Localization of strongly correlated electrons as Jahn-Teller polarons in manganites

A realistic modeling of manganites should include the Coulomb repulsion between $e_g$ electrons, the Hund's rule coupling to $t_{2g}$ spins, and Jahn-Teller phonons. Solving such a model by dynamical mean field theory, we report large magnetoresistances and spectra in good agreement with experiments. The physics of the unusual, insulating-like paramagnetic phase is determined by correlated electrons which are-due to strong correlations-easily trapped as Jahn-Teller polarons.Comment: 4 pages, 3 figure

An interactive graphics system to facilitate finite element structural analysis

The characteristics of an interactive graphics systems to facilitate the finite element method of structural analysis are described. The finite element model analysis consists of three phases: (1) preprocessing (model generation), (2) problem solution, and (3) postprocessing (interpretation of results). The advantages of interactive graphics to finite element structural analysis are defined

Comparing pertinent effects of antiferromagnetic fluctuations in the two and three dimensional Hubbard model

We use the dynamical vertex approximation (D$\Gamma$A) with a Moriyaesque $\lambda$ correction for studying the impact of antiferromagnetic fluctuations on the spectral function of the Hubbard model in two and three dimensions. Our results show the suppression of the quasiparticle weight in three dimensions and dramatically stronger impact of spin fluctuations in two dimensions where the pseudogap is formed at low enough temperatures. Even in the presence of the Hubbard subbands, the origin of the pseudogap at weak-to-intermediate coupling is in the splitting of the quasiparticle peak. At stronger coupling (closer to the insulating phase) the splitting of Hubbard subbands is expected instead. The $\mathbf{k}$-dependence of the self energy appears to be also much more pronounced in two dimensions as can be observed in the $\mathbf{k}$-resolved D$\Gamma$A spectra, experimentally accessible by angular resolved photoemission spectroscopy in layered correlated systems.Comment: 10 pages, 12 figure

Projective Quantum Monte Carlo Method for the Anderson Impurity Model and its Application to Dynamical Mean Field Theory

We develop a projective quantum Monte Carlo algorithm of the Hirsch-Fye type for obtaining ground state properties of the Anderson impurity model. This method is employed to solve the self-consistency equations of dynamical mean field theory. It is shown that the approach converges rapidly to the ground state so that reliable zero-temperature results are obtained. As a first application, we study the Mott-Hubbard metal-insulator transition of the one-band Hubbard model, reconfirming the numerical renormalization group results.Comment: 4 pages, 4 figure

Reply to a Comment on ``Projective Quantum Monte Carlo Method for the Anderson Impurity Model and its Application to Dynamical Mean Field Theory''

In our reply, we show that the objections put forward in cond-mat/0508763 concerning our paper, Phys. Rev. Lett. 93, 136405 (2004), are not valid: (i) There is no orthogonality catastrophe (OC) for our calculations, and it is also generally not ``unpractical'' to avoid it. (ii) The OC does not affect our results.Comment: 1 page, 1 figure, Phys. Rev. Lett. in print; also note cond-mat/050944

Orbital-selective Mott-Hubbard transition in the two-band Hubbard model

Recent advances in the field of quantum Monte Carlo simulations for impurity problems allow --within dynamical mean field theory-- for a more thorough investigation of the two-band Hubbard model with narrow/wide band and SU(2)-symmetric Hund's exchange. The nature of this transition has been controversial, and we establish that an orbital-selective Mott-Hubbard transition exists. Thereby, the wide band still shows metallic behavior after the narrow band became insulating -not a pseudogap as for an Ising Hund's exchange. The coexistence of two solutions with metallic wide band and insulating or metallic narrow band indicates, in general, first-order transitions.Comment: 4 pages, 3 figures; 2nd version as published in Phys. Rev. B (R); minor corrections, putting more emphasis on differences in spectra when comparing SU(2) and Ising Hund's exchang

Pressure-induced metal-insulator transition in LaMnO3 is not of Mott-Hubbard type

Calculations employing the local density approximation combined with static and dynamical mean-field theories (LDA+U and LDA+DMFT) indicate that the metal-insulator transition observed at 32 GPa in paramagnetic LaMnO3 at room temperature is not a Mott-Hubbard transition, but is caused by orbital splitting of the majority-spin eg bands. For LaMnO3 to be insulating at pressures below 32 GPa, both on-site Coulomb repulsion and Jahn-Teller distortion are needed.Comment: 4 pages, 3 figure

Dynamical mean field theory for manganites

Doped and undoped manganites are modeled by the coupling between itinerant $e_g$ electrons and static $t_{2g}$ spins, the Jahn-Teller and breathing phonon modes, and the Coulomb interaction. We provide for a careful estimate of all parameters and solve the corresponding Hamiltonian by dynamical mean field theory. Our results for the one-electron spectrum, the optical conductivity, the dynamic and static lattice distortion, as well as the Curie temperature show the importance of all of the above ingredients for a realistic calculation as well as for describing the unusual dynamical properties of manganites including the insulating parent compound and the insulating-like paramagnetic state of doped manganites.Comment: 11 pages, 18 figures In the 2nd version the only change is to correct one (important) referenc

On the Number of Iterations for Dantzig-Wolfe Optimization and Packing-Covering Approximation Algorithms

We give a lower bound on the iteration complexity of a natural class of Lagrangean-relaxation algorithms for approximately solving packing/covering linear programs. We show that, given an input with $m$ random 0/1-constraints on $n$ variables, with high probability, any such algorithm requires $\Omega(\rho \log(m)/\epsilon^2)$ iterations to compute a $(1+\epsilon)$-approximate solution, where $\rho$ is the width of the input. The bound is tight for a range of the parameters $(m,n,\rho,\epsilon)$. The algorithms in the class include Dantzig-Wolfe decomposition, Benders' decomposition, Lagrangean relaxation as developed by Held and Karp [1971] for lower-bounding TSP, and many others (e.g. by Plotkin, Shmoys, and Tardos [1988] and Grigoriadis and Khachiyan [1996]). To prove the bound, we use a discrepancy argument to show an analogous lower bound on the support size of $(1+\epsilon)$-approximate mixed strategies for random two-player zero-sum 0/1-matrix games