Superconducting ground state study of the valence-skipped compound AgSnSe2

Valence-skipped superconductors are natural candidates for unconventional superconductivity because they can exhibit a negative effective, attractive interaction for electron pairing. This work reports comprehensive x-ray diffraction, magnetization, specific heat, and muon spin rotation and relaxation measurements ($\ensuremath{\mu}\mathrm{SR}$) on the valence-skipped compound ${\mathrm{AgSnSe}}_{2}$. The temperature dependences of the electronic specific heat $[{C}_{\mathrm{el}}(T)]$ and the upper critical field $[{H}_{c2}(T)]$ provide evidence of two-gap superconductivity, which is also confirmed by our transverse-field $\ensuremath{\mu}\mathrm{SR}$ measurements. Zero-field $\ensuremath{\mu}\mathrm{SR}$ measurements reveal at most a slight increase $[\ensuremath{\sim}0.002(1)\phantom{\rule{3.33333pt}{0ex}}\ensuremath{\mu}{\text{s}}^{\ensuremath{-}1}]$ in relaxation in the superconducting state, much less than reported in a range of superconductors with broken time-reversal symmetry. Further measurements with greatly improved statistics will be required to make a definitive determination of the possible presence of broken time-reversal symmetry.