Gustau Catalan

ICREA and ICN2-Institut Català de Nanociència i Nanotecnologia
Fundamentals of Flexoelectricity
The generation of polarization under mechanical stress or, conversely, the generation of deformation in response to a voltage, are a traditional preserve of ferroelectric and piezoelectric materials. Piezoelectricity, however, is not the only electromechanical property of materials. Flexoelectricity, the coupling between polarization and strain gradients, is increasingly being recognized as a very important, sometimes even dominant, electromechanical property. Contrary to piezoelectricity, flexoelectricity is symmetry-allowed in all materials, including of course ferroelectrics, but also centrosymmetric materials. It turns out also that this property even exists in semiconductors and photovoltaic materials, metals, and biomaterials. It is thus truly universal. Moreover, because strain gradients tend to grow in inverse proportion to physical size, flexoelectricity can be the dominant electromechanical coupling at the nanoscale. Additionally, when it coexists with other functional properties (piezoelectricity in ferroelectrics, photovoltaic effects in semiconductors), it generates new functional properties that would not be possible without such coexistence. Understanding flexoelectricity is therefore important and potentially useful if we manage to harness these new flexoelectrically-induced functionalities. The present tutorial will attempt to give a complete overview of flexoelectricity, from its definition and physical principles, to its effect on different materials and the consequences for devices. At every step of the way, the working principles will be illustrated with examples. The aim is to provide a good foundation for those who are new or curious about flexoelectricity, and perhaps also to inspire new directions of research for those who already primed about it.
Presenter Bio

Gustau Catalan is a physicist and ICREA Research Professor at ICN2, in Barcelona, where he leads the Oxide Nanophysics laboratory. His research covers emerging properties of oxides at the nanoscale, with emphasis on ferroelectric and antiferroelectric materials. One such property is flexoelectricity, the theme of his tutorial talk for the ISAF meeting, and a property for which he has especial fondness and a dedicated laboratory, one of the first specialized laboratories of flexoelectricity in the world. His experience in his field goes a long way back, starting with his discovery in 2004 that flexoelectricity was a key contributor to the so-called “size effect” of ferroelectric thin films. Since then, he’s participated in several notable advances in the field, such as flexoelectric-induced mechanical writing and mechanical reading of ferroelectric domains, the existence flexoelectricity in semiconductors and bones and, most recently, the photo-flexoelectric effect of halide perovskites.

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