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

Temperature-dependent reflectivity studies on the non-magnetic kagome metal RbV${}_3$Sb${}_5$ 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 $T_{\rm CDW}=102$ 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, $\sim$ 800 cm${}^{-1}$ (100 meV) and 360 cm${}^{-1}$ (45 meV), the latter visible 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 $T_{\mathrm{C}\mathrm{D}\mathrm{W}}$. They show strong anomalies already above $T_{\mathrm{C}\mathrm{D}\mathrm{W}}$ with a further renormalization across the transition, suggesting the importance of the electron-phonon coupling in RbV${}_3$Sb${}_5$ 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 RbV${}_3$Sb${}_5$. 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 RbV${}_3$Sb${}_5$.