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 (μSR) on the valence-skipped compound AgSnSe2. The temperature dependences of the electronic specific heat [Cel(T)] and the upper critical field [Hc2(T)] provide evidence of two-gap superconductivity, which is also confirmed by our transverse-field μSR measurements. Zero-field μSR measurements reveal at most a slight increase [∼0.002(1)μs-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.