Polarons in oxides: a model hamiltonian and ab initio study

Charge carriers placed in an ionic deformable material interact with ion vibrations through the electronphonon interaction. As a consequence of this interaction, the ions can adjust their positions slightly and give rise to a polarization locally centered on the charge carrier. The carrier together with the induced polarization is considered as one entity, a quasiparticle which is called polaron. There are "large" and "small" polarons, defined by whether or not the polarization cloud (i.e. the polaron radius) is much larger than the interatomic spacing in the material. Despite their fundamental similarities, there is not a unique theory explaining togethr large and small polarons. The basic features of small and large polarons are described by two conceptually distinct theoretical schemes: ab-initio and model-Hamiltonains, respectively. Polarons play a crucial role in many physical mechanisms such as charge transfer, transport and optical excitations which are of fundamental importance in technologically applications related to energy conversion, catalysis, optoelectronics and photonics. This project proposes an ambitious research program to tackle challenging issues in understanding the nature and characteristics of polarons and their impact on the properties and functionalities of transition metal oxide materials, among the most promising class of materials for present and future technology. The first goal of our research is to formulate, design, and test an integrated ab-initio and model Hamiltonian approach that will allow an unified theoretical description of small and large polarons within the same theoretical framework. The second objective is to apply this novel computational machinery to realistic problems in material science. Specifically we aim to to explain and understand the formation and dynamics of polarons in transition metal oxides. To this end, we gather together in a synergic collaboration two complementary groups that are leading in model Hamiltonian (Belgium) and ab-inito schemes (Austria) and equipped with the most advanced theoretical methodologies and with outstanding expertise on electron-phonon interaction and polarons.