Competing s-wave and d-wave order in A15 superconductors

TL;DR

This paper presents evidence that A15 superconductors exhibit competing s-wave and d-wave orders, explaining various experimental phenomena and revealing the potential for multiple gaps under strain.

Contribution

It provides microscopic arguments for the coexistence of s-wave and d-wave orders in A15 superconductors, unifying diverse experimental observations.

Findings

Identification of d-wave order competing with s-wave in A15 superconductors

Explanation of Raman mode, vortex core shrinking, and critical field anisotropy

Prediction of multiple gaps under strain in A15 superconductors

Abstract

Competing pairing channels have been proposed in superfluid $^3$He, in the high temperature cuprate superconductors, and pnictide superconductors. While strong experimental support exists for this in superfluid $^3$He, the case for superconductors is weaker. Here, we show that $A15$ superconductors provide a compelling example for competing superconducting order. We present microscopic arguments that $A15$ superconductors have a competing d-wave order in addition to usual s-wave superconductivity and show that this d-wave order provides a unifying explanation for a series of diverse experimental results. These include: the emergence of a sharp mode in the superconducting state seen by Raman spectroscopy; the observed shrinking of the vortex core at anomalously low fields; and the large anisotropy measured in the upper critical field. We further show that multiple superconducting gaps can appear when these $A15$ superconductors are strained.