Electron and phonon interactions with two-level-tunneling systems in the high-Tc superconductor YBa2Cu3O7- delta and in niobium.

Abstract

We have measured ultrasonic velocity changes with temperature (T\ensuremath{\le}1 K) to study two-level-tunneling systems (TLS) interactions with electrons and with phonons in the high-${\mathrm{T}}_{\mathrm{c}}$ superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$ and in a niobium compact. In ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{7\mathrm{\ensuremath{-}}\mathrm{\ensuremath{\delta}}}$, at T\ensuremath{\ll}${\mathrm{T}}_{\mathrm{c}}$, TLS relaxation is dominated by conduction electrons, as in normal metallic glasses. In niobium, at T\ensuremath{\ll}${\mathrm{T}}_{\mathrm{c}}$, this relaxation is dominated by phonons, as in other disordered, conventional superconductors. We also review recent specific-heat measurements in light of these results.