LOW-TEMPERATURE ACOUSTIC PROPERTIES IN YBA2CU3O7-DELTA

Temperature dependencies of acoustic losses Q(-1) and of relative sound velocity change Delta v/v in YBa2Cu3O7-delta up to similar to 60 It are calculated by the tunneling model theory. The tunneling systems are related to the off-centered positions of the apical oxygen atoms O(A) and are described through the pseudo-Jahn-Teller effect. Tunneling systems' parameters are distributed in narrow range of values and are in correspondence with the experimentally observed infrared phonon spectra and thermal ellipsoids of O(A). Respective relaxation times are calculated by the adapted reaction rate method. The calculated Q(-1)(T) and Delta upsilon(T)/upsilon dependencies are in good agreement with the experimental data, which is an additional support to the conclusion about the existence of tunneling systems in YBa2Cu3O7-delta due to the pseudo Jahn-Teller effect.