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

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.

Observation of Unconventional Charge Density Wave without Acoustic Phonon Anomaly in Kagome Superconductors

The study by H. X. Li et al. investigates a notably unconventional manifestation of charge density waves (CDWs) in the kagome superconductors AV₃Sb₅ (A=Rb,Cs). The research presents an insightful examination of CDWs and their underlying mechanisms in a topological context, neglecting conventional acoustic phonon anomaly contributions often associated with such systems.

Synopsis

The research focuses on discerning the nature of CDW formation in kagome metals, which exhibit intricate electronic structures marked by non-trivial Z₂ topological invariants. Using hard x-ray scattering, the study identifies a three-dimensional (3D) 2x2x2 CDW superstructure in RbV₃Sb₅ and CsV₃Sb₅, which intriguingly lacks phonon anomalies typically observed in such materials. The findings suggest an unconventional particle-hole condensation mechanism, diverging from the traditional electron-phonon coupling (EPC) models.

Key Findings

The investigation employs a suite of advanced techniques, including inelastic x-ray scattering (IXS), Raman spectroscopy, and angle-resolved photoemission spectroscopy (ARPES), to explore the CDW state. Noteworthy findings include:

1. Lack of Acoustic Phonon Anomalies: Contrary to expectations in conventional CDW systems, the AV₃Sb₅ compounds did not exhibit CDW-induced anomalies in longitudinal and transverse acoustic phonon modes.

2. New Raman Mode: A novel Raman mode emerges at the CDW transition temperature, quickly damping into a broad continuum, indicating potential electron-driven dynamics rather than phonon-driven ones.

3. 3D CDW Superstructure: The study confirms a long-range ordered 3D CDW spanning a large correlation length (>1000 Å), suggesting a distinct mechanism from the two-dimensionality previously reported by scanning tunneling microscopy (STM) studies.

4. Electronic Band Influences: ARPES data reveal band inversions near the Fermi level in the vicinity of the CDW transition, indicating an intricate interplay between electronic band structure and CDW formation, likely featuring a topological component influencing superconductivity.

Implications

The discovery of a CDW that evades inducing phonon anomalies and the observation of a novel Raman mode imply a potentially electronic origin for the CDW in these materials. This phenomenon could signal a new class of particle-hole condensates that evade the established phonon-mediated frameworks, possibly expanding the understanding of quantum materials and topological superconductivity.

The interplay of CDWs, superconductivity, and topological band structures in kagome materials could bear significantly on theories of unconventional superconductivity, potentially driving further research into novel quantum states and mechanisms.

Future Prospects

This research opens pathways for future investigations into the role of electronic-driven phase transitions in unconventional superconductors and their relationship with topological properties. Further explorations should focus on:

• Developing theoretical models to understand the electron-driven CDW mechanisms and their implications on superconductivity comprehensively.
• Investigating the potential applications and implications of such topological materials in quantum computing and advanced electronic devices.
• Extending the study to other materials with similar structural and electronic properties to extrapolate the findings to broader classes of quantum materials.

In conclusion, the work by H. X. Li et al. provides an essential contribution to ongoing efforts to decode the intricate phenomena in topological superconductors, challenging the pre-existing paradigms surrounding CDW formation and highlighting the rich potential for discovering novel electronic phases in condensed matter physics.