Unusual Electron Correlations in NaxCoO2Due to the Spin-State Quasidegeneracy of Cobalt Ions

Abstract

Recent studies exposed many remarkable properties of layered cobaltates NaxCoO2. Surprisingly, many-body effects have been found to increase at sodium-rich compositions of NaxCoO2 where one expects a simple, nearly free motion of the dilute S = 1/2 holes doped into a band insulator NaCoO2. Here we discuss the origin of enigmatic correlations that turn a doped NaCoO2 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 CoO6 octahedral complex – which has been known, e.g., in the context of an unusual physics of LaCoO3 compound. Another important ingredient of the model is the 90° Co-O-Co bonding in NaxCoO2 which allows nearest-neighbor t2g-eg hopping. This hopping introduces a dynamical mixture of electronic configurations t2g6, S = 0 and t2g5eg1, 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 Co3+ is essential for the pairing mechanism. Implications of the model for magnetic orderings are also discussed.