Upper critical field of the noncentrosymmetric superconductor BiPd

TL;DR

This paper clarifies the superconducting properties of BiPd, establishing its bulk upper critical field and confirming it as a nearly-weak-coupled BCS s-wave superconductor, resolving previous discrepancies.

Contribution

It provides definitive measurements of the bulk upper critical field and clarifies the superconducting nature of BiPd, addressing prior conflicting reports.

Findings

The bulk upper critical field corresponds to the lower of previous reports.

BiPd behaves as a nearly-weak-coupled BCS s-wave superconductor.

Discrepancies in previous upper critical field measurements are discussed.

Abstract

The superconducting parameters and upper critical field of the noncentrosymmetric superconductor BiPd have proven contentious. This material is of particular interest because it is a singular example of a $4f$-electron-free noncentrosymmetric superconductor of which crystals may be grown and cleaved, enabling surface-sensitive spectroscopies. Here, using bulk probes augmented by tunnelling data on defects, we establish that the lower of the previously reported upper critical fields corresponds to the bulk transition. The material behaves as a nearly-weak-coupled BCS s-wave superconductor, and we report its superconducting parameters as drawn from the bulk upper critical field. Possible reasons behind the order-of-magnitude discrepancy in the reported upper critical fields are discussed.