Superconducting and normal-state anisotropy of the doped topological insulator Sr_0.1Bi₂Se₃

Sr_xBi₂Se₃ and the related compounds Cu_xBi₂Se₃ and Nb_xBi₂Se₃ have attracted considerable interest, as these materials may be realizations of unconventional topological superconductors. Superconductivity with T_c ~3 K in Sr_xBi₂Se₃ arises upon intercalation of Sr into the layered topological insulator Bi₂Se₃. Here we elucidate the anisotropy of the normal and superconducting state of Sr_0.1Bi₂Se₃ with angular dependent magnetotransport and thermodynamic measurements. High resolution x-ray diffraction studies underline the high crystalline quality of the samples. We demonstrate that the normal state electronic and magnetic properties of Sr_0.1Bi₂Se₃ are isotropic in the basal plane while we observe a large two-fold in-plane anisotropy of the upper critical field in the superconducting state. Our results support the recently proposed odd-parity nematic state characterized by a nodal gap of Eu symmetry in Sr_xBi₂Se₃.