Pressure weakens coupling strength in In and Sn elemental superconductors

The pressure dependence of the thermodynamic critical field Bc in elemental indium (In) and tin (Sn) superconductors was studied by means of muon-spin rotation/relaxation. Pressure enhances the deviation of Bc(T) from the parabolic behavior expected for a typical type-I superconductor, suggesting a weakening of the coupling strength α = kbTc ( is the average value of the superconducting energy gap, Tc is the transition temperature, and kB is the Boltzmann constant). As pressure increases from 0.0 to = 3.0 GPa, alfa decreases linearly, approaching the limiting weak-coupling Bardeen-Cooper-Schrieffer (BCS) value alfaBCS =1.764. Analysis of the data within the framework of Eliashberg theory reveals that only part of the pressure effect on alfa can be attributed to the hardening of the phonon spectra, reflected by a decrease in the electron-phonon coupling constant. Nearly 40% of the effect is caused by an increased anisotropy of the superconducting energy gap.