Electron-doping-induced insulator-to-superconductor transition in a BiS$_{2}$-based superconductor Sr$_{1-x}$La$_{x}$FBiS$_{2}$*

Recently, materials with BiS$_{2}$ layers have attracted much attention as a new family of layered superconductors. Superconductivity was first reported in Bi$_{4}$S$_{4}$O$_{3}$, followed by $R$O$_{1-x}$F$_{x}$BiS$_{2}$, Sr$_{0.5}$La$_{0.5}$FBiS$_{2}$, and Bi$_{3}$O$_{2}$S$_{3}$. So far, however, comprehensive studies about the dependence on carrier concentration have been still lacking. In this study, we have systematically synthesized polycrystalline Sr$_{1-x}$La$_{x}$FBiS$_{2}$ ($0\!\le\! x\!\le\! 0.6$) to reveal the electronic phase diagram associated with the superconductivity in the BiS$_{2}$ layer. Since the density of states of the Sr, La and F orbitals is negligibly small near the Fermi level, this series of compounds would allow the rigid-band carrier doping and provide an ideal arena to study the detailed concentration dependence. The obtained phase diagram is characterized by an insulator-superconductor transition with a steep phase boundary at $x\!\sim\!0.4$. This is markedly different from that for $R$O$_{1-x}$F$_{x}$BiS$_{2}$, indicating a strong impact of the blocking layer on the superconductivity. Unusual increase in $T_{\rm c}$ has been also revealed as the carrier concentration decreases toward the critical point [1]. [1] H. Sakai {\it et al.} JPSJ (accepted)