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Recently it was demonstrated that Sr intercalation provides a new route toinduce superconductivity in the topological insulator Bi$_2$Se$_3$. Topologicalsuperconductors are predicted to be unconventional, with mixed even and oddparity Cooper pairs states. An adequate probe to test for unconventionalsuperconductivity is the upper critical field, $B_{c2}$. For a standard BCSlayered superconductor $B_{c2}$ shows an anisotropy when the magnetic field isapplied parallel and perpendicular to the layers, but is isotropic when thefield is rotated in the plane of the layers. Here we report measurements of theupper critical field of superconducting Sr$_x$Bi$_2$Se$_3$ crystals ($T_c =3.0$~K). Surprisingly, field-angle dependent magnetotransport measurementsreveal a large anisotropy of $B_{c2}$ when the magnet field is rotated in thebasal plane. The large two-fold anisotropy, while six-fold is anticipated,cannot be explained with the Ginzburg-Landau anisotropic effective mass modelor flux flow induced by the Lorentz force. The rotational symmetry breaking of$B_{c2}$ indicates unconventional superconductivity with an odd-paritypolarized triplet Cooper pair state ($\Delta_4$-pairing) recently proposed forrhombohedral topological superconductors, or might have a structural nature,such as self-organized stripe ordering of Sr atoms.

Rotational symmetry breaking in the topological superconductor Sr$_x$Bi$_2$Se$_3$ probed by upper-critical field experiments