First-principles theory of flexoelectricity: methods and applications
- Autores: Andrea Maria Enrico Schiaffino
- Directores de la Tesis: Massimiliano Stengel (dir. tes.)
- Lectura: En la Universitat Autònoma de Barcelona ( España ) en 2019
- Idioma: español
- Tribunal Calificador de la Tesis: Ivo Souza (presid.), Gustau Catalán Bernabé (secret.), Raffaele Resta (voc.)
- Programa de doctorado: Programa de Doctorado en Física por la Universidad Autónoma de Barcelona
- Materias:
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- Resumen
This thesis focus on the recent developments of the microscopic flexoelectric theory and its practical applications to study the physical response in concrete systems. Flexoelectricity is the electromechanical property of all insulators of developing a polar response to a uniform strain gradient perturbation.A formal theory of such effect has been recently established in the framework of density functional perturbation theory (DFPT). Still, in order to reach a valuable methodology to facilitate the calculation of the bulk flexoelectric response for a generic crystal, some conceptual and practical subtleties remains. They are related with the intrinsic change of symmetry resulting from a mechanical deformation. In this thesis we will deal with such issue; The solution is based on the introduction of a novel crystal perturbation, the metric perturbation, which is defined as an acoustic phonon described in the reference frame that is co-moving with the atoms. After presenting its formal implementation in the context of the DFPT within the separable pseudopotential approximation, we will employ such new tool to perform extensive calculations of the flexoelectric tensor for selected materials.The previous achievements are followed by a practical application of the flexoelectric theory to engineering the design of “flexovoltaic” devices, i.e. photovoltaic devices that base their fundamental working principles on the flexoelectric effect. In practice, thanks to the close connection between the flexoelectric theory and the theory of absolute deformation potentials, we show how the electron band profile of bended nanodevices can be calculated, in a mesoscopic semiclassical framework, using few and well defined parameters. Such parameters vary with respect to the type of electrostatic boundary conditions imposed. Finally, we will focus our attention on the polar response shown by the ferroeastic domain walls (DWs) in SrTiO3, recently detected. In general, DWs are considered a promising source of unusual physical effects that can be of practical interest for electronic device applications. For improving the applicability of such physical effects it is essential to understand their origin. Inspired by the theory of flexoelectricity we develop an original methodology to study the case of the ferrolastic DWs in SrTiO3, which is based on the construction of an effective energy model, full from first principles, and that includes the effects of gradient mediated couplings between different crystal distortions. The effects of each identified contribution to the total polarization at the DWs is discussed.
