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 half-filled one-dimensional Holstein-Hubbard model presents, at zero temperature, a charge-density-wave (CDW) phase and a Mott insulator phase. Recent results have shown that the transition from one phase to the other might proceed through an intermediate metallic phase. In this work, we determine the CDW phase boundary using the variational cluster approximation. Using exact diagonalization and cluster perturbation theory, we study both the pair susceptibility and the spectral gaps in the non-CDW part of the phase diagram. We cannot rule out the existence of an intermediate metallic phase.
Phys. Rev. B 77, 134501 ͑2008͔͒, Sun and Ando estimate that the phonon mean free path at low temperature is roughly half the width of the single crystal used in our study, from which they argue that phonon scattering cannot be dominated by sample boundaries. Here we show that their use of specific-heat data on Nd 2 CuO 4 , which contains a large magnetic contribution at low temperature that is difficult to reliably extract, leads to an underestimate of the mean free path by a factor 2 compared to an estimate based on the specific-heat data of the nonmagnetic isostructural analog Pr 2 CuO 4 . This removes the apparent contradiction raised by Sun and Ando.
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