Competing s-wave and d-wave order in A15 superconductors
Abstract
Competing pairing channels have been proposed in superfluid 3He, in the high temperature cuprate superconductors, and pnictide superconductors. While strong experimental support exists for this in superfluid 3He, 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.