Normal-state and superconducting anisotropy in the doped topological insulator Sr_0.1Bi₂Se₃

The topological insulator Bi₂Se₃ becomes superconducting when doped with Cu, Nb (Tc~3.5 K) or Sr (Tc~3.0K); these materials are candidate topological superconductors. We observe twofold in-plane anisotropy in H_c2 in Sr_0.1Bi₂Se₃ via magnetotransport and thermodynamic measurements despite the trigonal crystal structure. We show that the direction of current does not affect the axis of anisotropy and that the normal state electronic and magnetic properties are isotropic in the basal plane. Synchrotron x-ray diffraction shows no distortions from the ideal crystal structure. We conclude that the anisotropy arises from the superconducting gap structure itself, consistent with the recently proposed odd-parity nematic superconducting state characterized by a nodal gap of E_u symmetry.