10,323 research outputs found
Localization of strongly correlated electrons as Jahn-Teller polarons in manganites
A realistic modeling of manganites should include the Coulomb repulsion
between electrons, the Hund's rule coupling to 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 (DA) with a Moriyaesque 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 -dependence of the self energy appears to be also much more
pronounced in two dimensions as can be observed in the -resolved
DA 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
electrons and static 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 random 0/1-constraints
on variables, with high probability, any such algorithm requires
iterations to compute a
-approximate solution, where is the width of the input.
The bound is tight for a range of the parameters .
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
-approximate mixed strategies for random two-player zero-sum
0/1-matrix games
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