High-pressure study of the basal-plane anisotropy of the upper critical field of the topological superconductor SrxBi2Se3

We report a high-pressure transport study of the upper-critical field, $B_{c2}(T)$, of the topological superconductor Sr${}_{0.15}$Bi${}_2$Se${}_3$ ($T_c = 3.0$K)\mathrm{.}$B_{c2}(T)$ was measured for magnetic fields directed along two orthogonal directions, $a$ and $a^{\ast }$, in the trigonal basal plane. While superconductivity is rapidly suppressed at the critical pressure $p_c\sim 3.5$ GPa, the pronounced two-fold basal-plane anisotropy $B_{c2}^a/B_{c2}^{a^*} = 3.2$at$T=0.3$ K, recently reported at ambient pressure (Pan et al., 2016), is reinforced and attains a value of $\sim 5$ at the highest pressure (2.2 GPa). The data reveal that the unconventional superconducting state with broken rotational symmetry is robust under pressure.