236 research outputs found
Charge Disproportionation and Spin Ordering Tendencies in Na(x)CoO2
The strength and effect of Coulomb correlations in the (superconducting when
hydrated) x~1/3 and ``enhanced'' x~2/3 regimes of Na(x)CoO2 are evaluated using
the correlated band theory LDA+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, (4) in a Co(3+)-Co(4+) ordered state, antiferromagnetic
coupling is favored over ferromagnetic, while below U_c 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 U_eff <= U/\sqrt(3) ~ W and
correlation effects are substantially reduced.Comment: 10 pages, 8 figures, corrected a few typos and changed reference
NaCoO in the x -> 0 Regime: Coupling of Structure and Correlation effects
The study of the strength of correlations in NaCoO is extended to the
x=0 end of the phase diagram where Mott insulating behavior has been widely
anticipated. Inclusion of correlation as modeled by the LDA+U approach leads to
a Mott transition in the subband if U is no less than U=2.5 eV. Thus
U smaller than U is required to model the metallic, nonmagnetic CoO
compound reported by Tarascon and coworkers. The orbital-selective Mott
transition of the state, which is essentially degenerate with the
states, occurs because of the slightly wider bandwidth of the
bands. The metal-insulator transition is found to be strongly coupled to the
Co-O bond length, due to associated changes in the bandwidth, but the
largest effects occur only at a reduced oxygen height that lies below the
equilibrium position.Comment: 8 pages with 9 embedded figure
Band structure renormalization and weak pseudogap behavior in Na_{0.33}CoO_2: Fluctuation exchange study based on a single band model
Based on a single band Hubbard model and the fluctuation exchange
approximation, the effective mass and the energy band renormalization in
NaCoO is elaborated. The renormalization is observed to exhibit
certain kind of anisotropy, which agrees qualitatively with the angle-resolved
photoemission spectroscopy (ARPES) measurements. Moreover, the spectral
function and density of states (DOS) in the normal state are calculated, with a
weak pseudogap behavior being seen, which is explained as a result of the
strong Coulomb correlations. Our results suggest that the large Fermi surface
(FS) associated with the band plays likely a central role in the
charge dynamics.Comment: 5 pages, 5 figure
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