Superconductivity, electronic phase diagram, and pressure effect in Sr 1− x Pr x FBiS 2

Based on a combination of X-ray diffraction, electrical transports, magnetic susceptibility, specific heat, and pressure-effect measurements, we report the results of experiments on a series of BiS2-based Sr1−xPrxFBiS2 superconductors with the maximum Tc of 2.7 K for x=0.5 and at ambient pressure. Superconductivity appears only for 0.4≤x≤0.7 whereas the normal-state resistivity shows the semiconducting-like behaviors. The magnetic susceptibility χ(T) displays the low superconducting shielding volume fractions and C(T) shows no distinguishable anomaly near Tc, which suggests a filamentary superconductivity in the Pr-doped polycrystalline samples. By varying doping concentrations, an electronic phase diagram is established. Upon applying pressure on the optimally doped Sr0.5Pr0.5FBiS2 system, Tc is abruptly enhanced, reaches 8.5 K at the critical pressure of Pc=1.5 GPa, and increases slightly to 9.7 K at 2.5 GPa. Accompanied by the enhancement of superconductivity from the low- to the high-Tc phases, the normal state undergoes a semiconductor-to-metal transition when under pressure. This scenario may be linked to enhanced overlap of the Bi-6p and S-p orbitals, which contributes to the enhanced superconductivity above Pc. The pressuretemperature phase diagram for Sr0.5Pr0.5FBiS2 is also presented.