Relativistic effects in the optical response of low-dimensional structures : new developments and applications within a time-dependent density functional theory framework

Abstract

The characterization of the electronic response of nanostructures to external electromagnetic fields is of great importance, both from the theoretical and technological points of view. In contrast to light elements, new physical processes and phenomena appear in heavy elements, where relativistic effects, like spin-orbit coupling cannot be ignored. In this work we develop the theoretical framework and the numerical tools needed to address those processes within time-dependent density functional theory (TDDFT). We first apply this methodology to the study of the photoabsorption cross-section of small cationic xenon clusters. Overall, we find our results to be in good agreement with experiment, except for the incorrect prediction of the relative intensities of the peaks. As a second application, we investigate the effect of spin-orbit coupling in the optical response of small gold clusters. We find that this effect is always noticeable, but its importance depends on the dimensionalities of the clusters (the effect is larger for wires than for 2D and 3D structures) and on the size of the clusters (the effect is “quenched” with increasing cluster size). Finally, we study the role of spin noncollinearity in the excited state properties of small chromium and iron clusters and its interplay with spin-orbit coupling. In particular, we compare the dipole and spin-dipole responses of clusters with collinear and noncollinear spin arrangements. We find that, in all cases, the different electronic structure of collinear and noncollinear configurations are reflected to some extent in the spectra. On the contrary, no direct evidence of spin-orbit coupling can be found in the spectra.