Disproportionation, metal-insulator transition, and critical interaction strength in Na(1/2)CoO2

Quantum Monte Carlo analysis of a charge ordered insulating antiferromagnet: the Ti4O7 Magnéli phase

Spin density surfaces in the low temperature phase of Ti₄O₇: the ferromagnetic state (left panel), and the lowest-energy antiferromagnetic state (right panel). The figures were generated using self-interaction corrected density functional theory.

An ab initio evaluation of the local effective interactions in the NaxCoO2 family

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.

Pressure-induced structural phase transitions on Na(0.5)CoO(2): a first principles study

First principles calculations have been carried out to study the pressure effects on the structural, electronic and magnetic properties of Na(0.5)CoO(2). An unexpected Na ion structural transformation has been predicted from our calculations. The main factor for such a transformation has been attributed to the increasing Coulomb repulsion between the Na1 and Co ions with the increase of pressure. The experimentally observed insulator-metal transition has been found to be induced by this structural transformation. Furthermore, due to the strong coupling between Na ordering and Co magnetic ordering, a magnetic phase transition follows after the structural transformation.

Correlation-driven charge order at the interface between a Mott and a band insulator

To study digital Mott insulator LaTiO3 and band insulator SrTiO3 interfaces, we apply correlated band theory within the local density approximation including a Hubbard U to (n, m) multilayers, 1<or=n, m<or=9 using unit cells with larger lateral periodicity. If the on-site repulsion on Ti is big enough to model the Mott insulating behavior of undistorted LaTiO3, the charge imbalance at the interface is found in all cases to be accommodated by disproportionation (Ti4++Ti3+), charge ordering, and Ti3+ dxy-orbital ordering, with antiferromagnetic exchange coupling between the spins in the interface layer. Lattice relaxations lead to conducting behavior by shifting (slightly but importantly) the lower Hubbard band, but the charge and orbital order is robust against relaxation.

Charge and spin order on the triangular lattice: NaxCoO2 at x=0.5

The nature of electronic states due to strong correlation and geometric frustration on the triangular lattice is investigated in connection to the unconventional insulating state of NaxCoO2 at x=0.5. We study an extended Hubbard model using a spatially unrestricted Gutzwiller approximation. We find a new class of charge and spin ordered states at x=1/3 and x=0.5 where antiferromagnetic (AFM) frustration is alleviated via weak charge inhomogeneity. At x=0.5, we show that the square root of 3a x 2a off-plane Na dopant order induces weak square root of 3a x 1a charge order in the Co layer. The symmetry breaking enables successive square root of 3a x 1a AFM and 2a x 2a charge- or spin-ordering transitions at low temperatures. The Fermi surface is truncated by the 2a x 2a hexagonal zone boundary into small electron and hole pockets. We study the phase structure and compare to recent experiments.

NMR study of the magnetic and metal-insulator transitions in Na0.5CoO2: a nesting scenario

Co and Na NMR are used to probe the local susceptibility and charge state of the two Co sites of the Na-ordered orthorhombic Na(0.5)CoO(2). Above T(N) = 86 K, both sites display a similar T dependence of the spin shift, suggesting that there is no charge segregation into Co(3+) and Co(4+) sites. Below T(N), the magnetic long range commensurate order found is only slightly affected by the metal-insulator transition at T(MIT) = 51 K. Furthermore, the electric field gradient at the Co site does not change at these transitions, indicating the absence of charge ordering. All these observations can be explained by successive spin-density wave induced by two nestings of the Fermi surface specific to the x = 0.5 Na ordering.

Charge and spin ordering in insulating Na0.5CoO2: effects of correlation and symmetry

Ab initio band theory including correlations due to intra-atomic repulsion is applied to study charge disproportionation and charge and spin ordering in insulating Na0.5CoO2. Various ordering patterns (zigzag and two striped) for four-Co supercells are analyzed before focusing on the observed "out-of-phase stripe" pattern of antiferromagnetic Co4+ spins along charge-ordered stripes. This pattern relieves frustration and shows distinct analogies with the cuprate layers: a bipartite lattice of antialigned spins, with axes at 90degrees angles. Substantial distinctions with cuprates are also discussed, including the tiny gap of a new variant of "charge-transfer" type within the Co 3d system.

Neutron scattering study of novel magnetic order in Na0.5CoO2

We report polarized and unpolarized neutron scattering measurements of the magnetic order in single crystals of Na0.5CoO2. Our data indicate that below TN=88 K the spins form a novel antiferromagnetic pattern within the CoO2 planes, consisting of alternating rows of ordered and nonordered Co ions. The domains of magnetic order appear to be closely coupled to the domains of Na ion order, consistent with such a twofold symmetric spin arrangement. Magnetoresistance and anisotropic susceptibility measurements further support this model for the electronic ground state.

Effect of dynamical coulomb correlations on the fermi surface of Na0.3CoO2

The t(2g) quasiparticle spectra of Na(0.3)CoO(2) are calculated within the dynamical mean field theory. It is shown that as a result of dynamical Coulomb correlations charge is transferred from the nearly filled e(g(')) subbands to the a(1g) band, thereby reducing orbital polarization among Co t(2g) states. Dynamical correlations therefore stabilize the small e(g(')) Fermi surface pockets, in contrast to angle-resolved photoemission data, which do not reveal these pockets.