We develop a theory for the electronic excitations in UPt 3 which is based on the localization of two of the 5f electrons. The remaining f electron is delocalized and acquires a large effective mass by inducing intra-atomic excitations of the localized ones. The measured deHaas-vanAlphen frequencies of the heavy quasiparticles are explained as well as their anisotropic heavy mass. A model calculation for a small cluster reveals why only the largest of the different 5f hopping matrix elements is operative causing the electrons in other orbitals to localize.
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