Superconductivity of topological insulator SbTeSeunder pressure.

The crystal structures of SbTeSe(= 0.6 and= 1.2) at 0-24 GPa were investigated by synchrotron x-ray diffraction. The stoichiometry of SbTeSeused in this study was determined to be SbTeSefor= 0.6 and SbTeSefor= 1.2, on the basis of energy-dispersive x-ray spectroscopy. The sample of SbTeSeshowed a structural phase transition from a rhombohedral structure (space group No. 166,3¯) (phase I) to a monoclinic structure (space group No. 12,2/) (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 group2/(No. 15). Finally, a 9/10-fold monoclinic structure (space group No. 12,2/) (phase III) was observed at 21.8 GPa. In contrast, the sample of SbTeSeprovided only phase I (space group No. 166,3¯) and phase II (space group No. 12,2/), 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 ('s) of 2 and 4 K was observed above 6 GPa in phase I of SbTeSe. In this sample, thevalues of 6 and 9 K were observed in phase II and phase II' or III of SbTeSe, respectively. Superconductivity with's of 4 and 5 K suddenly emerged in SbTeSeat 13.6 GPa, which corresponds to phase II, and it evolved to 6.0 K under further increased pressure. Avalue of 9 K was finally found above 15 GPa. The magnetic field dependence ofin phase II of SbTeSeand SbTeSefollowed a-wave polar model, suggesting topologically nontrivial superconductivity.