Unusual Electron Correlations in Na_xCoO_2 Due to the Spin-State Quasidegeneracy of Cobalt Ions(Interaction and Nanostructural Effects in Low-Dimensional Systems)

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Recent studies exposed many remarkable properties of layered cobaltates Na_xCoO_2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of Na_xCoO_2 where one expects a simple, nearly free motion of the dilute S=1/2 holes doped into a band insulator NaCoO_2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO_2 into a strongly correlated electronic system. A minimal model including orbital degeneracy is proposed and its predictions are discussed. The model is based on a key property of cobalt oxides - the spin-state quasidegeneracy of CoO_6 octahedral complex - which has been known, e.g., in the context of an unusual physics of LaCoO_3 compound. Another important ingredient of the model is the 90° Co-O-Co bonding in Na_xCoO_2 which allows nearest-neighbor t_<2g>-e_g hopping. This hopping introduces a dynamical mixture of electronic configurations t^6_<2g>, S=0 and t^5_<2g>e^1_g, S=1 of neighboring cobalt ions. We show that scattering of charge carriers on spin-state fluctuations suppresses their coherent motion and leads to the spin-polaron physics at x〜1. At larger doping when coherent fermionic bands are formed, the model predicts singlet superconductivity of extended s-wave symmetry. The presence of low-lying spin states of Co^<3+> is essential for the pairing mechanism. Implications of the model for magnetic orderings are also discussed.
2009-02-05