In a metal with multiple Fermi pockets, the formation of $s$-wave superconductivity can be conventional due to electron-phonon coupling or unconventional due to spin fluctuations. We analyze the hexagonal diboride ${\text{CrB}}_2$, which is an itinerant antiferromagnet at ambient conditions and turns superconducting upon increasing pressure. While the high-pressure behavior of $T_c$ suggests conventional $s$-wave pairing, we find that spin fluctuations promoting unconventional $s$-wave pairing become important 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 ${\text{CrB}}_2$ is a realization of a hybrid $s$-wave superconductor where unconventional and conventional $s$-wave mechanisms team up to form a joint superconducting dome.

• Received 8 November 2022
• Revised 12 June 2023
• Accepted 13 June 2023

DOI:https://doi.org/10.1103/PhysRevB.108.L020501