Superconductivity of topological insulator Sb2Te3-ySey under pressure

The crystal structures of Sb2Te3-y Se (y) (y = 0.6 and y = 1.2) at 0-24 GPa were investigated by synchrotron x-ray diffraction. The stoichiometry of Sb2Te3-y Se (y) used in this study was determined to be Sb2Te2.19(9)Se0.7(2) for y = 0.6 and Sb2Te1.7(1)Se1.3(3) for y = 1.2, on the basis of energy-dispersive x-ray spectroscopy. The sample of Sb2Te2.19(9)Se0.7(2) showed a structural phase transition from a rhombohedral structure (space group No. 166, R (3) over bar m) (phase I) to a monoclinic structure (space group No. 12, C2/m) (phase II), with increasing pressure up to similar to 9 GPa. A new structural phase (phase II ') emerged at 17.7 GPa, a monoclinic structure with the space group C2/c (No. 15). Finally, a 9/10-fold monoclinic structure (space group No. 12, C2/m) (phase III) was observed at 21.8 GPa. In contrast, the sample of Sb2Te1.7(1)Se1.3(3) provided only phase I (space group No. 166, R (3) over barm) and phase II (space group No. 12, C2/m), showing one structural phase transition from 0-19.5 GPa. These samples were not superconductors at ambient pressure, but superconductivity suddenly appeared with increasing pressure. Superconductivity with superconducting transition temperatures (T-c's) of 2 and 4 K was observed above 6 GPa in phase I of Sb2Te2.19(9)Se0.7(2). In this sample, the T-c values of 6 and 9 K were observed in phase II and phase II' or III of Sb2Te2.19(9)Se0.7(2), respectively. Superconductivity with T-c's of 4 and 5 K suddenly emerged in Sb2Te1.7(1)Se1.3(3) at 13.6 GPa, which corresponds to phase II, and it evolved to 6.0 K under further increased pressure. A T-c value of 9 K was finally found above 15 GPa. The magnetic field dependence of T-c in phase II of Sb2Te2.19(9)Se0.7(2) and Sb2Te1.7(1)Se1.3(3) followed a p-wave polar model, suggesting topologically nontrivial superconductivity.