The effect of disorder on the superconducting state of the candidate topological superconductor Nb$_x$Bi$_2$Se$_3$

The effects of disorder and electron scattering in topological superconductors are largely unknown. Aiming at eliminating this deficiency we study the influence of proton irradiation on the candidate topological superconductor Nb$_x$Bi$_2$Se$_3$. As the irradiation-deposited defect density increases we observe a rapid suppression of $T_c$ and a strong increase in residual resistivity in both magnetization and transport measurements. These results are inconsistent with a fully gaped superconductor; they rather suggest a nodal order parameter, in agreement with penetration depth measurements [1]. We interpret our data in terms of a nematic superconducting state with $E_u$ symmetry. [1] M. P. Smylie {\it et al.}, arXiv:1608.08164.