Rotation symmetry breaking in the normal state of a kagome superconductor KV3Sb5

Recently discovered superconductors AV3Sb5 (A = K, Rb, Cs)1,2 provide a fresh opportunity to study correlation-driven electronic phenomena on a kagome lattice. The observation of an unusual charge density wave (CDW) in the normal state of all the members of the AV3Sb5 family2–10 has prompted a large effort to identify any ‘hidden’ broken symmetries associated with it. We use spectroscopic-imaging scanning tunnelling microscopy to reveal pronounced intensity anisotropy between the different directions of hexagonal CDW in KV3Sb5. In particular, we find that one of the CDW directions is distinctly different compared with the other two. This observation points to an intrinsic rotation-symmetry-broken electronic ground state where the symmetry is reduced from sixfold to twofold. Furthermore, in contrast to previous reports3, we find that the CDW phase is insensitive to the magnetic-field direction, regardless of the presence or absence of atomic defects. Our experiments, combined with earlier observations of stripe charge ordering in CsV3Sb5, establish correlation-driven rotation symmetry breaking as a unifying feature of AV3Sb5 kagome superconductors.