Theoretical study of the electronic structure in β-pyrochlore oxides

Abstract We study the electronic state of the metallic β -pyrochlore osmium-oxides theoretically. The model Hamiltonian with 5d and 2p electrons on osmium and oxygen ions, espectively, is examined by means of a tight-binding approximation. The tight-binding parameters are deduced from the first-principle band calculation. Based on the one-electron consideration, an effective Hamiltonian is derived where the t 2 g electrons with orbital degeneracy and localized spins interact each other. We find an instability of the electronic state due to the competition between the superexchange and double-exchange interaction on the (frustrated) pyrochlore lattice. The chemical pressure effects and the recently observed high-pressure effects on the superconducting osmates are discussed in the light of the theoretical results.