Anisotropic phonon thermal conductivity of YBa 2Cu 3O −7δ

Abstract

Experimental data on the anisotropic thermal conductivity of single crystals of several high- T c superconductors have recently been reported, but the existing theory of this quantity is based on an isotropic Debye model of the phonon spectrum. We have used a shell model which gives accurate frequencies for all phonons in the Brillouin zone to calculate the anisotropy of the defect-limited phonon thermal conductivity of YBa 2Cu 3O 7−δ. We find that this anisotropy occurs because phonons propagating along the c-axis have lower velocities on the average, leading to a c-axis phonon thermal conductivity several times smaller than that in the ab-plane, in agreement with experiment.