Theoretical study of the electronic structure in β-pyrochlore oxides

We study the electronic state of the metallic beta-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(2g) 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. (c) 2006 Elsevier B.V. All rights reserved.