Direct Visualization of the Nematic Superconductivity in Cu_xBi₂Se₃

Cu_xBi₂Se₃ hosts both topological surface states and bulk superconductivity. It has been identified recently as a topological superconductor (TSC) with an extraordinary nematic, i. e. C₂-symmetric, superconducting state and odd-parity pairing. Here, using scanning tunneling microscopy (STM), we directly examine the response of the superconductivity of Cu_xBi₂Se₃ to magnetic field. Under out-of-plane fields (B_⊥), we discover elongated magnetic vortices hosting zero-bias conductance peaks consistent with the Majorana bound states expected in a TSC. Under in-plane fields (B_//), the average superconducting gap exhibits two-fold symmetry with field orientation; the long C₂ symmetry axes are pinned to the dihedral mirror planes under B_//=0.5 T but rotate slightly under B_//=1.0 T. Moreover, a nodeless Δ_4x gap structure is semi-quantitatively determined for the first time. Our data paint a microscopic picture of the nematic superconductivity in Cu_xBi₂Se₃ and pose strong constraints on theory.