Bulk superconductivity in a four-layer-type Bi-based compound La2O2Bi3Ag0.6Sn0.4S5.7Se0.3

Recently, we reported the observation of superconductivity at similar to 0.5 K in a La2O2M4S6-type (M: metal) layered compound La2O2Bi3AgS6, which is a layered system related to the BiS2-based superconductor system but possesses a thicker Bi3AgS6-type conducting layer. In this study, we have developed the La2O2Bi3AgS6-type materials by element substitutions to increase the transition temperature (T-c) and to induce bulk nature of superconductivity. A resistivity anomaly observed at 180 K in La2O2Bi3AgS6 was systematically suppressed by Sn substitution for the Ag site. By the Sn substitution, T-c increased, and the shielding volume fraction estimated from magnetization measurements also increased. The highest T-c (=2.3 K) and the highest shielding volume fraction (similar to 20%) was observed for La2O2Bi3Ag0.6Sn0.4S6. The superconducting properties were further improved by Se substitutions for the S site. By the combinational substitutions of Sn and Se, bulk-superconducting phase of La2O2Bi3Ag0.6Sn0.4S5.7Se0.3 with a T-c of 3.0 K (T-c(onset) = 3.6 K) was obtained.