Superconductivity of topological insulator Sb 2 Te 3 - y Se y under pressure.

The crystal structures of Sb ₂ Te _{3- y} Se _y ( y = 0.6 and y = 1.2) at 0-24 GPa were investigated by synchrotron x-ray diffraction. The stoichiometry of Sb ₂ Te _{3- y} Se _y used in this study was determined to be Sb ₂ Te _2.19(9) Se _0.7(2) for y = 0.6 and Sb ₂ Te _1.7(1) Se _1.3(3) for y = 1.2, on the basis of energy-dispersive x-ray spectroscopy. The sample of Sb ₂ Te _2.19(9) Se _0.7(2) showed a structural phase transition from a rhombohedral structure (space group No. 166, R 3¯ m ) (phase I) to a monoclinic structure (space group No. 12, C 2/ 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 C 2/ c (No. 15). Finally, a 9/10-fold monoclinic structure (space group No. 12, C 2/ m ) (phase III) was observed at 21.8 GPa. In contrast, the sample of Sb ₂ Te _1.7(1) Se _1.3(3) provided only phase I (space group No. 166, R 3¯ m ) and phase II (space group No. 12, C 2/ 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 Sb ₂ Te _2.19(9) Se _0.7(2) . In this sample, the T _c values of 6 and 9 K were observed in phase II and phase II' or III of Sb ₂ Te _2.19(9) Se _0.7(2) , respectively. Superconductivity with T _c 's of 4 and 5 K suddenly emerged in Sb ₂ Te _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. A T _c value of 9 K was finally found above 15 GPa. The magnetic field dependence of T _c in phase II of Sb ₂ Te _2.19(9) Se _0.7(2) and Sb ₂ Te _1.7(1) Se _1.3(3) followed a p -wave polar model, suggesting topologically nontrivial superconductivity.
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