Surface-sensitive fragile twofold symmetry in the kagome-lattice superconductor CsV3Sb5

The newly discovered kagome superconductor CsV3Sb5 exhibits numerous exotic phenomena and offers a rare platform to study the interplay of topological state, charge-density wave order, and superconductivity. Here we systematically investigated the twofold symmetry in the superconducting state through the measurements of off-plane resistivity under in-plane rotating magnetic field. It is found that the distinct twofold symmetry exists in the superconducting phase. Furthermore, this symmetry breaking is very fragile and sensitive to the surface degradation. When the sample is exposed to the atmosphere for several days, the twofold symmetry tends to be weakened and eventually disappears with increasing exposing time. However, it can be recovered after sample surface exfoliation. Combined with other measurements, we propose that this fragile twofold symmetry should be related to the surface condition. The surface degradation may be caused by Cs atom vacancies on the surface, which could destroy the topological surface state. We discussed the possible relationship between the fragile twofold symmetry and the topological surface state discovered by recent angle-resolved photoemission spectroscopy experiments.