Pressure-induced superconductivity in Bi2−xSbxTe3−ySey

We systematically investigated the pressure dependence of electrical transport and the crystal structure of topological insulator, $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}\mathrm{T}{\mathrm{e}}_{3\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{y}$, which showed no superconductivity down to 2.0 K at ambient pressure. The $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}\mathrm{T}{\mathrm{e}}_{3\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{y}$ crystal showed two structural phase transitions under pressure, from rhombohedral structure (space group No. 166, $R\overline{3}m$, termed phase I) to monoclinic structure (space group No. 12, $C2/m$, termed phase II), and from phase II to another monoclinic structure (space group No. 12, $C2/m$, termed phase III). Superconductivity appeared when applying pressure; actually the superconductivity of all $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}\mathrm{T}{\mathrm{e}}_{3\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{y}$ samples emerged in phase I. The superconducting transition temperature, ${T}_{\mathrm{c}}$, increased against pressure in a pressure range of 0--15 GPa for all $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}\mathrm{T}{\mathrm{e}}_{3\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{y}$ samples, and the maximum ${T}_{\mathrm{c}}$ was 5.45 K, recorded at 13.5 GPa in $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}\mathrm{T}{\mathrm{e}}_{3\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{y}$ at $x=0$ and $y=1.0$. The magnetic field (H) dependence of the R--T plot for $\mathrm{B}{\mathrm{i}}_{2\ensuremath{-}x}\mathrm{S}{\mathrm{b}}_{x}\mathrm{T}{\mathrm{e}}_{3\ensuremath{-}y}\mathrm{S}{\mathrm{e}}_{y}$ was measured to characterize the superconducting pairing mechanism of pressure-induced superconducting phase.