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Electronic excitations in complex systems: beyond density functional theory for real materials

Authors: S. Botti

Ref.: Mémoire de Habilitation à Diriger des Recherches, Université Claude Bernard - Lyon 1 (2010)

Abstract: Today, the use of approaches such as GW , BSE and TDDFT is continuously growing, in all areas where interactions are important but the direct solution of the Schrodinger equation is unfeasible. The present manuscript contains a fairly condensed overview of MBPT and TDDFT, and some examples of their applications to complex systems in the fields of nanotechnology, data storage and photovoltaics. These constitute the main research topics of the Author for the past years.

The manuscript is organized as follows.

Chapters 2 and 3 deal with fundamentals of theoretical approaches, such as GW and TDDFT, based on DFT and going beyond its limitations. In particular, we discuss the available functionals and approximations and the accuracy of results that they give in typical calculations. In chapter 4 we introduce a class of model kernels, derived from the BSE, that are successful for the description of optical absorption in some classes of solids, with a strongly reduced computational impact.

Chapters 5 and 6 deal with applications of GW and TDDFT within linear response. The main objective is to obtain reliable spectra (usually absorption) from ab initio calculations. By comparing theoretical spectra with experimental curves, one is usually able to deduce important information that is not directly available from experiment. Moreover, a better understanding of the excitation properties of the systems opens the way for the design of new materials with improved performances.

Chapter 7 is concerned with the important issue of van der Waals interactions, and how to extract, from TDDFT calculations, relevant parameters to describe them. We will discuss both the interaction between two finite systems, and between a finite system and a semiconducting surface.