Enhancement of superconducting Tsub(c) by two-level systems in metallic glasses

A model is developed for the change in superconducting transition temperature of metallic glasses due to the presence of two-level systems (TLS). The effect of such low-energy excitations is described by an electron-TLS interaction, which results in the addition of a TLS-mediated term to the usual phonon-mediated BCS electron-electron interaction. The model gives good qualitative agreement between calculated and measured superconducting Tsub(c): in particular, it leads to a non-negligible enhancement by the TLS, and corroborates the hypothesis of a reduction of the TLS density of states upon thermal relaxation. (author)