  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.

Superconductivity;Strongly Correlated Electrons