Conductivity of twin walls - surface junctions in ferroelastics: interplay of deformation potential, octahedral rotations, improper ferroelectricity and flexoelectric coupling

TL;DR

This paper investigates how twin wall-surface junctions in ferroelastics influence local conductivity through deformation potential, octahedral rotations, improper ferroelectricity, and flexoelectric effects, revealing both direct and indirect mechanisms.

Contribution

It provides analytical insights into the interplay of structural and electronic phenomena at twin wall-surface junctions, highlighting mechanisms behind static conductivity in ferroelastics and related materials.

Findings

Elastic strains reduce local band gap via deformation potential and flexoelectric coupling.

Flexo-roto fields induce carrier accumulation near twin walls.

Both direct and indirect mechanisms contribute to domain wall conductivity.

Abstract

Electronic and structural phenomena at the twin domain wall-surface junctions in the ferroelastic materials are analyzed. Carriers accumulation caused by the strain-induced band structure changes originated via the deformation potential mechanism, structural order parameter gradient, rotostriction and flexoelectric coupling is explored. Approximate analytical results show that inhomogeneous elastic strains, which exist in the vicinity of the twin walls - surface junctions due to the rotostriction coupling, decrease the local band gap via the deformation potential and flexoelectric coupling mechanisms. This is the direct mechanism of the twin walls static conductivity in ferroelastics and, by extension, in multiferroics and ferroelectrics. On the other hand, flexoelectric and rotostriction coupling leads to the appearance of the improper polarization and electric fields proportional to the structural order parameter gradient in the vicinity of the twin walls - surface junctions. The "flexo-roto" fields leading to the carrier accumulation are considered as indirect mechanism of the twin walls conductivity. Comparison of the direct and indirect mechanisms illustrates complex range of phenomena directly responsible for domain walls static conductivity in materials with multiple order parameters.