Revealing Rotational Symmetry Breaking Charge-density Wave Order in Kagome Superconductor AV₃Sb₅ by Ultrafast Pump-probe Experiment

The recently discovered Kagome metals AV₃Sb₅ (A = K, Rb, Cs) has stimulated widespread research interest due to its interplay of non-trivial topology and unconventional correlated physics including charge-density waves (CDW) and superconductivity. The essential prerequisite to understand the microscopic mechanism of this complex electronic landscape is to unveil the configuration and symmetry of the charge-density wave order, while little consensus has been made whether the rotational symmetry is broken. Herein, we clarify the microscopic structure and symmetry breaking of the CDW phase in AV₃Sb₅ by ultrafast time-resolved reflectivity. Our approach is based on extracting coherent phonon spectra induced by three-dimensional CDW and comparing to calculated phonon frequencies via density-functional theory, which shows marked difference of the CDW structure on composition and doping. The CDW structure of CsV₃Sb₅ shows the coexistence of Star-of-David and inverse Star-of-David distortions combined with six-fold rotational symmetry breaking, which can be tuned by Sn-doping. The CDW structure of Rb- and KV₃Sb₅ prevailing up to T_CDW is the 2 × 2 × 2 staggered inverse Star-of-David pattern with interlayer π phase shift. These observations thus corroborate six-fold rotational symmetry breaking throughout the CDW phase of AV₃Sb₅.