Theoretical spectroscopy of realistic condensed matter systems

TL;DR

This thesis advances ab initio methods for calculating ground and excited state properties of condensed matter systems, including magnetic and surface phenomena, using density functional theory and its time-dependent extension.

Contribution

It introduces an original method for response function calculation and generalizes existing code to include spin, enabling detailed studies of magnetic and surface properties.

Findings

Validated the method by analyzing surface spectra and ruling out certain surface reconstructions.

Highlighted challenges in describing open-shell excitations within TDDFT.

Achieved good agreement with experimental data for the optical properties of magnetic materials.

Abstract

This thesis is devoted to ab initio calculations of ground and excited state properties of different systems within density functional theory and time dependent density functional theory. From the numerical point of view we implemented an original method in the DP code to calculate the independent particle response function. Moreover, we generalized the DP code to the spin degree of freedom in order to study the magnetic properties of realistic condensed matter systems. We studied reflectance anisotropy and energy loss spectra of a clean and oxidized surface, and we performed an analysis of the origin of the main spectral features. Thanks to the comparison between experimental and theoretical energy loss spectra, we roule out the p(2x1) reconstruction for the Si(100) surface. Moreover, in the case of a simple BeH molecule we evidenced the problem of correctly describing the excitation spectra for open shell systems within the TDDFT framework. In the second part of the thesis, we presented the study of the optical properties of magnetic systems such as FeS2, CoS2 or NiS2, interesting materials for possible technological applications in the growing field of spintronics. Within this context we calculated ground state properties and optical condictivity of BCC bulk iron, for which we found a nice agreement with available experimental data.