Rotational Symmetry Breaking in a Trigonal superconductor Nb-doped Bi$_{2}$Se$_{3}$

Topological superconductors (TSC) have been attracting huge interest due to their potential applications to topological quantum computation. While it has been challenging to confirm TSC, recently it has been predicted that superconducting doped Bi$_2$Se$_3$ shows a nematic order in the TSC state. In this study we probed the rotational symmetry of a TSC candidate Nb-doped Bi$_2$Se$_3$ in both normal and superconducting states by torque magnetometry. The magnetic field was applied in-plane and the symmetry of magnetic anisotropic susceptibility as well as hysteresis loop was measured. While $\sin 6\phi$ dependence was observed in the normal state, $\sin 2\phi$ and $\sin 4\phi$ components become dominant instead of vanishing $\sin 6\phi$ component in the superconducting state. This indicates rotational symmetry breaking in the superconducting state, suggesting nematic order as predicted.