Pressure-induced superconductivity in $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$

We systematically investigated the pressure dependence of electrical transport and the crystal structure of topological insulator, $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$, which showed no superconductivity down to 2.0 K at ambient pressure. The $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ 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 $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ 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 $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ samples, and the maximum $T_{\mathrm{c}}$ was 5.45 K, recorded at 13.5 GPa in $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ at $x=0$ and $y=1.0$. The magnetic field (H) dependence of the R–T plot for $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ was measured to characterize the superconducting pairing mechanism of pressure-induced superconducting phase.

Figure 1

(a) Optical image and (b) EDX spectrum of a crystal-like lump of $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ ($x=0.25$ and $y=1.0$). (c) R–T plot and (d) M/H–T plot of $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ (nominal $x=0$ and $y=1.0$) recorded at 2.73 and 0 GPa, respectively.

Figure 2

Pressure-dependent powder XRD patterns of $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ with nominal $x$ values of (a) 0, (b) 0.25, (c) 0.5, and (d) 1.0; nominal $y$ value is 1.0 for all samples. Measurement temperature and pressure are 297 K and 0–30 GPa, respectively.

Figure 3

Crystal structures of phase I, phase II, phase II’, and phase III in $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$.

Figure 4

(a) Schematic representation of evolution of crystal structure against pressure, and (b) $x$ dependence of pressure causing structural transitions (phase I to phase II, and phase II to phase III) in $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$.

Figure 5

Pressure dependence of V of $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ (V/Z) with nominal $x$ values of (a) 0, (b) 0.25, (c) 0.5, and (d) 1.0; nominal $y$ value is 1.0 for all samples. Chemical composition for each sample is listed in Table 1. Z is the number of asymmetry unit.

Figure 6

(a) R/R(10 K)–T plots at different pressures for $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ with nominal $x$ value of 0 and nominal $y$ of 1.0. Chemical composition is “$\mathrm{B}{\mathrm{i}}_{2.1\left(1\right)}\mathrm{T}{\mathrm{e}}_{1.8\left(2\right)}\mathrm{S}{\mathrm{e}}_{1.2\left(2\right)}$, ” which is determined from EDX. (b) $T_{\mathrm{c}}-p$ plots for $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ with nominal $x$ value of 0, 0.25, 0.5m and 1.0; nominal $y$ is 1.0. Chemical compositions for the above samples determined from EDX are listed in Table 1. Red and blue circles for $x=0$ refer to the first and second measurements, which were recorded in increasing pressure. Red circle and green triangle for $x=0.25$, $x=0.5$, and $x=1.0$ refer to plots recorded in increasing and decreasing pressure.

Figure 7

R/R(10 K)–T plots for $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ ($x=0.25$, $y=1.0$) under magnetic field H at (a) 7.69 GPa and (c) 10.3 GPa. $h^{*}-t$ plots for $\mathrm{B}{\mathrm{i}}_{2-x}\mathrm{S}{\mathrm{b}}_x\mathrm{T}{\mathrm{e}}_{3-y}\mathrm{S}{\mathrm{e}}_y$ ($x=0.25$, $y=1.0$) at (b) 7.69 GPa and (d) 10.3 GPa. Chemical composition of the sample is “$\mathrm{B}{\mathrm{i}}_{1.75\left(4\right)}\mathrm{S}{\mathrm{b}}_{0.25\left(4\right)}\mathrm{T}{\mathrm{e}}_{1.89\left(7\right)}\mathrm{S}{\mathrm{e}}_{1.11\left(7\right)}$, ” which is determined from EDX. A very small kink observed at around 2.0 K in (a) and (c) is due to the problem of equipment, which never affects the $T_{\mathrm{c}}$ determination necessary for drawing the graphs in (b) and (d).