Anisotropic, multiband, and strong-coupling superconductivity of the Pb0.64Bi0.36 alloy

TL;DR

This study combines experimental and theoretical approaches to explore the anisotropic, multiband, and strong-coupling superconductivity of Pb0.64Bi0.36 alloy, revealing complex gap structures and the effects of disorder on its superconducting properties.

Contribution

It provides a comprehensive analysis of the superconducting state in Pb-Bi alloy, highlighting its multiband nature, strong electron-phonon coupling, and the impact of chemical disorder, which are novel insights.

Findings

Pb0.64Bi0.36 exhibits one of the strongest coupling among ambient pressure superconductors.

The alloy shows deviations from single-gap BCS behavior, indicating complex gap structures.

Chemical disorder reduces the critical temperature through strong electron scattering.

Abstract

This paper presents theoretical and experimental studies on the superconductivity of Pb${_{0.64}}$Bi$_{0.36}$ alloy, which is a prototype of strongly coupled superconductors and exhibits one of the strongest coupling under ambient pressure among the materials studied so far. The critical temperature, the specific heat in the superconducting state, and the magnetic critical fields are experimentally determined. Deviations from the single-gap s-wave BCS-like behavior are observed. The electronic structure, phonons and electron-phonon interactions are analyzed in relation to the metallic Pb, explaining why the Pb-Bi alloy exhibits such a large value of the electron-phonon coupling parameter $\lambda \simeq 2$. Superconductivity is studied using the isotropic Eliashberg formalism as well as the anisotropic density functional theory for superconductors. We find that while Pb is a two-gap superconductor with well-defined separate superconducting gaps, in the Pb-Bi alloy an overlapped three-gap-like structure is formed with a strong anisotropy. Furthermore, the chemical disorder, inherent to this alloy, leads to strong electron scattering, which is found to reduce the critical temperature.