Low-temperature phonon thermal conductivity of single-crystalline Nd2CuO4:: Effects of sample size and surface roughness

The effect of sample size and surface roughness on the phonon thermal conductivity kappa(p) of Nd(2)CuO(4) single crystals was studied down to 50 mK. At 0.5 K, kappa(p) is proportional to root A, where A is the cross-sectional area of the sample. This demonstrates that kappa(p) is dominated by boundary scattering below 0.5 K or so. However, the expected T(3) dependence of kappa(p) 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 mirrorlike sample surfaces. We propose an empirical power law fit to kappa(p)similar to T(alpha) (where alpha < 3) in cuprate single crystals. Using this method, we show that recent thermal conductivity studies of Zn doping in YBa(2)Cu(3)O(y) reaffirm the universal heat conductivity of d-wave quasiparticles at T -> 0.