Charge Disproportionation and Spin-Ordering Tendencies in Na(x)CoO2 at x=1/3

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The strength and effect of Coulomb correlations in the (superconducting when hydrated) x 1/3 regime of Na(x)CoO(2) have been evaluated using the correlated band theory LDA+U method. Our results, neglecting quantum fluctuations, are: (1) there is a critical Uc = 3 eV, above which charge ordering occurs at x=1/3, (2) in this charge-ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below Uc, ferromagnetism is favored; and (3) carrier conduction behavior should be very asymmetric for dopings away from x=1/3. For x < 1/3, correlated hopping of parallel spin pairs is favored, suggesting a triplet superconducting phase.