Correlated electrons in heavy fermion and double exchange systems

The metamagnetic state of the Anderson model is investigated with the intent of explaining anomalous de Haas-van Alphen experiments on the heavy fermion system CeRu2Si2. Working in the Landau quantised basis a variational quasiparticle ansatz modelling magnetic polaron formation is used to obtain the self-energy. The oscillatory magnetisation M-tilde is calculated following Luttinger and the usual Lifschitz-Kosevich formula is obtained. Owing to the large mass enhancement the oscillatory part of the self-energy Σ-tilde is not negligible here however, and the effects of Σ-tilde on M-tilde are calculated. A new oscillatory paramagnetic term and a splitting of the spectral peaks into subharmonics is predicted, but the anomalous experimental results are not explained. The double exchange model is studied in order to resolve controversy over whether it suffices to model the colossal magnetoresistance manganites or whether additional physics is required. The one-electron Green function is obtained using a Hubbard-III type equation of motion decoupling approximation approximation exact in the atomic limit, regarded as a dynamic many-body coherent potential approximation. The paramagnetic state resistivity is calculated and found to be much smaller than that measured, and the reason for disagreement over its magnitude is explained. The magnetic susceptibility is studied for spin 1/2 and it is found that ferromagnetism does not exist in this approximation. At low temperature and half-filling the susceptibility deviates from the physical Curie law behaviour and methods of remedying this problem are considered. (author)