Influence of the Phonon Spectra of In-Tl Alloys on the Superconducting Transition Temperatures

Abstract

A tunneling investigation of the effective phonon density of states through the four distinct crystal structures of the In-Tl alloy series has been performed. The resultant ${\alpha }^2\left(\omega \right)F\left(\omega \right)$'s extracted from these studies reflect the characteristics of each individual structure. The data obtained from these experiments serve as a test of existing theoretical expressions for determining $T_c$ of a superconductor from normal-state parameters. It is found that the agreement is somewhat disappointing. In addition, a study of the phonon distribution, or more explicitly, an average phonon energy $〈\omega 〉$ in the region of the face-centered cubic to face-centered tetragonal phase transition, suggests an instability in at least one of the phonon modes. This result appears to support the suggestion that this transition is second-order.