ELECTRONIC HAMILTONIAN FOR TRANSITION-METAL OXIDE COMPOUNDS

An effective electronic Hamiltonian for transition-metal oxide compoun ds is presented. For Mn oxides, the Hamiltonian contains spin-2 ''spin s'' and spin-3/2 ''holes'' as degrees of freedom. The model is constru cted from the Kondo-lattice Hamiltonian for mobile e(g) electrons and localized t(2g) spins, in the limit of a large Hund's coupling. The ef fective electron-bond-hopping amplitude fluctuates in sign as the tota l spin of the bond changes. In the large spin limit, the hopping ampli tude for electrons aligned with the core ions is complex and a Berry p hase is accumulated when these electrons move in loops. The model is c ompared with the standard double-exchange Hamiltonian. Both have ferro magnetic ground states at finite hole density and low temperatures, bu t their critical temperatures could be substantially different due to the frustration effects induced by the Berry phase.