High-pressure study of the basal-plane anisotropy of the upper critical field of the topological superconductor Sr_xBi₂Se₃

We report a high-pressure transport study of the upper-critical field Bc₂(T) of the topological superconductor Sr_0.15Bi₂Se₃ (T_c=3.0 K). Bc₂(T) was measured for magnetic fields directed along two orthogonal directions, a and a∗, in the trigonal basal plane. While superconductivity is rapidly suppressed at the critical pressure p_c∼3.5 GPa, the pronounced two-fold basal-plane anisotropy B^a_c2/B^a∗_c2=3.2 at T=0.3 K, recently reported at ambient pressure [Pan et al., Sci. Rep. 6, 28632 (2016)], is reinforced and attains a value of ∼5 at the highest pressure (2.2 GPa). The data reveal that the unconventional superconducting state with broken rotational symmetry is robust under pressure.