Polarons in oxides: a model hamiltonian and ab initio study

Charge carriers placed in an ionic deformable material interact with ion vibrations through the electron- phonon 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.

This project has studied the interaction between electrons and phonons, basic concepts in material physics. Electrons are subatomic particles that orbit the nucleus of an atom, whereas phonons are related to the oscillations of an atom in a crystal. Under certain conditions an electron can be captured by these atomic oscillations and form a composite quantum object called polaron. In this project we have combined the distinct expertises of two research groups (Vienna and Antwerp) to study the formation and properties of polarons in materials. This was done using theoretical models and computer aided calculations. The main question that we have addressed is: Is it possible to describe the physics of polarons without relying on any real-world observations or adjustable parameters, i.e. from a purely theoretical level (typically refer to as 'ab initio')? The answer is yes: by merging established theories and methodologies we have conceptualized and constructed a novel procedure to achieve this goal. The main three ingredients were Mathematical analysis, advanced numerical techniques and so-called quantum Monte Carlo methods based on random numbers and probability distribution. The development of an optimally merged computational scheme connecting the ab initio and quantum-theoretical descriptions represents an absolute novelty in theoretical condensed matter physics,and was only possible thanks to the continuous collaboration between the Vienna team and the Antwerp team. This is a further confirmation that interdisciplinary interactions are essential for advancing our knowledge and go beyond the present limits. The obtained results show an excellent account of polaron properties such as the conditions for the formation of a polaron and its dynamic. Moreover, to assess the predicting power of theory we have conducted joint studies with experimental colleagues at TU Wien to actually observe polarons in real materials and to study their effects. Also in this case the outcome was very satisfactory. The theoretical predictions have been confirmed by the measurements and, in addition to that, new types of polaron effects have been discovered. For instance we found that polarons can group together and modify the structure of a materials and, when located at the surface of a material, can attract and interact with external molecules such as CO, thereby demonstrating the importance of polarons in catalysis (acceleration of a chemical reaction) and in energy production applications.