UC Santa Barbara

Kagome metals have emerged as a frontier in condensed matter physics due to their potential to host exotic quantum states. Among these materials, CsV3Sb5 has attracted significant attention for its unusual coexistence of charge density wave (CDW) order and superconductivity, presenting an ideal system for exploring novel electronic and phononic phenomena. However, the nature of CDW formation in CsV3Sb5 has been a subject of considerable debate.

Unlike well-studied CDW materials, CsV3Sb5 exhibits unconventional CDW behavior. Previous studies have suggested that the mechanism driving the CDW transition in CsV3Sb5 is distinct from conventional ones such as strong electron-phonon coupling (EPC) and Fermi surface nesting. Notably, the CDW transition in CsV3Sb5 is a first-order phase transition rather than the typical second-order transition observed in most CDW materials. It is commonly proposed that van Hove singularities (VHS) close to the Fermi level are central to CDW formation, providing a high density of states that could promote electronic instability. Additionally, antimony (Sb) bands have been suggested to play an essential role in enhancing CDW phase stability. Consequently, CsV3Sb5 is often thought to represent a novel type of CDW material where mechanisms beyond simplenesting and EPC govern the transition.

In this thesis, we investigate the complex mechanisms underlying the CDW transition in CsV3Sb5 by combining ab initio simulations, finite-temperature lattice dynamics, and detailed electron-phonon coupling analysis. Our methodologies reveal that the CDW formation in CsV3Sb5 is indeed driven by electron-phonon coupling—a conventional mechanism—with quantum fluctuations playing a significant role in explaining its unconventional behaviors. Furthermore, we find that the VHS points and Sb bands are NOT the dominant k points contributing to the electron-phonon coupling 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. This zero-point motion smears the CDW energy 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, leading to a weak first-order phase transition. We further discuss experimental implicationsand use simulations 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 significant role of quantum effects in influencing the 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, thereby aiding in the understanding of the CDW phase in quantum materials.

Building upon the possibility that strong EPC induces the CDWin CsV3Sb5 by quantum fluctuations, we further employ a self-developed constrained occupation approach to identify specific electron states contributing to CDW stability. By demonstrating how uniform hole doping eliminates the CDWphase through stabilizing the pristine structure, we selectively tune the occupations of specific k points. Systematic testing of the van Hove singularities and Sb bands provides evidence that they do NOT play a major role in the CDW formation of CsV3Sb5. This approach, combined with finite-temperatureEPC calculations, shows how significant k points with large EPC strengths effectively impact phonon instabilities. These findings underscore the flexibility and applicability of the method for exploring CDW mechanisms in CsV3Sb5, potentially assisting further theoretical explanations in fundamental physics and experimental developments related to superconductivity.