Superconductivity in Bi_2-xSb_xTe_3-ySe_y(x= 1.0 andy= 2.0) under pressure

The crystal structure of BiSbTeSe₂(Bi_2-xSb_xTe_3-ySe_y(x= 1.0 andy= 2.0)) at 0-29 GPa is investigated through synchrotron x-ray diffraction (XRD) and two structural phase transitions are discovered. The stoichiometry of BiSbTeSe₂employed in this study is Bi_1.19(4)Sb_0.81(4)Te_0.83(4)Se_2.17(4), as determined from energy-dispersive x-ray spectroscopy. The sample demonstrated structural transitions, from a rhombohedral structure (space group no 166,R3¯m) (phase I) to a monoclinic structure (space group no 12,C2/m) (phase II), and from phase II to a 9/10-fold monoclinic structure (space group no 12,C2/m) (phase III). The temperature dependence of resistance (R-Tplot) exhibited a semiconducting behavior in a low pressure range and changed from semiconducting to metallic behavior with increasing pressure. Pressure-driven superconductivity is observed above 9.1 GPa in Bi_1.19(4)Sb_0.81(4)Te_0.83(4)Se_2.17(4). The pressure phase corresponds to phase II. The superconducting transition temperature,T_c, increased with pressure. The maximumT_cvalue is 8.3 K at 19.1 GPa. The magnetic field dependence ofT_cin phase II of Bi_1.19(4)Sb_0.81(4)Te_0.83(4)Se_2.17(4)is proceeded by ap-wave polar model, indicating topologically nontrivial superconductivity. In addition, the emergence of superconductivity and the change in superconducting behavior are closely associated with the structural transitions.