The effect of sample size and surface roughness on the phonon thermal conductivity κp of Nd2CuO4 single crystals was studied down to 50 mK. At 0.5 K, κp is proportional to √ A, where A is the cross-sectional area of the sample. This demonstrates that κp is dominated by boundary scattering below 0.5 K or so. However, the expected T 3 dependence of κp is not observed down to 50 mK. Upon roughing the surfaces, the T 3 dependence is restored, showing that departures from T 3 are due to specular reflection of phonons off the mirror-like sample surfaces. We propose an empirical power law fit, to κp ∼ T α (where α < 3) in cuprate single crystals. Using this method, we show that recent thermal conductivity studies of Zn doping in YBa2Cu3Oy re-affirm the universal heat conductivity of d-wave quasiparticles at T → 0. To understand the pairing mechanism in a superconductor, it is essential to know the symmetry of the order parameter. In this context, measurements of lowtemperature thermal conductivity κ, which probes the low-energy quasiparticle excitations, has emerged as a powerful probe of the order parameter in superconductors. For conventional s-wave superconductors with a fully gapped excitation spectrum, the linear-temperature electronic contribution to thermal conductivity is zero at T → 0, i.e. the residual linear term κ 0 /T = 0. This can be seen in the single-gap s-wave superconductor Nb,
The in-plane resistivity rho and thermal conductivity kappa of single crystal Na0.7CoO2 were measured down to 40 mK. Verification of the Wiedemann-Franz law, kappa/T=L(0)/rho as T-->0, and observation of a T2 dependence of rho at low temperature establish the existence of a well-defined Fermi-liquid state. The measured value of coefficient A reveals enormous electron-electron scattering, characterized by the largest Kadowaki-Woods ratio A/gamma(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.
The magnetic and transport properties are systematically studied on the single crystal N a0.55CoO2 with charge ordering and divergency in resistivity below 50 K. A long-range ferromagnetic ordering is observed in susceptibility below 20 K with the magnetic field parallel to Co-O plane, while a negligible behavior is observed with the field perpendicular to the Co-O plane. It definitely gives a direct evidence for the existence of in-plane ferromagnetism below 20 K. The observed magnetoresistance (MR) of 30 % at the field of 6 T at low temperatures indicates an unexpectedly strong spin-charge coupling in triangle lattice systems.
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