Non-adiabatic phonon renormalization in metallic versus insulating rutile oxides

Abstract

We present a comparative Raman scattering study of metallic rutile oxides (RuO${}_2$ and IrO${}_2$) and insulating rutiles (TiO${}_2$ and SnO${}_2$). Temperature-dependent Raman spectra reveal that the metallic compounds exhibit pronounced phonon frequency hardening, $\omega (11\mathrm{K})-\omega (300\mathrm{K})=\Delta \omega \approx 6$-$10{\mathrm{cm}}^{-1}$, whereas the insulating rutiles show only modest hardening, $\Delta \omega \approx 1$-$3{\mathrm{cm}}^{-1}$. In contrast, the linewidth changes, $\Delta \Gamma \approx 1$--$7{\mathrm{cm}}^{-1}$, do not display a systematic metallic-insulating classification. Fits with the conventional Klemens anharmonic decay model reproduce the overall temperature trends but yield inconsistent anharmonic parameters for the metallic compounds when benchmarked against insulating rutile analogues. A modified Klemens framework, incorporating an additional $T^2$ correction to the phonon frequency arising from the electronic contribution to the phonon self-energy, quantitatively accounts for the enhanced renormalization observed in metallic systems. These results establish finite non-adiabatic electron-phonon coupling in metallic rutiles and demonstrate that phonon renormalization can be identified even in the absence of observable Fano asymmetry in the phonon line shapes.

Published in Phys. Rev. B as a regular article (https://doi.org/10.1103/4nhr-dvbn)