Thermal conductivity measurements were performed on single crystal samples of the superconducting filled-skutterudite compounds PrOs4Sb12 and PrRu4Sb12 both as a function of temperature and transverse magnetic field. In a zero magnetic field, the low temperature electronic thermal conductivity of PrRu4Sb12 is consistent with a fully gapped Fermi surface. For PrOs4Sb12, residual electronic conduction in the zerotemperature limit is consistent with the presence of nodes in the superconducting energy gap. The electronic thermal conductivity for both compounds shows a rapid rise at low magnetic fields. In PrRu4Sb12, this is interpreted in terms of multiband effects. In PrOs4Sb12, we consider the Doppler shift of nodal quasiparticles and multiband effects.

The specific heat C(T) of new iron-based high-Tc superconductor SmO1−xFxFeAs (0≤x≤0.2) was systematically studied. For undoped x=0 sample, a specific heat jump was observed at 130 K. This is attributed to the structural or spin-density-wave (SDW) transition, which also manifests on resistivity as a rapid drop. However, this jump disappears with slight F doping in x = 0.05 sample, although the resistivity drop still exists. The specific heat C/T shows clear anomaly near Tc for x = 0.15 and 0.20 superconducting samples. Such anomaly has been absent in LaO1−xFxFeAs. For the parent compound SmOFeAs, C(T) shows a sharp peak at 4.6 K, and with electron doping in x = 0.15 sample, this peak shifts to 3.7 K. It is interpreted that such a sharp peak results from the antiferromagnetic ordering of Sm3+ ions in this system, which mimics the electron-doped high-Tc cuprate Sm2−xCexCuO4−δ.

The thermal conductivity к of the heavy-fermion superconductor CeIrIn5 was measured as a function of temperature down to Tc/8, for current directions perpendicular (J‖a) and parallel (J‖c) to the tetragonal c axis. For J‖a, a sizable residual linear term к0/T is observed, as previously, which confirms the presence of line nodes in the superconducting gap. For J‖c, on the other hand,к/T → 0 as T → 0. The resulting precipitous decline in the anisotropy ratio кc/кa at low temperature rules out a gap structure with line nodes running along the c-axis, such as the d-wave state favoured for CeCoIn5, and instead points to a hybrid gap of Eg symmetry. It therefore appears that two distinct superconducting states are realized in the CeMIn5 family.

The thermal conductivity к of underdoped YBa2Cu3Oy was measured in the T→0 limit as a function of hole concentration p across the uperconducting critical point at psc=5.0 %. The evolution of bosonic and fermionic contributions to к was tracked as the doping level evolved continuously in each of our samples. For p≤psc, we observe a T3 component in к which we attribute to the boson excitations of a phase with long-range spin or charge order. Fermionic transport, observed as a T-linear term in к which persists unaltered through psc, violates the Wiedemann-Franz law,since the electrical resistivity varies as log(1/T) and grows with decreasing p.

Low-temperature heat transport was used to investigate the ground state of high-purity single crystals of the lightly doped cuprate a2Cu3O6:33. Samples were measured with doping concentrations on either side of the superconducting phase boundary. We report the observation of delocalized fermionic excitations at zero energy in the nonsuperconducting state, which shows that the ground state of underdoped cuprates is a thermal metal. Its low-energy spectrum appears to be similar to that of the d-wave superconductor, i.e., nodal. The insulating ground state observed in underdoped La2-xSrxCuO4 is attributed to the competing spin-density-wave order.

The in-plane resistivity ρ and thermal conductivity к of a single crystal of NaxCoO2 (x = 0.7) were measured down to 40 mK. Verification of the Wiedemann-Franz law, к/T = L0/ρ as T→0, and observation of a T2 dependence of ρ at low temperature, △ρ= AT2, establish the existence of a well-defined Fermi-liquid state. The measured value of coefficient A reveals enormous electronelectron scattering, characterized by the largest Kadowaki-Woods ratio A/γ 2 encountered in any material. The rapid suppression of A with magnetic field suggests a possible proximity to a magnetic quantum critical point. We also speculate on the possible role of magnetic frustration and proximity to a Mott insulator.