Revealing rotational symmetry breaking charge density wave order in the kagome superconductor (Rb,K)⁢V3⁢Sb5 by ultrafast pump-probe experiments

The recently discovered Kagome superconductor 𝐴⁢V3⁢Sb5 (where 𝐴 refers to K, Rb, Cs) has stimulated widespread research interest due to its interplay of nontrivial topology and unconventional correlated physics including charge-density waves (CDW) and superconductivity. The essential prerequisite to understanding the microscopic mechanisms of this complex electronic landscape is to unveil the configuration and symmetry of the charge-density wave order. As to now, little consensus has been made on what symmetry is broken. Herein, we clarify the microscopic structure and symmetry breaking of the CDW phase in RbV3⁢Sb5 and KV3⁢Sb5 by ultrafast time-resolved reflectivity. Our approach is based on extracting coherent phonon spectra induced by three-dimensional CDW and comparing them to calculated phonon frequencies via density-functional theory. The combination of these experimental results and calculations provides compelling evidence that the CDW structure of both compounds prevailing up to 𝑇CDW is the 2×2×2 staggered inverse Star-of-David pattern with interlayer 𝜋 phase shift, in which the sixfold rotational symmetry is broken. These observations thus corroborate sixfold rotational symmetry breaking throughout the CDW phase of RbV3⁢Sb5 and KV3⁢Sb5.