Origin of strong correlations and superconductivity in NaxCoO2

We propose a minimal model resolving a puzzle of enigmatic correlations observed in sodium-rich NaxCoO2 where one expects a simple, free motion of the dilute S= 1/2 holes doped into a band insulator NaCoO2. The model also predicts singlet superconductivity at experimentally observed compositions. The model is based on a key property of cobalt oxides-the spin-state quasidegeneracy of CoO6 octahedral complex-leading to an unusual physics of, e. g., LaCoO3. We show that correlated hopping between t(2g) and e(g) states leads to the spin-polaron physics at x similar to 1, and to an extended s-wave pairing at larger doping when coherent fermionic bands are formed.