Observation of anomalous amplitude modes in the kagome metal CsV$_3$Sb$_5$

TL;DR

This study reports the discovery of Raman-active amplitude modes in CsV$_3$Sb$_5$, revealing strong electron-phonon coupling and unconventional charge-density wave behavior without soft phonons, challenging traditional CDW theories.

Contribution

It provides the first observation of amplitude modes in CsV$_3$Sb$_5$, demonstrating unconventional CDW characteristics and strong electron-phonon interactions without soft phonon involvement.

Findings

Observation of Raman-active amplitude modes in CsV$_3$Sb$_5$

Strong hybridization and temperature-dependent shifts of modes

Indication of strong electron-phonon coupling in the CDW

Abstract

The charge-density wave (CDW) phase is often accompanied by the condensation of a soft acoustic phonon mode, giving rise to lattice distortion and charge density modulation. This picture was challenged for the recently discovered kagome metal CsV$_3$Sb$_5$, based on the evidence of absence of soft phonons. Here we report the observation of Raman-active CDW amplitude modes in this material, which are collective excitations typically thought to emerge out of frozen soft phonons. The amplitude modes strongly hybridize with other superlattice modes, imparting them with clear temperature-dependent frequency shift and broadening, rarely seen in other known CDW materials. Both the mode mixing and the large amplitude mode frequencies suggest that the CDW exhibits the character of strong electron-phonon coupling, a regime in which acoustic phonon softening can cease to exist. The observation of amplitude modes in the absence of soft phonons highlights the unconventional nature of the CDW in CsV$_3$Sb$_5$.