Anomalous phonon damping in insulating cuprates

Measurements of the in-plane (κ_ab) and out-of-plane (κ_c) thermal conductivity for insulating cuprate crystals are discussed along with new measurements for YBa₂Cu₃O₆, and PrBa₂Cu₃O₆, where both κ_ab and κ_c are twice the magnitude previously reported for this material. An unusual temperature (T) dependence of κ_ab in cuprates with apical oxygen indicates the onset of strong phonon damping for T < 200-250K. Along with dielectric and elastic anomalies reported in this regime, the data suggest the occurrence of a structural phase transition, involving rotations of the CuO polyhedra about an in-plane axis. The role of such local distortions in the thermal transport of superconducting compounds and the superconducting-state enhancement of κ_ab are discussed.