Spectral functions in doped transition metal oxides

We present experimental photoemission and inverse photoemission spectra of SrTiO$_{3- \delta}$ representing electron doped $d^0$ systems. Photoemission spectra in presence of electron doping exhibit prominent features arising from electron correlation effects, while the inverse photoemssion spectra are dominated by spectral features explainable within single-particle approaches. We show that such a spectral evolution in chemically doped correlated systems is not compatible with expectations based on Hubbard or any other similar model. We present a new theoretical approach taking into account the inhomogeneity of the `real' system which gives qualitatively different results compared to standard `homogeneous' models and is in quantitative agreement with experiments.Comment: 10 pages; 1 tex file+4 postscript files (to appear in Europhysics Letters

Evolution of photoemission spectral functions in doped transition metal oxides

We discuss the experimental photoemission and inverse photoemission of early transition metal oxides, in the light of the dynamical mean field theory of correlated electrons which becomes exact in the limit of infinite dimensions. We argue that a comprehensive description of the experimental data requires spatial inhomogeneities and present a calculation of the evolution of the spectral function in an inhomogenous system with various degrees of inhomogeneity. We also point out that comparaison of experimental results and large d calculations require that the degree of correlation and disorder is larger in the surface than in the bulk

Dynamical Mean Field Theory of the Antiferromagnetic Metal to Antiferromagnetic Insulator Transition

We study the antiferromagnetic metal to antiferromagnetic insulator using dynamical mean field theory and exact diagonalization methods. We find two qualitatively different behaviors depending on the degree of magnetic correlations. For strong correlations combined with magnetic frustration, the transition can be described in terms of a renormalized slater theory, with a continuous gap closure driven by the magnetism but strongly renormalized by correlations. For weak magnetic correlations, the transition is weakly first order.Comment: 4 pages, uses epsfig,4 figures,notational errors rectifie

Magnetotransport in the doped Mott insulator

We investigate the Hall effect and the magnetoresistance of strongly correlated electron systems using the dynamical mean-field theory. We treat the low- and high-temperature limits analytically and explore some aspects of the intermediate-temperature regime numerically. We observe that a bipartite-lattice condition is responsible for the high-temperature result $\sigma_{xy}\sim 1/T^2$ obtained by various authors, whereas the general behavior is $\sigma_{xy}\sim 1/T$, as for the longitudinal conductivity. We find that Kohler's rule is neither obeyed at high nor at intermediate temperatures.Comment: 9 pages, 7 figures, accepted for publication in Phys. Rev.

Persistent currents in mesoscopic rings with a quantum dot

Using the Anderson model in the Kondo regime, we calculate the persistent current j in a ring with an embedded quantum dot (QD) as a function of the Aharonov-Bohm flux Phi for different ring length L, temperature T and broadening of the conduction states delta . For T=delta =0 and L >> xi, where xi is the Kondo screening length, Lj tends to the value for a non interacting ideal ring, while it is suppressed for a side coupled QD. For any L/xi, Lj is also suppressed when either T or delta increase above a fraction of the level spacing which depends on Phi.Comment: 5 pages, 6 figures, submitted to Phys. Rev. B, (Refs. added

Thermoelectric properties of the degenerate Hubbard model

We investigate the thermoelectric properties of a system near a pressure driven Mott-Hubbard transition. The dependence of the thermopower and the figure of merit on pressure and temperature within a degenerate Hubbard model for integer filling n=1 is calculated using dynamical mean field theory. Quantum Monte Carlo method is used to solve the impurity model. Obtained results can qualitatively explain thermoelectric properties of various strongly correlated materials.Comment: RevTex, 7 pages, 6 figure

Thermoelectric Response Near the Density Driven Mott Transition

We investigate the thermoelectric response of correlated electron systems near the density driven Mott transition using the dynamical mean field theory.Comment: 4 pages, 2 embedded figure

Calculation of Optical Conductivity, Resistivity and Thermopower of Filled Skutterudite CeRu4_4Sb12_{12} based on a Realistic Tight-binding Model with Strong Correlation

The filled-skutterudite compound CeRu$_4$Sb$_{12}$ shows a pseudo-gap structure in the optical conductivity spectra similar to the Kondo insulators, but metallic behavior below 80 K. The resistivity shows a large peak at 80 K, and the Seebeck coefficient is positive and also shows a large peak at nearly the same temperature. In order to explain all these features, a simplified tight-binding model, which captures the essential features of the band calculation, is proposed. Using this model and introducing the correlation effect within the framework of the dynamical mean field approximation and the iterative perturbation theory, the temperature dependences of the optical conductivity, resistivity and the Seebeck coefficient are calculated, which can explain the experiments.Comment: 4 pages, 6 figure

Temperature-dependent electronic structure and ferromagnetism in the d=oo Hubbard model studied by a modfied perturbation theory

The infinite-dimensional Hubbard model is studied by means of a modified perturbation theory. The approach reduces to the iterative perturbation theory for weak coupling. It is exact in the atomic limit and correctly reproduces the dispersions and the weights of the Hubbard bands in the strong-coupling regime for arbitrary fillings. Results are presented for the hyper-cubic and an fcc-type lattice. For the latter we find ferromagnetic solutions. The filling-dependent Curie temperature is compared with the results of a recent Quantum Monte Carlo study.Comment: RevTeX, 5 pages, 6 eps figures included, Phys. Rev. B (in press), Ref. 16 correcte

Dynamical correlations in multiorbital Hubbard models: Fluctuation-exchange approximations

We study the two band degenerate Hubbard model using the Fluctuation Exchange approximation (FLEX) method and compare the results with Quantum Monte-Carlo calculations. Both the self-consistent and the non-self-consistent versions of the FLEX scheme are investigated. We find that, contrary to the one band case, in the multiband case, good agreement with the Quantum Monte-Carlo results is obtained within the electron-electron T-matrix approximation using the full renormalization of the one-particle propagators. The crossover to strong coupling and the formation of satellites is more clearly visible in the non-self-consistent scheme. Finally we discuss the behavior of the FLEX for higher orbital degeneracy.Comment: 18 pages with 12 PS figure