Esquinazi, P., C. Durán, C. Fainstein, and M. NÃºÃ±ez Regueiro. "Evidence of low-energy tunneling excitations in the high-Tc superconductor YBa2Cu3O7-x." Physical Review B 37, no. 1 (1988): 545–547.
Abstract: We have measured the internal friction Q-1 and the Young's-modulus sound velocity vE at 1 kHz for YBa2Cu3O7-x. Below 20 K the relative 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.