ANOMALOUS PHONON DAMPING IN INSULATING CUPRATES

Measurements of the in-plane (kappa(ab)) and out-of-plane (kappa(c)) thermal conductivity for insulating cuprate crystals are discussed along with new measurements for YBa2Cu3O6 and PrBa2Cu3O6, where both kappa(ab) and kappa(c) are twice the magnitude previously reported for this material. An unusual temperature (T) dependence of kappa(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 kappa(ab) are discussed.