Absence of Phonon Softening across a Charge Density Wave Transition due to Quantum Fluctuations

TL;DR

This study reveals that quantum zero-point lattice fluctuations prevent phonon softening in CsV3Sb5 during its charge density wave transition, challenging conventional understanding and highlighting quantum effects in heavy-element materials.

Contribution

It demonstrates that quantum fluctuations can stabilize the lattice structure, explaining the absence of phonon softening and the weak first-order nature of the CDW transition in CsV3Sb5.

Findings

Absence of phonon softening in CsV3Sb5 during CDW transition.

Quantum zero-point motion causes smearing of the CDW landscape.

Coexistence of pristine and CDW structures across the transition.

Abstract

Kagome metals have emerged as a frontier in condensed matter physics due to their potential to host exotic quantum states. Among these, CsV3Sb5 has attracted significant attention for the unusual coexistence of charge density wave (CDW) order and superconductivity, presenting an ideal system for exploring novel electronic and phononic phenomena. The nature of CDW formation in CsV3Sb5 has sparked considerable debate. Previous studies have suggested that the underlying mechanism driving the CDW transition in CsV3Sb5 is distinct from conventional ones, such as electron-phonon coupling and Fermi surface nesting. In this study, we examine the origin of the CDW state via ab initio finite-temperature simulations of the lattice dynamics associated with CDW structures in CsV3Sb5. Through a comparative study of CsV3Sb5 and 2H-NbSe2, we demonstrate that the experimental absence of phonon softening in CsV3Sb5 and the presence of a weakly first order transition can be attributed to quantum zero-point motion of the lattice, which leads to smearing of the CDW landscape and effectively stabilizes the pristine structure even below the CDW transition temperature. We argue that this surprising behavior could cause coexistence of pristine and CDW structures across the transition and lead to a weak first-order transition. We further discuss experimental implications and use the simulation to interpret coherent phonon spectroscopy results in single crystalline CsV3Sb5. These findings not only refine our fundamental understanding of CDW transitions, but also highlight the surprising role of quantum effects in influencing macroscopic properties of relatively heavy-element materials like CsV3Sb5. Our results provide crucial insights into the formation mechanism of CDW materials that exhibit little to no phonon softening, including cuprates, aiding in the understanding of the CDW phase in quantum materials.