Anisotropic thermal conductivity of YBa₂Cu₃O_7-δ

The thermal conductivity κ of high quality YBa₂Cu₃O_7-δ has been measured for deoxygenated, twinned and detwinned samples along the a and b axes. The normal state anisotropy κ_bκ_a is consistent with the anisotropy of electrical conduction if the Lorenz number takes on its maximum value (i.e. LL₀). Under this assumption, the phonon conduction is about the same for well oxygenated and deoxygenated samples. These results are discussed in connection with electronic versus phononic scenarios for the peak in κ(T) below T_c. At very low temperatures, a residual linear term in κ is indicative of uncondensed electrons, but the absence of anisotropy excludes the previous explanation in terms of non-superconducting chains.