Femtosecond studies of the phase transition inTi2O3

We report femtosecond time-resolved pump-probe DECP experiments using a colliding pulse mode-locked laser performed on ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$ as the sample lattice temperature ${\mathit{T}}_{\mathit{L}}$ is raised from 300 K through the ‘‘soft transition’’ at 450 K to a temperature of 570 K. We have observed DECP spectra through the transition, with oscillations in reflectivity of a few percent associated with the low frequency ${\mathit{A}}_{1\mathit{g}}$ mode. A thermodynamic relation is found between the low frequency ${\mathit{A}}_{1\mathit{g}}$ equilibrium displacement and the number of excited electrons removed from the valence band. When applied to ${\mathit{T}}_{\mathit{L}}$-dependent equilibrium coordinate data for ${\mathrm{Ti}}_2$ ${\mathrm{O}}_3$ obtained in x-ray experiments, the theory allows a determination of the band overlap vs lattice temperature. The band overlap at ${\mathit{T}}_{\mathit{L}}$=621 K is found to be ∼0.06 eV. The ${\mathit{A}}_{1\mathit{g}}$ mode frequency ${\nu }_{\mathrm{ph}}$, the electronic relaxation rate (1/${\mathrm{\tau }}_{\mathrm{el}}$), and the phonon relaxation rate (1/${\mathrm{\tau }}_{\mathrm{ph}}$) have all been followed through the transition. ${\nu }_{\mathrm{ph}}$ decreases, and shows a partial recovery in agreement with other Raman studies. The behavior of (1/${\mathrm{\tau }}_{\mathrm{}\mathrm{el}}$) can be understood as due to an increase in available states for interband electron-phonon scattering as the band crossing takes place. Applying a deformation potential model to the data for (1/${\mathrm{\tau }}_{\mathrm{el}}$) before band crossing, with the low frequency ${\mathit{A}}_{1\mathit{g}}$ mode as the dominant scattering mechanism, a value of |D|≃2.0 eV is obtained for the valence band deformation potential associated with this mode. (1/${\mathrm{\tau }}_{\mathrm{ph}}$) does not show a clearcut correlation with bandcrossing due to greater scatter in the data. The temperature dependence is partially explained by the two-phonon decay of the coherent phonon excited in DECP, and may also have a component due to interaction with hot electrons as well as a dephasing contribution. © 1996 The American Physical Society.