New functionalities from gradient couplings: Flexoelectricity and more

Flexoelectricity, the coupling between an inhomogeneous deformation and the electrical polarization, has emerged a "hot" topic in modern materials science due to its cross-cutting relevance to many phenomena of fundamental and technological interest. Understanding the intriguing physics that governs its behaviour at the nanoscale is crucial to harnessing the potential of strain gradients in practical applications, and such a progress requires a substantial support from theory. Due to the inherent breakdown of lattice periodicity that a strain gradient entails, however, first- principles calculations of flexoelectricity remain technically challenging at several levels. In this talk, I will discuss the recent methodological developments that have made such calculations feasible, and their application to realistic materials systems. In particular, I will focus on manifestations of the flexoelectric effect in SrTiO3, either at the level of the bulk crystal, surfaces or ferroelastic twin boundaries. More generally, I will emphasize the opportunities that spatial gradients (e.g. a nonuniform polarization, strain or magnetization field) offer for materials design, by enabling functionalities that would be otherwise forbidden in a uniform crystalline phase.