Superconductivity of topological insulator Sb2Te3− y Se y under pressure

Abstract

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 m) (phase I) to a monoclinic structure (space group No. 12, C2/m) (phase II), with increasing pressure up 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 m) 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.