Possible effects of charge frustration in NaxCoO2: Bandwidth suppression, charge orders, and resurrected resonating valence bond superconductivity

Charge frustration due to further neighbor Coulomb repulsion can have dramatic effects on the electronic properties of NaxCoO2 in the full doping range. It can significantly reduce the effective mobility of the charge carriers, leading to a low degeneracy temperature epsilonF<~T. Such strongly renormalized Fermi liquid has rather unusual properties—from the point of view of the ordinary metals with epsilonFT—but similar to the properties that are actually observed in the NaxCoO2 system. For example, we show that the anomalous thermopower and Hall effect observed in Na0.7CoO2 may be interpreted along these lines. If the repulsion is strong, it can also lead to charge order; nevertheless, away from the commensurate dopings, the configurational constraints allow some mobility for the charge carriers, i.e., there remains some "metallic" component. Finally, the particularly strong bandwidth suppression around the commensurate x = 1/3 can help resurrect the resonating valence bond superconductivity, which would otherwise not be expected near this high doping. These suggestions are demonstrated specifically for a tJ-like model with an additional nearest-neighbor repulsion.