Electron-electron correlations in Raman spectra of VO$_2$

It has been shown that, in single crystals and films of a strongly correlated material, namely, vanadium dioxide, upon a thermally stimulated phase transition from the low-temperature monoclinic phase to the high-temperature tetragonal phase, the narrow-line Raman spectrum of the insulating (monoclinic) phase transforms into the broad-band Raman spectrum, which contains two peaks at 500 and 5000 cm$^{-1}$ with widths of 400 and 3500 cm$^{-1}$, respectively. It has been found that, as the temperature of the monoclinic phase approaches the structural phase transition temperature (340 K), the line profile of soft-mode phonons at a frequency of 149 cm$^{-1}$ with $A_g$ symmetry and the line profile of phonons at a frequency of 201 cm$^{-1}$ with $A_g$ symmetry acquire an asymmetric shape with a Fano antiresonance that is characteristic of the interaction of a single phonon vibration with a continuum of strongly correlated electrons. It has been demonstrated that the thermal transformation of peaks in the Raman spectra of the VO$_2$ metallic phase is in quantitative agreement with the theory of Raman scattering in strongly correlated materials.