The role of Coulomb correlations in the iron pnictide LaFeAsO is studied by generalizing exact diagonalization dynamical mean field theory to five orbitals. For rotationally invariant Hund's rule coupling a transition from a paramagnetic Fermi-liquid phase to a non-Fermi-liquid metallic phase exhibiting frozen moments is found at moderate Coulomb energies. For Ising-like exchange, this transition occurs at a considerably lower critical Coulomb energy. The correlation-induced scattering rate as a function of doping relative to half-filling, i.e., δ = n/5 − 1, where n = 6 for the undoped material, is shown to be qualitatively similar to the one in the two-dimensional single-band Hubbard model which is commonly used to study strong correlations in high-Tc cuprates. In this scenario, the parent Mott insulator of LaFeAsO is the half-filled n = 5 limit, while the undoped n = 6 material corresponds to the critical doping region δc ≈ 0.2 in the cuprates, on the verge between the Fermi-liquid phase of the overdoped region and the non-Fermi-liquid pseudogap phase in the underdoped region.PACS. 71.20.Be Transition metals and alloys -71.27+a Strongly correlated electron systems

Dynamical mean field theory (DMFT), combined with finite-temperature exact diagonalization, is one of the methods used to describe electronic properties of strongly correlated materials. Because of the rapid growth of the Hilbert space, the size of the finite bath used to represent the infinite lattice is severely limited. In view of the increasing interest in the effect of multi-orbital and multi-site Coulomb correlations in transition metal oxides, high-T(c) cuprates, iron-based pnictides, organic crystals, etc, it is appropriate to explore the range of temperatures and bath sizes in which exact diagonalization provides accurate results for various system properties. On the one hand, the bath must be large enough to achieve a sufficiently dense level spacing, so that useful spectral information can be derived, especially close to the Fermi level. On the other hand, for an adequate projection of the lattice Green's function onto a finite bath, the choice of the temperature is crucial. The role of these two key ingredients in exact diagonalization DMFT is discussed for a wide variety of systems in order to establish the domain of applicability of this approach. Three criteria are used to illustrate the accuracy of the results: (i) the convergence of the self-energy with the bath size, (ii) the quality of the discretization of the bath Green's function, and (iii) comparisons with complementary results obtained via continuous-time quantum Monte Carlo DMFT. The materials comprise a variety of three-orbital and five-orbital systems, as well as single-band Hubbard models for two-dimensional triangular, square and honeycomb lattices, where non-local Coulomb correlations are important. The main conclusion from these examples is that a larger number of correlated orbitals or sites requires a smaller number of bath levels. Down to temperatures of 5-10 meV (for typical bandwidths W ≈ 2 eV) two bath levels per correlated impurity orbital or site are usually adequate.

A new concept is proposed for the paramagnetic metal insulator transition in the layer perovskite Ca2−xSrxRuO4. Whereas the pure Sr compound is metallic up to very large Coulomb energies due to strong orbital fluctuations, structural changes induced by doping with Ca give rise to a interorbital charge transfer which makes the material extremely sensitive to local correlations. Using dynamical mean field theory based on finite temperature multi-band exact diagonalization it is shown that the combination of crystal field splitting and onsite Coulomb interactions leads to complete filling of the dxy band and to a Mott transition in the half-filled dxz,yz bands.DOI:

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The phase diagrams of isotropic and anisotropic triangular lattices with local Coulomb interactions are evaluated within cluster dynamical mean-field theory. As a result of partial geometric frustration in the anisotropic lattice, short-range correlations are shown to give rise to re-entrant behavior which is absent in the fully frustrated isotropic limit. The qualitative features of the phase diagrams including the critical temperatures are in good agreement with experimental data for the layered organic charge-transfer salts -͑BEDT-TTF͒ 2 Cu͓N͑CN͒ 2 ͔Cl and -͑BEDT-TTF͒ 2 Cu 2 ͑CN͒ 3 .