Superconductivity and abnormal pressure effect in Sr${}_{0.5}$La${}_{0.5}$FBiSe₂ superconductor

Through the solid state reaction method, we synthesized a new BiSe₂-based superconductor Sr${}_{0.5}$La${}_{0.5}$FBiSe₂ with superconducting transition temperature T ${}_c$ $\approx 3.8$ K. A strong diamagnetic signal below T _c in susceptibility $\chi \left(T\right)$ is observed indicating the bulk nature of superconductivity. Different to most BiS₂-based compounds where superconductivity develops from a semiconducting-like normal state, the present compound exhibits a metallic behavior down to T _c. Under weak magnetic field or pressure, however, a remarkable crossover from metallic to insulating behaviors takes place around T _min where the resistivity picks up a local minimum. With increasing pressure, T ${}_c$  decreases monotonously and T _min shifts to high temperatures, while the absolute value of the normal state resistivity at low temperatures first decreases and then increases with pressure up to 2.5 GPa. These results imply that the electronic structure of Sr${}_{0.5}$La${}_{0.5}$FBiSe₂ may be different to those in the other BiS₂-based systems. (letter)