Fundamental absorption edge ofSrTiO3at high temperatures

Transmission measurements on single-crystal ${\mathrm{SrTiO}}_{3}$ between room temperature and 1200?deC are reported. Urbach broadening of the absorption edge is observed at all temperatures. Below 200?deC the broadening is typical of the lowest indirect band gap. Above 400?deC the dominating broadening is due to the lowest direct gap located at 3.5 eV. This transition from indirect to direct broadening is related to the proximity of the direct and indirect gaps and should be observed in other narrow-band indirect-gap materials. The small temperature dependence of the band gaps is ascribed to the small magnitude of both the implicit and explicit electron-phonon contributions rather than to accidental cancellation of both terms as has been suggested by Redfield and Burke. We argue that, in indirect narrow-band materials, the explicit term of the direct gap may be small owing to intervalley scattering that compensates for the regular intravalley scattering.