Roto-flexoelectric coupling impact on the phase diagrams and pyroelectricity of thin SrTiO3 films

TL;DR

This study uses LGD theory to explore how flexoelectric and rotostriction couplings influence phase diagrams and induce pyroelectricity in thin SrTiO3 films, revealing surface-localized improper polarization and pyroelectric effects.

Contribution

It demonstrates the significant role of flexoelectric and rotostriction couplings in generating improper polarization and pyroelectricity in SrTiO3 films, highlighting surface effects and phase diagram modifications.

Findings

Gradients induce improper polarization and pyroelectricity.

Surface regions exhibit maximal roto-flexoelectric fields.

Penetration depth of improper phases can reach several nanometers.

Abstract

The influence of the flexoelectric and rotostriction coupling on the phase diagrams of ferroelastic-quantum paraelectric SrTiO3 films was studied using Landau-Ginzburg-Devonshire (LGD) theory. The phase diagrams in coordinates temperature - film thickness were calculated for different epitaxial misfit strains. Tensile misfit strains stimulate appearance of the spontaneous out-of-plane structural order parameter (displacement vector of an appropriate oxygen atom from its cubic position) in the structural phase. Compressive misfit strains stimulate appearance of the spontaneous in-plane structural order parameter. Gradients of the structural order parameter components, which inevitably exist in the vicinity of film surfaces due to the termination and symmetry breaking, induce improper polarization and pyroelectric response via the flexoelectric and rotostriction coupling mechanism. Flexoelectric and rotostriction coupling results in the roto-flexoelectric field that is antisymmetric inside the film, small in the central part of the film, where the gradients of the structural parameter are small, and maximal near the surfaces, where the gradients of the structural parameter are highest. The field induces improper polarization and pyroelectric response. Penetration depths of the improper phases (both polar and structural) can reach several nm from the film surfaces. An improper pyroelectric response of thin films is high enough to be registered with planar-type electrode configurations by conventional pyroelectric methods.