Nematicity of the superconducting gap in Cu_xBi₂Se₃ studied by scanning tunneling microscopy

The search for unconventional superconductor is always a hotspot in condensed matter physics. Cu_xBi₂Se₃ is a strong candidate given both the topological insulator nature and superconductivity. Nematic superconducting state has been observed in Cu_xBi₂Se₃ by NMR study, and subsequently confirmed by thermodynamic study, torque magnetometry and transport measurements. Here by using ultralow temperature and vector magnetic field equipped STM, we directly measured the in-plane magnetic field angular dependence of the superconducting gap in Cu_xBi₂Se₃. Fully gapped superconducting gap is observed, which is insensitive to defects and step edges, excluding the existence of nodes. Under the in-plane magnetic fields of 0.5 T and 1.0 T, the angular dependence of superconducting gap size exhibits two-fold symmetry. The orientation of the two-fold symmetry of SC gap is the same in one domain, but differs in different SC domains. In addition, the magnitude of magnetic field may alter the orientation of the two-fold symmetry of SC gap. The direct observation of the anisotropy of the superconducting gap and its response to magnetic field may lay a foundation for further understanding the superconducting state of Cu_xBi₂Se₃.