Evidence of low-energy tunneling excitations in the high-Tc superconductor YBa2Cu3O7-x.

Abstract

We have measured the internal friction Q/sup -1/ and the Young's-modulus sound velocity v/sub E/ at 1 kHz for YBa/sub 2/Cu/sub 3/O/sub 7-//sub x/. Below 20 K the relatiave change in the sound velocity and the temperature dependence of the internal friction perfectly resemble the behavior obtained for Suprasil W. Below 1 K we observe a logarithmic temperature dependence for the sound velocity and between 1 and 5 K a well-defined plateau in the internal friction is obtained. We find quantitative agreement with the predictions of the standard tunneling model which is widely used to explain the low-temperature properties of amorphous materials. The implications of the existence of tunneling systems in these materials are discussed.