Charge disproportionation and spin ordering tendencies inNaxCoO2

Abstract

The strength and effect of Coulomb correlations in the (superconducting when hydrated) x ≈1/3 and "enhanced" x ≈2/3 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 Uc = 3 eV, above which charge disproportionation occurs for both x=1/3 and x=2/3, (2) allowing antiferromagnetic order at x=1/3, Uc drops to 1 eV for disproportionation, (3) disproportionation and gap opening occur simultaneously, (4) in a Co 3+ -Co 4+ ordered state, antiferromagnetic coupling is favored over ferromagnetic, while below Uc ferromagnetism is favored. Comparison of the calculated Fermi level density of states compared to reported linear specific heat coefficients indicates enhancement of the order of five for x ∼0.7, but negligible enhancement for x ∼0.3. This trend is consistent with strong magnetic behavior and local moments (Curie-Weiss susceptibility) for x >0.5 while there 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 U >> W for x > 0.5 into a three-band regime for x < 0.5, where U → U ef f ≤ U/ √ 3 ∼ W and correlation effects are substantially reduced.

Magnetic phase transition at88KinNa0.5CoO2Pedrini1, Gavilano2, Felder3 et al. 2005Phys. Rev. B131140View full textAdd to dashboardCite

“…The electronic structure of these compounds was calculated in the local (spin) density approximation (LSDA). The inclusion of a Hubbard repulsion parameter U in the calculation yields the two magnetically different regimes for x < 0.5 and x > 0.5 [14] that are observed experimentally. The calculation also explains [15] the absence of pockets of the Fermi surface, as was registered in angle-resolved photoemission (ARPES) experiments for x = 0.3 [16], x = 0.6 [17] and x = 0.7 [18].…”

Orbital-ordering driven structural distortion in metallicSrCrO3

“…It is this difference in the band center (i.e. the on-site energy) that drives this transition, as happened in Na x CoO 2 [17] or V 2 O 3 . [18] Keeping the structure (and symmetry) cubic inhibits such an orbital-ordering transition, implying close interplay between structural distortion (even though tiny) and orbital ordering in LDA+U calculations in this system as suggested in CaCrO 3 .…”

Origin of electron disproportionation in metallic sodium cobaltates

“…The magnetic moments of the d-shells serve as indicators of the redistribution of the d-electron spin and charge density between the Co ions in their triangular lattice. 12,24,25,27,29 In the previous study we have reported about the LS "phase" of the electrons, 29 see Fig. 1(c).…”

“…In subsequent calculations it was established, that when the coupling with such order is taken into account, the LS states with hole disproportionation indeed appear, possessing charge/spin patterns of various types (the zig-zag, striped, honeycomb, and hexagonal) which depend on the presumed Na arrangement. [24][25][26][27][28][29] However the ordering of Na + ions drastically changes the electronic low-energy states, 24 and the KSL structure expected for the phase x = 2/3 was not obtained in the LS-states of the lattice even with the realistic sodium structure. 29 The aim of the present study is to investigate conditions for the origin of a metallic state with the KSLstructure of the d-electron charge and spin disproportionation in cobaltates, employing the ab-initio calculations for the system with ordered Na ions.…”