Coherent phonon pairs and rotational symmetry breaking of charge density wave order in the kagome superconductor CsV₃Sb₅

In this work we perform ultrafast time-resolved reflectivity measurements to study the symmetry breaking in the charge-density wave (CDW) phase of CsV₃Sb₅. By extracting the coherent phonon spectrum in the CDW phase of CsV₃Sb₅, we discover close phonon pairs near 1.3 THz and 3.1 THz, as well as a new mode at 1.84 THz. The 1.3 THz phonon pair and the 1.84 THz mode are observed up to the CDW transition temperature. Combining density-functional theory calculations, we show that these phonon pairs arise from the coexistence of Star of David and inverse Star of David distortions combined with sixfold rotational symmetry breaking. An anisotropy in the magnitude of transient reflectivity change is also revealed at the onset of CDW order. Our results thus indicate broken sixfold rotational symmetry in the charge-density wave state of CsV₃Sb₅, along with the absence of nematic fluctuation above T_CDW. Meanwhile, the measured coherent phonon spectrum in the CDW phase of CsV₃Sb_5−xSn_x with x = 0.03–0.04 matches the staggered inverse Star of David with interlayer π phase shift. This CDW structure contrasts with undoped CsV₃Sb₅ and explains the evolution from a phonon pair to a single mode at 1.3 THz by x = 0.03–0.04 Sn doping.