Observation of anomalous amplitude modes in the kagome metal CsV3Sb5

Abstract

The kagome lattice provides a fertile platform to explore novel symmetry-breaking states. Charge-density wave (CDW) instabilities have been recently discovered in a new kagome metal family, commonly considered to arise from Fermi-surface instabilities. Here we report the observation of Raman-active CDW amplitude modes in CsV3Sb5, which are collective excitations typically thought to emerge out of frozen soft phonons, although phonon softening is elusive experimentally. 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 phonon softening can cease to exist. Our work highlights the importance of the lattice degree of freedom in the CDW formation and points to the complex nature of the mechanism.

Anomalous enhancement of charge density wave in kagome superconductor CsV3Sb5 approaching the 2D limit

“…3d, e, which can be understood as a reduced EPC derived from the CDW partially gaping out the Fermi surfaces. This phononic renormalization across the TCDW is consistent with earlier results on the bulk sample and provides clear evidence of a significant electronphonon interaction on the thick flake side 50,51 .…”

“…3c. The solid black line is the fitting curve using an anharmonic phonon decay model 60 and the dramatic change in slope across the transition of the A1g mode clearly deviates from it, indicating a strong EPC 50,51 . Concomitantly, the corresponding line width and amplitude sharply decrease below TCDW, as shown in Fig.…”

“…However, recent angleresolved photoemission spectroscopy (ARPES) measurements revealed that the CDW band splitting are consistent with the electronic band structures with an imposed distortion from softening phonon modes [47][48][49] . The variation of phonon modes across the CDW transition from neutron and Raman scattering indicates their important role in promoting CDW ordering [50][51][52] . Therefore, combining all the available evidences, electronic interaction and electron-phonon coupling (EPC) may cooperate to generate the CDW.…”

Charge density wave and superconductivity in the kagome metal CsV$_3$Sb$_5$ around a pressure-induced quantum critical point

“…During this QPT, we find that there are several interesting features in the electronic structure and phonon spectrum of the CDW phase, such as the degeneracy breaking of electronic states around E F due to the formation of quasimolecular states [24] originating from V trimers and hexamers, the resulting reduction of DOS at E F , and the phonon softening associated with the periodic lattice distortion of V trimers and hexamers that are accompanied by the vertical displacements of Sb 1 and Sb 2 atoms, as will be discussed later. It is thus likely that lattice effects such as Jahn-Teller-like distortion [24] and strong EPC [19][20][21][22][23] are intimately related to the emergence of the CDW order.…”

“…This CDW order has been revealed to be unconventional with a chiral anisotropy breaking time-reversal symmetry [1], which leads to a large anomalous Hall effect in the absence of long-range magnetic ordering [5,6]. However, the origin of CDW is currently being debated whether it is ascribed to the Fermi surface nestingdriven Peierls-like electronic instability [7][8][9][10][11][12][13][14][15][16][17], many-body correlations and excitonic effects with particle-hole condensation [18], momentum-dependent electron-phonon coupling (EPC) [19][20][21][22][23], or Jahn-Teller-like distortion with the formation of quasimolecular states [24]. Upon cooling down to temperatures below ∼3 K, SC [25][26][27] emerges, indicating the coexistence of CDW and SC.…”