Pressure-induced reemergence of superconductivity in topological insulator ${\text{Sr}}_{0.065}{\text{Bi}}_2{\text{Se}}_3$

The recently discovered ${\mathrm{Sr}}_x{\mathrm{Bi}}_2{\mathrm{Se}}_3$ superconductor provides an alternative and ideal material base for investigating possible topological superconductivity. Here, we report that in ${\mathrm{Sr}}_{0.065}{\mathrm{Bi}}_2{\mathrm{Se}}_3$, the ambient superconducting phase is gradually depressed upon the application of external pressure. At high pressure, a second superconducting phase emerges at above 6 GPa, with a maximum $T_c$ value of $\sim 8.3$ K. The joint investigations of the high-pressure synchrotron x-ray diffraction and electrical transport properties reveal that the reemergence of superconductivity in ${\mathrm{Sr}}_{0.065}{\mathrm{Bi}}_2{\mathrm{Se}}_3$ is closely related to the structural phase transition from an ambient rhombohedral phase to a high-pressure monoclinic phase around 6 GPa, and further to another high-pressure tetragonal phase above 25 GPa.

Figure 1

(a) Temperature-dependent $ab$-plane resistance for ${\text{Sr}}_{0.065}{\text{Bi}}_2{\text{Se}}_3$ at low-pressure region. (b) Temperature-dependent $ab$-plane resistance for ${\text{Sr}}_{0.065}{\text{Bi}}_2{\text{Se}}_3$ at high-pressure region. (c) An enlarged view of the reemergent superconductivity near the transition temperature (run 1). (d) The temperature dependence of resistance for the ${\text{Sr}}_{0.065}{\text{Bi}}_2{\text{Se}}_3$ at run 2, reaching a maximum pressure of 80 GPa.

Figure 2

(a) Temperature dependence of resistance under different magnetic fields up to 5.0 T. The applied pressure is fixed at 19.5 GPa. (b) Temperature dependence of the reduced upper critical field $h^{*}$ (the red circles) and the fittings according to a $p$-wave polar state and the $s$-wave clean limit, respectively. The $T_c$ at specific magnetic field is determined as a 90% drop of the normal state resistance.

Figure 3

(a) Synchrotron radiation x-ray diffraction patterns at various applied pressures. For clarity, the backgrounds have been subtracted by using the dioptas program. (b) The refined lattice parameters $a$ (squares), $b$ (diamonds), and $c$ (circles) as a function of pressure. Three structural phases, i.e., $R\text{−}3m,C2/m,I4/mmm$, with two overlap pressure regions are revealed upon compression.

Figure 4

(a) The compression data $(V$ versus $P)$ were fitted by the third-order Birch-Murnaghan equation of state (solid red line), which yields the bulk modulus 58(1), 85(1), 116(2) GPa for the $R\text{−}3m,C2/m$, and $I4/mmm$ phases, respectively. (b) The pristine superconducting phase, SC-I, occurs in the ambient rhombohedral $\left(R\text{−}3m\right)$ phase. The reemergent superconducting phase, SC-II, occurs in the monoclinic $(C2/m)$ phase and in the tetragonal $I4/mmm$ phase.