Charge disproportionation and spin ordering tendencies in NaxCoO2 -: art. no. 045104

The strength and effect of Coulomb correlations in the (superconducting when hydrated) xapproximate to1/3 and enhanced xapproximate to2/3 regimes of NaxCoO2 are evaluated using the correlated band theory LDA+U (local density approximation of Hubbard U) method. Our results, neglecting quantum fluctuations, are (1) allowing only ferromagnetic order, there is a critical U-c=3 eV, above which charge disproportionation occurs for both x=1/3 and x=2/3, (2) allowing antiferromagnetic order at x=1/3, U-c drops to 1 eV for disproportionation, (3) disproportionation and gap opening occur simultaneously, and (4) in a Co3+-Co4+ ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below U-c ferromagnetism is favored. A comparison of the calculated Fermi level density of states compared to reported linear specific heat coefficients indicates enhancement of the order of five for xsimilar to0.7, but negligible enhancement for xsimilar to0.3. This trend is consistent with strong magnetic behavior and local moments (Curie-Weiss susceptibility) for x>0.5 while there is no magnetic behavior or local moments reported for x<0.5. We suggest that the phase diagram is characterized by a crossover from effective single-band character with UW for x>0.5 into a three-band regime for x<0.5, where U-->U(eff)less than or equal toU/root3similar toW and correlation effects are substantially reduced.