Optical detection of charge-density-wave instability in the non-magnetic kagome metal KV$_3$Sb$_5$

Abstract

Optical spectroscopy and density-functional calculations reveal electronic properties of the nonmagnetic kagome metal KV3Sb5. Temperature and frequency-dependent optical measurements down to 10 K and up to 2 eV energy range confirm bulk nature of the charge-density-wave (CDW) state below 78 K and gauge the charge gap of ∆CDW ≈ 60 meV at 10 K. We further detect strong phonon anomalies and the prominent low-energy localization peak indicative of the unconventional charge transport caused by electron-phonon or electron-electron interactions. Possible CDW structures of KV3Sb5, the star and hexagon (inverse star), are strongly reminiscent of p-wave states expected in the Hubbard model on the kagome lattice at the filling level of the van Hove singularity. The proximity to this regime may have intriguing and far-reaching implications for the physics of KV3Sb5 and related materials.

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

“…In addition to the classical Lorentzian and Drude contributions, a Fano-like shaped peak around 400 cm −1 , as well as a strongly temperature-dependent absorption feature in the same energy range, are observed. In line with the previous optical studies of the kagome metals, this feature is assigned to a so-called localization peak [27,28,38] that signals hindered electron dynamics.…”

“…The effects that hinder electron dynamics seem to be purely thermal in nature and vanish in the T → 0 limit. A similar behavior is observed in CsV 3 Sb 5 [27], but not in KV 3 Sb 5 where the localization peak also shifts toward low energies upon cooling, yet it saturates at around 300 cm −1 and does not reach zero energy even at 10 K [28].…”

“…[6] for the undistorted RbV 3 Sb 5 structure, whereas possible CDW structures were obtained by a structural relaxation in VASP [35,36] similar to Ref. [28]. Spin-orbit coupling was included for the calculations of band structure and optical conductivity.…”

“…Here, C is a constant, is the reduced Planck constant, k B the Boltzmann constant, and τ is the elastic scattering time of the standard Drude model. While its origin is still under debate, a temperaturedependent low-energy absorption feature is commonly found in strongly correlated electronic systems and is also observed in the sibling compounds, KV 3 Sb 5 and CsV 3 Sb 5 [27,28,40]. The formalism, as presented in Ref.…”

“…Despite many similarities shared by all three compounds -KV 3 Sb 5 , RbV 3 Sb 5 , and CsV 3 Sb 5 -they feature somewhat different energy scales and, possibly, different underlying interactions. Density-functional-theory (DFT) band-structure calculations could perfectly reproduce the optical response of CsV 3 Sb 5 [27], but not of KV 3 Sb 5 [28], suggesting that deviations from the band picture may be enhanced across the series as the size of the alkaline metal is reduced. This motivates a comparative optical study of RbV 3 Sb 5 as the possible intermediate case.…”