Coherent Phonon Pairs and Rotational Symmetry Breaking of Charge Density Wave Order in the Kagome Superconductor CsV${}_3$Sb${}_5$

Abstract

In this work, we perform ultrafast time-resolved reflectivity measurements to study the symmetry breaking in the charge-density wave (CDW) phase of CsV${}_3$Sb${}_5$. By extracting the coherent phonon spectrum in the CDW phase of CsV${}_3$Sb${}_5$, 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 point out these phonon pairs arise from the coexistence of Star-of-David and inverse Star-of-David distortions combined with six-fold 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 six-fold rotational symmetry in the charge-density wave state of CsV${}_3$Sb${}_5$, along with the absence of nematic fluctuation above T${}_{\text{CDW}}$. Meanwhile, the measured coherent phonon spectrum in the CDW phase of CsV${}_3$Sb${}_{5-\text{x}}$Sn${}_{\text{x}}$ with x = 0.03-0.04 matches with staggered inverse Star-of-David with interlayer $\pi$ phase shift. This CDW structure contrasts with undoped CsV${}_3$Sb${}_5$ and explains the evolution from phonon pair to a single mode at 1.3 THz by x = 0.03-0.04 Sn-doping.

Published in Physical Review B

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Cited by in corpus (2)

• Symmetry-broken ground state and phonon mediated superconductivity in Kagome CsV${}_3$Sb${}_5$
• Persistence of charge density wave fluctuations in the absence of long-range order in a hole-doped kagome metal