Anomalous variation of phonon Raman intensities near the metal-to-Mott-insulator transition in titanium oxide systems

Raman-scattering spectra were investigated for strongly correlated electron systems, ${\mathrm{La}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Sr}}_{\mathit{x}}$ ${\mathrm{TiO}}_3$ and ${\mathrm{Y}}_{1\mathrm{-}\mathit{x}}$ ${\mathrm{Ca}}_{\mathit{x}}$ ${\mathrm{TiO}}_3$, which undergo a metal-to-Mott-insulator transition under variation of the band filling. The phonon spectra decrease in their intensity as the system approaches the metal-insulator phase boundary, accompanied by a critical enhancement of the value of the effective mass, and almost disappear in the insulating phase. We found that the phonon spectral intensity in the metallic region approximately scales with (n/m*${)}^2$, n and m* being the density and effective mass of conduction carriers.