A ‘soft electronic band’ and the negative thermooptic effect in strontium titanate

The optical functions of strontium titanate (SrTiO 3 ) (STO) single crystals were determined with spectroscopic ellipsometry in the 1–6 eV spectral region and the 4.2–300 K temperature region. On approaching the band gap edge, the optical absorption shows Urbach tail behavior followed by the onset of indirect optical transitions starting at ∼3.2 eV and extending up to ∼3.68 eV; then direct transitions are very well seen. The refractive index and its temperature and spectral dependences were obtained. In the visible region, the refractive index rises on cooling (negative thermooptic effect) down to 50 K, saturating at lower temperatures. The energy of indirect interband optical transitions increases on cooling up to 3.23 eV at 100 K. The observed magnitude ∂ E /∂ T =−1.7 × 10 −4 eV K −1 is typical of ionic crystals. Surprisingly, the absorption edge in the region of the lowest direct interband optical transitions 3.7–4 eV shifts to lower energies on cooling. It is concluded that the presence of this 'soft electronic band' together with the rise in absorption intensity at 4.8 eV is responsible for the increase of refractive index on cooling, elucidating the origin of the negative proper thermooptics in STO.