 25 K. © 2003 The American Physical Society.