Ab-initio calculations of the charge-density response in complex materials

by 1976- Restrepo, Oscar Dario

Our main goal is to have a realistic description of the charge excitations in complex materials in the range of energies such as the coulomb energy U, which is in the eV range. Spectra of these charge excitations, for a large range of wave vector transfers, may provide signatures of the underlying electronic structures. The charge-density response function calculated within Time-dependent Density functional (TDDFT) is an ideal theoretical framework for the calculation of these excitations, since comparison with experimental data (in particular with non-resonant inelastic x-ray scattering experiments) can be done in absolute units (without the need of any adjustable parameters). The large phase space considered gives a rich testing ground of theory vs. experiment. In Chapter 3 we study the collective excitations in MgB2. We find a sharp feature in the charge-excitation spectra of single-crystal MgB2 showing striking, periodic, energy dispersion with momentum transfer q along the c-axis. This excitation was observed for the first time by non-resonant inelastic x-ray scattering in collaborative work. Our Timedependent density-functional theory calculations show that the observed spectra arises from a strong coupling between single-particle and collective degrees of freedom, mediated by large crystal local fields. As a result, the small-q collective mode residing in the single-particle excitation gap of the B ? bands reappears periodically in higher Brillouin zones, a phenomenon which is traced to the layered structure of MgB2. In Chapter 4 we argue that the large density of states associated with the narrow bands of strongly-correlated materials may drive the formation of a collective electron-hole state, with an excitation energy on the order of the “Hubbard U.” Whether the state is actually realized (as we predict for manganites) or not (for monoxides), depends on details of the hybridization between the correlated d orbitals and the oxygen-derived p orbitals. The crystal local fields play a crucial role in the appearance of this excitation. We discuss in Chapter 5 the electron-hole excitations in the hydrated and non-hydrated Sodium Cobaltates compounds for sodium concentrations of 1/3. In both types of compounds we find a collective excitation at 9 eV induced by the Crystal Local Fields for iv a large range of momentum transfers. It is shown how the inclusion of the water affects the screening at small energies allowing single-particle transitions to be present in the loss spectrum for small momentum transfers. For large values of momenta the loss function for the hydrated and non-hydrated systems are very similar due to the reduced polarizability of the water molecules.