Topological superconductivity revealed by scanning tunneling spectroscopy

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. Using scanning tunneling microscopy (STM), we directly examine the response of the superconductivity of Cu_xBi₂Se₃ to magnetic field. Under out-of-plane fields, we discover elongated magnetic vortices hosting a zero-bias conductance peak (ZBCP) consistent with the Majorana zero mode (MZM) 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 slightly rotate 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. [1]

The MZMs in Cu_xBi₂Se₃ are neither clean nor robust, most likely due to contamination from impurity states or other closely-packed Caroli-de Gennes-Matricon (CdGM) states, which hampers further manipulations of Majorana fermions. We show that a ZBCP well separated from the other discrete CdGM states exists ubiquitously in all cores of free vortices in the defect free regions of (Li_0.84Fe_0.16)OHFeSe, which has a superconducting transition temperature of 42 K. Moreover, a Dirac-cone-type surface state is observed by angle-resolved photoemission spectroscopy, and its topological nature is confirmed by band calculations. The observed ZBCP can be naturally attributed to a MZM arising from this chiral topological surface states of a bulk superconductor. (Li_0.84Fe_0.16)OHFeSe thus provides an ideal platform for studying MZMs and topological quantum computing. [2]

References
[1]. R. Rao et al. Phys. Rev. X 8, 041024 (2018).
[2]. Q. Liu et al. arXiv: 1807.01278