Electric-Field-Induced Soft-Mode Hardening inSrTiO3Films

We have studied electric-field-induced Raman scattering in ${\mathrm{SrTiO}}_3$ thin films using an $\mathrm{indium}-\mathrm{tin}-\mathrm{oxide}/{\mathrm{SrTiO}}_3/{\mathrm{SrRuO}}_3$ structure grown by pulsed laser deposition. The soft mode polarized along the field becomes Raman active. Experimental 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 existence of local polar regions.