Quasi-particles and excitons: Models of structure and correlation

This chapter uses simple models, which include disordered and externally driven systems, to show that the electronic excitation spectrum of a material depends on its structure, range and type of electronic correlations and the nature of the reference state considered. Discusses basic concepts of quantum field theory (quantization of classical fields, Schroedinger field, occupation number representation, self-interaction, sources, effective vacuum) and the models (Empty Band Model, Fermi Gas Model, Tight Binding Model, models with particle interactions, Random Cell Model, driven systems). Concludes that categories which can be used to classify excitation spectra include the nature of quasi-particles, existence of gaps, the type of quasi-particle interactions (excitons) and the relation to phase transition (Anderson-type, superconducting/normal, metal/insulator)