Distortion-Enhanced Optical Absorption in SrTiO3at the Cubic-to-Tetragonal Transition

We present thermomodulated optical absorption data near the fundamental edge of SrTi${\mathrm{O}}_3$ which gives, at the cubic-to-tetragonal phase transition, the first evidence of a second-order transition affecting the electronic energy levels. Enhanced absorption at the indirect gap energy is observed and attributed to relaxation of momentum conservation, via electron scattering at fluctuating domain boundaries. This extra absorption peaks at the transition temperature, where phase fluctuations are maximum.