Observation of Unconventional Charge Density Wave without Acoustic Phonon Anomaly in Kagome Superconductors AV3Sb5 (A=Rb,Cs)

TL;DR

This study reveals an unconventional charge density wave in kagome superconductors AV3Sb5 that does not involve typical phonon anomalies, suggesting an electronic-driven mechanism linked to the material's topological properties.

Contribution

It demonstrates a 3D CDW in AV3Sb5 without acoustic phonon anomalies, indicating a novel electronic mechanism distinct from conventional electron-phonon coupling.

Findings

Identified a 3D 2*2*2 superstructure CDW in AV3Sb5.

Observed absence of acoustic phonon anomalies at the CDW wavevector.

Detected a new Raman mode associated with the CDW.

Abstract

The combination of non-trivial band topology and symmetry breaking phases gives rise to novel quantum states and phenomena such as topological superconductivity, quantum anomalous Hall effect and axion electrodynamics. Evidence of intertwined charge density wave (CDW) and superconducting order parameters has recently been observed in a novel kagome material AV3Sb5 (A=K,Rb,Cs) that features a Z2 topological invariant in the electronic structure. However, the origin of the CDW and its intricate interplay with topological state has yet to be determined. Here, using hard x-ray scattering, we demonstrate a three-dimensional (3D) CDW with 2*2*2 superstructure in (Rb,Cs)V3Sb5. Unexpectedly, we find that the CDW fails to induce acoustic phonon anomalies at the CDW wavevector but yields a novel Raman mode which quickly damps into a broad continuum below the CDW transition temperature. Our observations exclude strong electron-phonon coupling driven CDW in AV3Sb5 and point to an unconventional and electronic-driven mechanism that couples the CDW and the topological band structure.