Hybrid s-wave superconductivity in CrB₂

In a metal with multiple Fermi pockets, the formation of s-wave superconductivity can both be conventional due to electron-phonon coupling or unconventional due to spin fluctuations. We analyze the hexagonal diboride CrB₂ which is found to be an itinerant antiferromagnet at ambient conditions and turn superconducting upon increasing pressure. While the high pressure behavior of Tc suggests conventional s-wave pairing, we find that spin fluctuations mediating unconventional s-wave pairing form a significant contribution in the vicinity of the antiferromagnetic dome. As the symmetry class of the s-wave state is independent of its underlying mechanism, we argue that CrB₂ is a realization of a hybrid s-wave superconductor where unconventional and conventional s-wave mechanisms team up to form a joint superconducting dome.