ANOMALOUS PHONON DAMPING AND THERMAL-CONDUCTIVITY IN INSULATING CUPRATES

We examine the in-plane thermal conductivity (K-ab) for different classes of insulating cuprate crystals, and present measurements for YBa2Cu3O6+x and PTBa2CU3O6+x (x less than or equal to 0.11). An unusual, nonmonotonic temperature (T) dependence of K-ab, previously observed in La2CuO4+delta, is demonstrated to be a characteristic of the 123 compounds as well. This feature signals the onset of anomalous damping for in-plane, heat-carrying phonons at T less than or equal to 250 K that is highly sensitive to light oxygen doping in YBa2Cu3O6+x. Its absence in Nd-2 CuO4+delta and similarity to the behavior of SrTiO3 suggests that the phenomenon in the cuprates is connected with tilt distortions of the CuO polyhedra. We discuss the implications of these results for interpretations of the superconducting-state enhancement of K-ab, widely observed in superconducting compounds.