Techn. Hochsch., Diss., 2002--Aachen.

Zugl.: Stuttgart, Univ., Diss., 2005.

Both cuprates and manganites belong to the transition metal oxides. The physics of these compounds is characterized by a dualism of local electron interaction and itinerant charge motion. In the present work, several key issues of metallic cuprates and manganites are addressed on a theoretical level, while close connection to recent experimental work is kept. The work is based on the notion of spin and orbital liquids, representing elegant tools to handle the strongly correlated nature of the metallic state in an efficient and transparent manner. A concise introduction to the physics of cuprates and manganites as well as to the methods employed is presented at the beginning of the work. In a subsequent part, we show that the peculiar magnetic response of metallic cuprates upon impurity doping can be successfully explained within a spin-liquid picture. The remainder of the work is devoted to the metallic state of manganites. Elaborating on the notion of an orbital liquid, the interplay of electron correlations, orbital degeneracy, and double exchange is studied. Thereby, the unconventionally large incoherent optical spectrum of metallic manganites and the pronounced softening of the magnon spectrum observed in experiment can be explained. Finally, a theory of the metal-insulator transition of manganites is presented which is based upon the newly introduced notion of orbital polarons. In general, we believe the close agreement of our results with experiment to strongly support the validity of our approach, giving new insight into the spectacular and sometimes as-tonishing physics of transition metal oxides.

Both cuprates and manganites belong to the transition metal oxides. The physics of these compounds is characterized by a dualism of local electron interaction and itinerant charge motion. In the present work, several key issues of metallic cuprates and manganites are addressed on a theoretical level, while close connection to recent experimental work is kept. The work is based on the notion of spin and orbital liquids, representing elegant tools to handle the strongly correlated nature of the metallic state in an efficient and transparent manner. A concise introduction to the physics of cuprates and manganites as well as to the methods employed is presented at the beginning of the work. In a subsequent part, we show that the peculiar magnetic response of metallic cuprates upon impurity doping can be successfully explained within a spin-liquid picture. The remainder of the work is devoted to the metallic state of manganites. Elaborating on the notion of an orbital liquid, the interplay of electron correlations, orbital degeneracy, and double exchange is studied. Thereby, the unconventionally large incoherent optical spectrum of metallic manganites and the pronounced softening of the magnon spectrum observed in experiment can be explained. Finally, a theory of the metal-insulator transition of manganites is presented which is based upon the newly introduced notion of orbital polarons. In general, we believe the close agreement of our results with experiment to strongly support the validity of our approach, giving new insight into the spectacular and sometimes as-tonishing physics of transition metal oxides.