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 have studied the electrical resistivity ρ of the Pr-based cubic compound PrPb3 with the Γ3 doublet ground state. The temperature dependence of the resistivity is found to exhibit a non-Fermi liquid behavior with convex curve at high temperatures, as suggestive of the putative realization of the quadrupole Kondo effect. At low temperatures under magnetic fields, we observe anomalies associated with antiferroquadrupole (AFQ) ordering, incommensurate/commensurate transition inside the AFQ phase, and field-induced phase transition. The constructed phase diagram based on these observations well reproduces the one determined by the specific heat and magnetization measurements. Deep inside each phase, the resistivity turns to show the Fermi liquid behavior with the quadratic temperature dependence. The estimated slope of the T2 term is almost independent of field in contrast to our previous work of PrIr2Zn20 and PrRh2Zn20 in which the slope is largely enhanced at the edge of the AFQ phase.

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.

Low-temperature Seebeck coefficient S/T measurements have been performed on Pr-based 1-2-20 system, PrTr2X20 (Tr = Ti, Ta, V, Ir, X = Al, Zn) with non-Kramers doublet ground states. For PrTr2X20 with X = Al, we find a large S/T, which amounts to those of heavy fermion metals. By contrast, S/T for PrIr2Zn20 is found to be considerably small as the same order of magnitude as those of ordinary metals, despite the commonly enhanced Sommerfeld coefficient γ throughout the system. A satisfactory of the quasi-universal relation between S/T and γ as well as the Kadowaki-Woods relation demonstrates that the mass enhancement is realized in PrTr2Al20 due to the hybridization between f-electrons and the conduction electrons. We also find that the small S/T of PrIr2Zn20 is enhanced at low temperatures under the fields on the verge of quadrupole ordered phase, and in the same regime, the electrical resistivity follows the quadratic temperature dependence with a steep slope as a characteristic of Fermi liquid. The results imply an emergence of a nontrivial coherent state with sizable mass enhancement associated with the quadrupole degree of freedom.

The thermal conductivity of YbRh_{2}Si_{2} 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 study the transport coefficients of PrIr2Zn20 and PrRh2Zn20 with non-Kramers doublet ground state. From the resistivity measurement, we commonly found a convex temperature dependence at high temperatures. At low temperatures, an anomaly at T* is discovered near the boundary of the antiferro-quadrupole (AFQ) ordering phase in both systems. The Fermi liquid behavior with a large electron mass is observed below T* Thus a non-trivial heavy Fermion state would be formed there. In contrast, from the quantum oscillation of the Seebeck coefficient of PrRh2Zn20, the electron mass is found to be small below the critical field for B || [110], which is consistent with negligibly mass Seebeck coefficient of PrIr2Zn20. Since these features are detected in both compounds, they are expected to be universal properties of non-Kramers systems.

The thermal conductivity measurements are performed on the heavy-fermion compound YbRh(2)Si(2) down to 0.04 K and under magnetic fields through a quantum critical point (QCP) at B(c)=0.66  T∥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 YbRh(2)Si(2).

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.

The thermoelectric coefficients have been measured down to a very low temperature for the Yb-based heavy-fermion compounds β-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 β-YbAlB4, but drastically reduced in YbRh2Si2. The ratio of thermopower to specific heat remains constant for β-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.

An electric current flowing through a conductor in a magnetic field produces a transverse voltage drop known as the Hall effect. In the absence of the field, this effect also appears in ferromagnets in a plane normal to its spontaneous magnetization vector owing to the spin-orbit coupling. Generally, it may also detect a nontrivial order parameter breaking the time-reversal symmetry on a macroscopic scale, for example, scalar spin chirality. Here, we present our recent results in the study of the frustrated magnetism and Hall transport of the metallic magnet Pr2Ir2O7. Strikingly, a spontaneous Hall effect is observed in the absence of both an external magnetic field and conventional magnetic long-range order. This strongly suggests the existence of a chiral spin liquid, a spin-liquid phase breaking the time-reversal symmetry. Both our measurements indicate that spin-ice correlations in the liquid phase lead to a non-coplanar spin texture forming a uniform but hidden order parameter: the spin chirality.

We report the results of the low-temperature specific heat measurements on the β-pyrochlore oxide superconductor RbOs 2 O 6 ( T c =6.3 K) in magnetic field under high pressure. We investigated the upper critical field H c2 at several pressures and found that the slope of H c2 at T c monotonically increases with increasing pressure in low-pressure superconducting phase SC1, indicating that the effective mass m * is enhanced by the pressure. Moreover, the jump of the specific heat at T c is also enhanced by the application of the pressure in SC1, suggesting that the pressure gives rise to the enhancement of the electron–phonon coupling and consequently enhances T c .

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.

We report the low-temperature thermoelectric power S ( T ) on high-quality single crystals of β-YbAlB 4 down to 40 mK. β-YbAlB 4 is the first Yb-based heavy fermion superconductor with the transition temperature of T c ∼ 80 mK, and stoichiometrically quantum critical at ambient pressure and under zero magnetic field. In the vicinity of a quantum critical point, S ( T )/ T exhibits a dramatic enhancement and takes large negative values at low temperatures. By contrast, when the system goes into the Fermi liquid state by applying the magnetic field, S ( T )/ T becomes constant. From the comparative study using the different quality samples, we confirm that the thermoelectric power is a good probe to elucidate the low-energy electronic excitations of the strongly correlated metals. The apparently different behaviors of the dimensionless ratio q of the thermoelectric power S and the specific heat C between β-YbAlB 4 and YbRh 2 Si 2 toward the QCP suggest that the systems belong to distinct classes of the quan...

Thermal conductivity tensor has been measured using single crystalline Co-doped BaFe2As2 down to 0.1K and under magnetic fields up to 7T. We observe peak anomalies both in the thermal conductivity and the thermal Hall conductivity in the superconducting state as an indication of enhancement of the quasiparticle mean-free path. Furthermore, we find a residual T-linear term in the thermal conductivity possibly due to a finite quasiparticle density of states in the superconducting gap induced by impurity pair-breaking.

The first Yb-based heavy fermion superconductor β-YbAlB4 with Tc = 80 mK is stoichiometrically quantum critical under ambient pressure and under zero magnetic field. We report the low-temperature thermoelectric power S(T) on a high-quality single crystal of β-YbAlB4 down to 40 mK. In zero field, S(T)/T exhibits a dramatic enhancement and takes large negative values at low temperatures, followed by a fast drop below 80 mK as an indication of the superconducting transition. Under the field of B = 25 mT (≥ Bc2), −S(T)/T is found to continuously increase down to the lowest temperature, which is highly in contrast to what is expected for the Fermi liquids (S/T ~ const.).

We report the results of the low-temperature specific heat measurements of the single crystalline β-pyrochlore oxide superconductors AOs2O6 (A=K, Rb, and Cs) under high pressure up to 13GPa. We find that superconducting transition temperature (Tc) monotonically increases for CsOs2O6 and RbOs2O6, while the one for KOs2O6 decreases by applying the pressure. With further increasing the pressure, Tc is suddenly suppressed at the same lattice volume for all compounds, concomitant with the first-order structural phase transition.

Thermal transport measurements have been made on the Fe-based superconductor Lu2Fe3Si5 (T(c) ∼ 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.

We report the results of the specific heat measurements for beta-pyrochlore oxide superconductor RbOs2O6 with superconducting transition temperature T-c similar to 6.3 K in magnetic field and under pressure. We find that T-c rises up to 9.3 K by applying the pressure concomitant with an increase of an initial slope of the upper critical field H-c2 and a jump of the specific heat at T-c. From the analysis of our data, the effective mass of conduction electrons and the electron-phonon coupling appear to increase under the pressure.

Geometrically frustrated magnets are of interest because of the novel phenomena that arise from their exotic ground states and low-lying excitations. Muon spin rotation and relaxation (mu SR) is a sensitive probe of magnetism on the local (atomic) distance scale, and is an attractive tool for the study of frustrated magnets. The muon carries a unit electric charge, however, which can have an appreciable effect on local properties. We discuss two cases where such an effect might be involved. In the 2D triangular antiferromagnet NiGa2S4 the 'magnetic' specific heat is field-independent up to 7 T, suggesting nonmagnetic excitations, but mu SR experiments reveal Ni spin freezing below similar to 9 K and strong magnetic fluctuations down to 25 mK. Comparison with Ga nuclear quadrupole resonance data suggests, however, that the muon charge does not cause this discrepancy. In the pyrochlore iridate Pr2Ir2O7 muon spin relaxation due to a distribution of quasistatic fields is observed over a wide temperature range. The data strongly suggest hyperfine-enhanced Pr-141 nuclear magnetism, which requires a nonmagnetic Pr3+ ground state. This may be due to lifting of the Pr3+ non-Kramers degeneracy by the muon electric field or, at least in part, a property of a spin-liquid-like many-body ground state.