Possible effects of charge frustration inNaxCoO2: Bandwidth suppression, charge orders, and resurrected resonating valence bond superconductivity

Abstract

Charge frustration due to further neighbor Coulomb repulsion can have dramatic effects on the electronic properties of ${\text{Na}}_{x}{\text{CoO}}_{2}$ in the full doping range. It can significantly reduce the effective mobility of the charge carriers, leading to a low degeneracy temperature ${ϵ}_{F}\ensuremath{\lesssim}T$. Such strongly renormalized Fermi liquid has rather unusual properties---from the point of view of the ordinary metals with ${ϵ}_{F}⪢T$---but similar to the properties that are actually observed in the ${\text{Na}}_{x}{\text{CoO}}_{2}$ system. For example, we show that the anomalous thermopower and Hall effect observed in ${\text{Na}}_{0.7}{\text{CoO}}_{2}$ 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.