Flexoelectric Effect in Solids

Piezoelectricity—the coupling between strain and electrical polarization, allowed 2 Zubko, Catalan, Tagantsev by symmetry in a restricted class of materials—is at the heart of many devices that permeate our daily life. Since its discovery in the 1880’s by Pierre and Jacques Curie, the piezoelectric effect has found use in everything from submarine sonars to cigarette lighters. By contrast, flexoelectricity—the coupling between polarization and strain gradients, allowed by symmetry in all materials—was largely overlooked for decades since its first proposal in the late 1950’s, due to the seemingly small magnitude of the effect in bulk. The development of nanoscale technologies, however, has renewed the interest in flexoelectricity, as the large strain gradients often present at the nanoscale can lead to dramatic flexoelectric phenomena. Here we review the fundamentals of the flexoelectric effect, discuss its presence in many nanoscale systems, and look at potential applications of this fascinating phenomenon. The review will also emphasize the many open questions and unresolved issues in this developing field.