Possible valence skipping superconductivity in doped Dirac metal*

We studied the superconductivity in NaCl-type structured AgSnSe2 (Tc~3-6 K). The material is perceived as Ag-doped to imaginary NaCl-type SnSe, a Dirac metal. By varying the ratio of Ag/Sn, i.e., changing the ratio of Sn valence states, 2+ and 4+, we found that the Sn 2+ state is crucial for the realization of superconductivity along with the lattice constant. This finding is supported by the XPS, showing the clear coexistence of Sn 2+ and Sn 4+ in the superconducting Ag-Sn-Se compounds, but not in non-superconducting Sn-Se. The NMR experiments on the same superconducting compounds, however, indicated the Sn state is almost 3+, as is expected from the neutrality. The disagreement on these experiments may come from the electronic states in the compound’s unique superconductivity.