724 research outputs found
Non-perturbative approaches to magnetism in strongly correlated electron systems
The microscopic basis for the stability of itinerant ferromagnetism in
correlated electron systems is examined. To this end several routes to
ferromagnetism are explored, using both rigorous methods valid in arbitrary
spatial dimensions, as well as Quantum Monte Carlo investigations in the limit
of infinite dimensions (dynamical mean-field theory). In particular we discuss
the qualitative and quantitative importance of (i) the direct Heisenberg
exchange coupling, (ii) band degeneracy plus Hund's rule coupling, and (iii) a
high spectral density near the band edges caused by an appropriate lattice
structure and/or kinetic energy of the electrons. We furnish evidence of the
stability of itinerant ferromagnetism in the pure Hubbard model for appropriate
lattices at electronic densities not too close to half-filling and large enough
. Already a weak direct exchange interaction, as well as band degeneracy, is
found to reduce the critical value of above which ferromagnetism becomes
stable considerably. Using similar numerical techniques the Hubbard model with
an easy axis is studied to explain metamagnetism in strongly anisotropic
antiferromagnets from a unifying microscopic point of view.Comment: 11 pages, Latex, and 6 postscript figures; Z. Phys. B, in pres
Realistic modeling of strongly correlated electron systems: An introduction to the LDA+DMFT approach
The LDA+DMFT approach merges conventional band structure theory in the local
density approximation (LDA) with a state-of-the-art many-body technique, the
dynamical mean-field theory (DMFT). This new computational scheme has recently
become a powerful tool for ab initio investigations of real materials with
strong electronic correlations. In this paper an introduction to the basic
ideas and the set-up of the LDA+DMFT approach is given. Results for the
photoemission spectra of the transition metal oxide La_{1-x}Sr_xTiO_3, obtained
by solving the DMFT-equations by quantum Monte-Carlo (QMC) simulations, are
presented and are found to be in very good agreement with experiment. The
numerically exact DMFT(QMC) solution is compared with results obtained by two
approximative solutions, i.e., the iterative perturbation theory and the
non-crossing approximation.Comment: 15 pages, 3 figures, SCES-Y2K Conference Proceeding
Orbital-selective Mott transitions in the anisotropic two-band Hubbard model at finite temperatures
The anisotropic degenerate two-orbital Hubbard model is studied within
dynamical mean-field theory at low temperatures. High-precision calculations on
the basis of a refined quantum Monte Carlo (QMC) method reveal that two
distinct orbital-selective Mott transitions occur for a bandwidth ratio of 2
even in the absence of spin-flip contributions to the Hund exchange. The second
transition -- not seen in earlier studies using QMC, iterative perturbation
theory, and exact diagonalization -- is clearly exposed in a low-frequency
analysis of the self-energy and in local spectra.Comment: 4 pages, 5 figure
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