9,232 research outputs found
Microscopic conditions favoring itinerant ferromagnetism: Hund's rule coupling and orbital degeneracy
The importance of Hund's rule coupling for the stabilization of itinerant
ferromagnetism is investigated within a two-band Hubbard model. The magnetic
phase diagram is calculated by finite-temperature quantum Monte Carlo
simulations within the dynamical mean-field theory. Ferromagnetism is found in
a broad range of electron fillings whereas antiferromagnetism exists only near
half filling. The possibility of orbital ordering at quarter filling is also
analyzed.Comment: 5 pages, 6 figures, RevTeX, final version contains an additional
phase diagram for smaller Hund's rule coupling. to appear in Eur. Phys. J. B
(1998
Correlated-Electron Theory of Strongly Anisotropic Metamagnets
We present the first correlated-electron theory of metamagnetism in strongly
anisotropic antiferromagnets. Quantum-Monte-Carlo techniques are used to
calculate the field vs. temperature phase diagram of the infinite-dimensional
Hubbard model with easy axis. A metamagnetic transition scenario with 1.~order
and 2.~order phase transitions is found. The apparent similarities to the phase
diagram of FeBr and to mean-field results for the Ising model with
competing interactions are discussed.Comment: 4 pages, RevTeX + one uuencoded ps-file including 3 figure
Double Exchange model for nanoscopic clusters
We solve the double exchange model on nanoscopic clusters exactly, and
specifically consider a six-site benzene-like nanocluster. This simple model is
an ideal testbed for studying magnetism in nanoclusters and for validating
approximations such as the dynamical mean field theory (DMFT). Non-local
correlations arise between neighboring localized spins due to the Hund's rule
coupling, favoring a short-range magnetic order of ferro- or antiferromagnetic
type. For a geometry with more neighboring sites or a sufficiently strong
hybridization between leads and the nanocluster, these non-local correlations
are less relevant, and DMFT can be applied reliably.Comment: 9 pages, 9 figures, 1 tabl
Two Aspects of the Mott-Hubbard Transition in Cr-doped V_2O_3
The combination of bandstructure theory in the local density approximation
with dynamical mean field theory was recently successfully applied to
VO -- a material which undergoes the f amous Mott-Hubbard
metal-insulator transition upon Cr doping. The aim of this sh ort paper is to
emphasize two aspects of our recent results: (i) the filling of the
Mott-Hubbard gap with increasing temperature, and (ii) the peculiarities of the
Mott-Hubbard transition in this system which is not characterized by a diver
gence of the effective mass for the -orbital.Comment: 2 pages, 3 figures, SCES'04 conference proceeding
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
Dynamical Mean-Field Theory for Molecular Electronics: Electronic Structure and Transport Properties
We present an approach for calculating the electronic structure and transport
properties of nanoscopic conductors that takes into account the dynamical
correlations of strongly interacting d- or f-electrons by combining density
functional theory calculations with the dynamical mean-field theory. While the
density functional calculation yields a static mean-field description of the
weakly interacting electrons, the dynamical mean-field theory explicitly takes
into account the dynamical correlations of the strongly interacting d- or
f-electrons of transition metal atoms. As an example we calculate the
electronic structure and conductance of Ni nanocontacts between Cu electrodes.
We find that the dynamical correlations of the Ni 3d-electrons give rise to
quasi-particle resonances at the Fermi-level in the spectral density. The
quasi-particle resonances in turn lead to Fano lineshapes in the conductance
characteristics of the nanocontacts similar to those measured in recent
experiments of magnetic nanocontacts.Comment: replaced with revised version; 11 pages; 9 figure
Absence of orbital-selective Mott transition in Ca_2-xSr_xRuO4
Quasi-particle spectra of the layer perovskite SrRuO are calculated
within Dynamical Mean Field Theory for increasing values of the on-site Coulomb
energy . At small the planar geometry splits the bands near
into a wide, two-dimensional band and two narrow, nearly
one-dimensional bands. At larger , however, the spectral
distribution of these states exhibit similar correlation features, suggesting a
common metal-insulator transition for all bands at the same critical
.Comment: 4 pages, 4 figure
Critical properties of the half-filled Hubbard model in three dimensions
By means of the dynamical vertex approximation (DA) we include
spatial correlations on all length scales beyond the dynamical mean field
theory (DMFT) for the half-filled Hubbard model in three dimensions. The most
relevant changes due to non-local fluctuations are: (i) a deviation from the
mean-field critical behavior with the same critical exponents as for the three
dimensional Heisenberg (anti)-ferromagnet and (ii) a sizable reduction of the
N\'eel temperature () by for the onset of antiferromagnetic
order. Finally, we give a quantitative estimate of the deviation of the spectra
between DA and DMFT in different regions of the phase-diagram.Comment: 4 pages, 3 figures; accepted for publication in Phys. Rev. Let
Comparative study of correlation effects in CaVO3 and SrVO3
We present parameter-free LDA+DMFT (local density approximation + dynamical
mean field theory) results for the many-body spectra of cubic SrVO3 and
orthorhombic CaVO3. Both systems are found to be strongly correlated metals,
but not on the verge of a metal-insulator transition. In spite of the
considerably smaller V-O-V bond angle in CaVO3 the LDA+DMFT spectra of the two
systems for energies E<E_F are very similar, their quasiparticle parts being
almost identical. The calculated spectrum for E>E_F shows more pronounced,
albeit still small, differences. This is in contrast to earlier theoretical and
experimental conclusions, but in good agreement with recent bulk-sensitive
photoemission and x-ray absorption experiments.Comment: 15 pages, 6 figure
Surface vs. bulk Coulomb correlations in photoemission spectra of perovskites
Recent photoemission spectra of the perovskite series SrCaVO
revealed strong modifications associated with surface contributions. To study
the effect of Coulomb correlations in the bulk and at the surface the
quasi-particle spectra are evaluated using the dynamical mean field theory. It
is shown that as a result of the reduced coordination number of surface atoms
correlation effects are stronger at the surface than in the bulk, in agreement
with experiment.Comment: 4 pages 3 figure
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