THERMAL-CONDUCTIVITY OF OXYGEN-DEFICIENT YBA2CU3O6+X

We have performed a systematic study of the temperature dependence of the in-plane thermal conductivity K(T) in oxygen-deficient single crys tals of YBa2Cu3O6+x (0<x<1). An anomalous dependence of K(T) observed in insulating YBa2Cu3O6 below 200 K is evidence for the existence of u nknown phonon relaxation process. This phonon relaxation seems to pers ist in superconducting samples. A strong temperature dependence of K(T ) in antiferromagnetic YBa2Cu3O6 at T>240 K suggests a strong spin-pho non coupling. For underdoped superconducting crystals, we have found t hat features in metallic normal state K(T) correlate with so-called sp in-gap behavior of the spin and charge excitation spectrum. We interpr et these features of K(T) in terms of a spin-phonon interaction.