Observation of Unconventional Charge Density Wave without Acoustic Phonon Anomaly in Kagome Superconductors AV3Sb5 (

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 that 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 particle-hole condensation mechanism that couples CDW, superconductivity and topological band structure.

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

“…Our Raman data are quite different from those in Ref. [27], and the cause of the discrepancy is unclear. The observation of the E2g main lattice mode in the LL configuration is due to its strong intensity and polarization leakage.…”

“…In this work, we focus on the CsV 3 Sb 5 compound, which shows a CDW transition temperature T CDW = 94 K [12]. The transition is accompanied by appreciable partial gapping of the Fermi surface [22][23][24][25][26] and reportedly the lack of Kohn anomalies [27], although density functional theory (DFT) calculations found two phonon instabilities at the M and L points of the Brillouin zone [28][29][30]. Currently, there is still no consensus on the structural configuration of the CDW ground state.…”

Chiral excitonic order from twofold van Hove singularities in kagome metals

“…In the case of the vanadium metals AV 3 Sb 5 , we discuss a few relevant aspects of experiment. Superconductivity exists alongside density wave order which sets in at a higher temperature T * ≈ 90 K than superconductivity (T c ≈ 3.5 K) and appears to compete with superconductivity [24][25][26][27][28][29][30][31][32][33]. The Fermi surface is reconstructed by the presence of the density wave order, but DFT calculations and magnetotransport indicate that the roughly hexagonal Fermi contours hosting the twofold van Hove singularities remain in the reconstructed band structure, which mainly undergoes changes near the K points [49].…”

Optical study of RbV$_3$Sb$_5$: Multiple density-wave gaps and phonon anomalies

“…One of the key features of the AV 3 Sb 5 family is the formation of a density-wave state accompanied by a superstructure [9]. While there is a consensus on the 2 × 2 in-plane modulation [22,[41][42][43][44][45], both two-fold and fourfold modulations along the c axis have been proposed [23,44,46,47]. This superstructure formation is concomitant with the reduction in the density of states at the Fermi level.…”