Pressure weakens coupling strength in In and Sn elemental superconductors

TL;DR

This study investigates how applying pressure affects the coupling strength in elemental In and Sn superconductors, revealing a weakening of coupling and increased gap anisotropy with pressure, using muon-spin rotation/relaxation data.

Contribution

It provides new insights into pressure-induced changes in superconducting coupling strength and gap anisotropy in elemental superconductors, analyzed through muon-spin techniques and Eliashberg theory.

Findings

Pressure decreases the coupling strength parameter α in In and Sn.

Pressure increases the anisotropy of the superconducting energy gap.

Only part of the pressure effect is due to phonon spectrum hardening.

Abstract

Pressure dependence of the thermodynamic critical field $B_{\rm c}$ in elemental indium (In) and tin (Sn) superconductors was studied by means of the muon-spin rotation/relaxation. Pressure enhances the deviation of $B_{\rm c}(T)$ from the parabolic behavior, expected for a typical type-I superconductor, suggesting a weakening of the coupling strengths $\alpha=\langle\Delta\rangle /k_{\rm B}T_{\rm c}$ ($\langle\Delta\rangle$ is the average value of the superconducting energy gap, $T_{\rm c}$ is the transition temperature and $k_{\rm B}$ is the Boltzmann constant). As pressure increases from 0.0 to $\simeq 3.0$ GPa $\alpha$ decreases linearly, by approaching the limiting weak-coupling BCS value $\alpha_{\rm BCS}=1.764$. Analysis of the data within the framework of the Eliashberg theory reveals that only part of the pressure effect on $\alpha$ can be attributed to the effect of hardening of the phonon spectra, which is reflected by a decrease of the electron-phonon coupling constant. Nearly 40% of the effect is caused by increased anisotropy of the superconducting energy gap.