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-degree 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 t2g5​eg1​,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.Comment: 27 pages, 19 figures, submitted to Progress of Theoretical Physics
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