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Optical study of RbV₃Sb₅: Multiple density-wave gaps and phonon anomalies

Wenzel, M.; Ortiz, B. R.; Wilson, S. D.; Dressel, M.; Tsirlin, A. A.; Uykur, E.


Temperature-dependent reflectivity studies on the non-magnetic kagome metal RbV₃Sb₅ in a broad energy range (50 cm$^{-1}$ - 20000 cm$^{-1}$, equivalent to 6 meV - 2.5 eV) down to 10 K are reported. Below T_CDW=102 K, the optical spectra demonstrate a prominent spectral-weight transfer from low to higher energies as the fingerprint of the charge-density wave (CDW) formation with the opening of a partial gap. A detailed analysis reveals two energy scales of, respectively, ~800 cm$^{-1}$ (100 meV) and 360 cm$^{-1}$ (45 meV), the latter appearing below 50 K only. Additionally, two modes at, respectively, 160 cm$^{-1}$ (20 meV) and 430 cm$^{-1}$ (53 meV) can be traced both above and below T_CDW. They show strong anomalies already above T_CDW with a further renormalization across the transition, suggesting the importance of the electron-phonon coupling in RbV₃Sb₅ in both normal and CDW states. While the 160 cm$^{-1}$mode can be attributed to the E_1u phonon, the 430 cm$^{-1}$ mode could not be reproduced in our phonon calculations. The antiresonance nature of this mode suggests a nontrivial electron-phonon coupling in RbV₃Sb₅. A distinct localization peak observed at all temperatures signals damped electron dynamics, whereas the reduced Drude spectral weight manifests moderate deviations from the band picture in RbV₃Sb₅.

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