Optical Gap of Strontium Titanate (Deviation from Urbach Tail Behavior)

The absorption of SrTi${\mathrm{O}}_3$ below the fundamental edge shows, near liquid-nitrogen temperature, an oscillatory behavior, for which an explanation is proposed in terms of indirect transitions ${\Gamma }_{15}\to X_3$, centered around 3.27 eV and involving a 51-meV LO phonon. There is no evidence of excitonic effects. The first direct transition is probably $X_{5^{\prime }}\to X_3$ and is located at ∼3.46 eV. Broadening effects mask the indirect structure at higher temperatures where the absorption curves gradually evolve into normal Urbach tailing.