Electric-field-induced soft-mode hardening in SrTiO3 films

We have studied electric-field-induced Raman scattering in SrTiO3 thin film s using an indium-tin-oxide/SrTiO3/SrRuO3 structure grown by pulsed laser d eposition. The soft mode polarized along the field becomes Raman active. Ex perimental data for electric-field-induced hardening of the soft modes and the tuning of the static dielectric. constant are in agreement described by the Lyddane-Sachs-Teller formalism. The markedly different behavior of the soft modes in thin films from that in the bulk is explained by the existen ce of local polar regions.