Anab initioevaluation of the local effective interactions in the Na_xCoO₂family

Abstract

We used quantum chemical ab initio methods to determine the effective parameters of Hubbard and t-J models for the Na(x)CoO(2) compounds (x = 0 and 0.5). As for the superconducting compound we found the a(1g) cobalt orbitals above the e'(g) ones by a few hundreds of meV due to the e'(g)-e(g) hybridization of the cobalt 3d orbitals. The correlation strength was found to increase with the sodium content x, whereas the in-plane AFM coupling decreases. The less correlated system was found to be the pure CoO(2), however it is still strongly correlated and very close to the Mott transition. Indeed we found U/t ∼ 15, which is the critical value for the Mott transition in a triangular lattice. Finally, we found the magnetic exchanges in the CoO(2) layers to be strongly dependent on the weak local structural distortions.

Electronic structure of the Ca3Co4O9compound fromab initiolocal interactions

“…), since it supports the Fermi level electrons 6,10 while the CoCa 2 O 3 subsystem was proved to be gapped. In addition, in the related Na x CoO 2 family the Fermi level properties were proved to be strongly dependant both on the doping 4 and on the local structural parameters 13 . The purpose of the present work will thus be to study the effect of the incommensurate modulations on the low energy degrees of freedom of the Ca 3 Co 4 O 9 system.…”

Theory of Carrier Concentration-Dependent Electronic Behavior in Layered Cobaltates

“…It has been suggested that while the a 1g -only description is valid for large x, holes occupy both a 1g and e ′ g orbitals at small x [13], and the onsite Hubbard correlation U is x-dependent. Quantum chemical configuration interaction calculations [14] find larger a 1g − e ′ g separation than LDA calculations [2], and (ii) many-body approaches that take Coulomb hole-hole repulsion into account [15][16][17] do not find the e ′ g pockets on the Fermi surface predicted within LDA calculations.x-dependence has also been ascribed to differences in the potential due to Na layers [11,18]. Experimentally, Li x CoO 2 [19] and Bi "misfit" cobaltates [20] do not exhibit ion ordering but nevertheless exhibit very similar carrier concentration-dependence [19,20], suggesting that this dependence is intrinsic to the CoO 2 layers, with the interlayer potential playing a secondary role.In the present Letter we show that a simple and natural explanation of the carrier concentration-dependence emerges within a 1g -only as well as multiband extended Hubbard models.…”

“…As in [16], we have ignored the Hund's rule coupling based on the very small hole occupation of e ′ g orbitals (see below). As both photoemission experiments [3][4][5] and many-body theories [14,15,17] find negligible role of e ′ g orbitals, we discuss the one-band limit of Eq. 1 first.…”

Original Layered OP4-(Li,Na)xCoO2 Phase: Insights on Its Structure, Electronic Structure, and Dynamics from Solid State NMR

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The OP4-(Li/Na)xCoO2 phase is an unusual lamellar oxide with a 1:1 alternation between Li and Na interslab spaces. In order to probe the local structure, electronic structure and dynamics, 7 Li and 23 Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy were performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. 7 Li MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons that is really unusual.

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“…In order to probe the local structure, electronic structure and dynamics, 7 Li and 23 Na magic angle spinning nuclear magnetic resonance (MAS NMR) spectroscopy were performed in complementarity to X-ray diffraction and electronic and magnetic properties measurements. 7 Li MAS NMR showed that NMR shifts result from two contributions: the Fermi contact and the Knight shifts due to the presence of both localized and delocalized electrons that is really unusual. 7 Li MAS NMR clearly shows several Li environments indicating that, moreover, Co ions with different local electronic structures are formed, probably due to the arrangement of the Na + ions in the next cationic layer.…”