Covalency, double-counting and the metal-insulator phase diagram in transition metal oxides

Dynamical mean field theory calculations are used to show that for late transition-metal-oxides a critical variable for the Mott/charge-transfer transition is the number of d-electrons, which is determined by charge transfer from oxygen ions. Insulating behavior is found only for a narrow range of d-occupancy, irrespective of the size of the intra-d Coulomb repulsion. The result is useful in interpreting 'density functional +U' and 'density functional plus dynamical mean field' methods in which additional correlations are applied to a specific set of orbitals and an important role is played by the 'double counting correction' which dictates the occupancy of these correlated orbitals. General considerations are presented and are illustrated by calculations for two representative transition metal oxide systems: layered perovskite Cu-based "high-Tc" materials, an orbitally non-degenerate electronically quasi-two dimensional systems, and pseudocubic rare earch nickelates, an orbitally degenerate electronically three dimensional system. Density functional calculations yield d-occupancies very far from the Mott metal-insulator phase boundary in the nickelate materials, but closer to it in the cuprates, indicating the sensitivity of theoretical models of the cuprates to the choice of double counting correction and corroborating the critical role of lattice distortions in attaining the experimentally observed insulating phase in the nickelates.Comment: 10+ pages, 5 figure

Mott transition at large orbital degeneracy: dynamical mean-field theory

We study analytically the Mott transition of the N-orbital Hubbard model using dynamical mean-field theory and a low-energy projection onto an effective Kondo model. It is demonstrated that the critical interaction at which the insulator appears (Uc1) and the one at which the metal becomes unstable (Uc2) have different dependence on the number of orbitals as the latter becomes large: Uc1 ~ \sqrt{N} while Uc2 ~ N. An exact analytical determination of the critical coupling Uc2/N is obtained in the large-N limit. The metallic solution close to this critical coupling has many similarities at low-energy with the results of slave boson approximations, to which a comparison is made. We also discuss how the critical temperature associated with the Mott critical endpoint depends on the number of orbitals.Comment: 13 pages. Minor changes in V

k-dependent spectrum and optical conductivity near metal-insulator transition in multi-orbital Hubbard bands

We apply the dynamical mean field theory (DMFT) in the iterative perturbation theory(IPT) to doubly degenerate eg bands and triply degenerate tg bands on a simple cubic lattice and calculate the spectrum and optical conductivity in arbitrary electron occupation. The spectrum simultaneously shows the effects of multiplet structure and DMFT together with the electron ionization and affinity levels of different electron occupations, coherent peaks at the Fermi energy in the metallic phase and a gap at an integer filling of electrons for sufficiently large Coulomb U. We also calculate the critical value of the Coulomb U for degenerate orbitals.Comment: 8 pages, 6 figure

Quantum impurity solvers using a slave rotor representation

We introduce a representation of electron operators as a product of a spin-carry ing fermion and of a phase variable dual to the total charge (slave quantum rotor). Based on this representation, a new method is proposed for solving multi-orbital Anderson quantum impurity models at finite interaction strength U. It consists in a set of coupled integral equations for the auxiliary field Green's functions, which can be derived from a controlled saddle-point in the limit of a large number of field components. In contrast to some finite-U extensions of the non-crossing approximation, the new method provides a smooth interpolation between the atomic limit and the weak-coupling limit, and does not display violation of causality at low-frequency. We demonstrate that this impurity solver can be applied in the context of Dynamical Mean-Field Theory, at or close to half-filling. Good agreement with established results on the Mott transition is found, and large values of the orbital degeneracy can be investigated at low computational cost.Comment: 18 pages, 15 figure

Effects of degenerate orbitals on the Hubbard model

Stability of a metallic state in the two-orbital Hubbard model at half-filling is investigated. We clarify how spin and orbital fluctuations are enhanced to stabilize the formation of quasi-particles by combining dynamical mean field theory with the quantum Monte Carlo simulations. These analyses shed some light on the reason why the metallic phase is particularly stable when the intra- and inter-band Coulomb interactions are nearly equal.Comment: 3 pages, To appear in JPSJ Vol. 72, No. 5 200

The Influence of Quantum Critical Fluctuations of Circulating Current Order Parameters on the Normal State Properties of Cuprates

We study a model of the quantum critical point of cuprates associated with the "circulating current" order parameter proposed by Varma. An effective action of the order parameter in the quantum disordered phase is derived using functional integral method, and the physical properties of the normal state are studied based on the action. The results derived within the ladder approximation indicate that the system is like Fermi liquid near the quantum critical point and in disordered regime up to minor corrections. This implies that the suggested marginal Fermi liquid behavior induced by the circulating current fluctuations will come in from beyond the ladder diagrams.Comment: 7pages, 1 figure included in RevTex file. To appear in Phys. Rev.

Selective separation and preconcentration of Th(IV) using organo-functionalized, hierarchically porous silica monoliths

The potential application of thorium (Th) as nuclear fuel, as well as the environmental and public health concerns associated with it, promotes the development of economic and sustainable materials for the separation and removal of Th(IV) from minerals and environmental samples. In this work, centimeter-size, porous silica monoliths exhibiting hierarchical macroporosity-mesoporosity -and a robust silica skeleton were prepared using a sol−gel process combined with post-synthetic hydrothermal treatment in ammonium hydroxide. Upon functionalization with diglycolamide (DGA), the monolithic silica was used as a column-type fixed bed sorbent for continuous flow extraction. An enhanced Th(IV) uptake from aqueous solution was achieved with high enrichment factor and selectivity in the presence of competitive ions such as rare earth elements (REEs) and uranium (U). Systematic mechanistic studies show that the hierarchical pore system is crucial for enhanced adsorption kinetics and capacity. Two mineral leachates were used to assess the performances of the hybrid material, and despite the complex ion matrix and high ionic composition, the sorbent shows highly efficient recovery of Th(IV). The material was able to undergo 10 extraction-stripping-regeneration cycles, which bode well for potential industrial applications