Improvement of superconducting properties by chemical pressure effect in Eu-doped La2-xEuxO2Bi3Ag0.6Sn0.4S6

We have investigated the substitution effect of Eu on the superconductivity in La2-xEuxO2Bi3Ag0.6Sn0.4S6. Recently, we reported an observation of superconductivity at 0.5 K in a layered oxychalcogenide La2O2Bi3AgS6. The Sn doping at the Ag site was found to raise the superconducting transition temperature, T-c, to 2.5 K in La2O2Bi3Ag0.6Sn0.4S6. To further improve the superconducting properties, we have partially substituted Eu for the La site to increase the chemical pressure in La2-xEuxO2Bi3Ag0.6Sn0.4S6 (x = 0.1-0.6). La2-xEuxO2Bi3Ag0.6Sn0.4S6 is crystallized in a tetragonal phase with the space group P4/nmm. With the increase in Eu concentration, x, the lattice constant a decreased, while the lattice constant c was marginally shortened, which suggests that the chemical pressure induced by the Eu doping is uniaxial along the a-axis. T-c increased with increasing x up to x = 0.4, and the suppression of superconductivity was observed for higher Eu concentrations of x = 0.5 and 0.6. From the magnetic susceptibility and resistivity measurements, a superconducting transition has been confirmed for x = 0.1-0.6 with T-c = 2.5-4.0 K, respectively. The estimated upper critical field (B-c2) was 3.5 T for x = 0.4, which also has the highest T. We observed the jump in the temperature dependence of heat capacity near T-c = 4.0 K for La1.6Eu0.4O2Bi3Ag0.6Sn0.4S6, which confirms the emergence of bulk superconductivity in La1.6Eu0.4O2Bi3Ag0.6Sn0.4S6.