Superconductivity of topological insulator Sb₂Te₃Seunder pressure

The crystal structures of Sb₂Te_3-Se(y= 0.6 andy= 1.2) at 0-24 GPa were investigated by synchrotron x-ray diffraction. The stoichiometry of Sb₂Te_3-Seused in this study was determined to be Sb₂Te_2.19(9)Se_0.7(2)fory= 0.6 and Sb₂Te_1.7(1)Se_1.3(3)fory= 1.2, on the basis of energy-dispersive x-ray spectroscopy. The sample of Sb₂Te_2.19(9)Se_0.7(2)showed a structural phase transition from a rhombohedral structure (space group No. 166,R3¯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 groupC2/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 Sb₂Te_1.7(1)Se_1.3(3)provided only phase I (space group No. 166,R3¯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 Sb₂Te_2.19(9)Se_0.7(2). In this sample, theT_cvalues of 6 and 9 K were observed in phase II and phase II' or III of Sb₂Te_2.19(9)Se_0.7(2), respectively. Superconductivity withT_c's of 4 and 5 K suddenly emerged in Sb₂Te_1.7(1)Se_1.3(3)at 13.6 GPa, which corresponds to phase II, and it evolved to 6.0 K under further increased pressure. AT_cvalue of 9 K was finally found above 15 GPa. The magnetic field dependence ofT_cin phase II of Sb₂Te_2.19(9)Se_0.7(2)and Sb₂Te_1.7(1)Se_1.3(3)followed ap-wave polar model, suggesting topologically nontrivial superconductivity.