1,804 research outputs found
Equivalence of Gutzwiller and slave-boson mean-field theories for multi-band Hubbard models
We demonstrate that a recently introduced slave-boson mean-field theory is
equivalent to our Gutzwiller theory for multi-band Hubbard models with general
onsite interactions. We relate the different objects that appear in both
approaches at zero temperature and discuss the limitations of both methods.Comment: 4 page
Perturbation theory for optical excitations in the one-dimensional extended Peierls--Hubbard model
For the one-dimensional, extended Peierls--Hubbard model we calculate
analytically the ground-state energy and the single-particle gap to second
order in the Coulomb interaction for a given lattice dimerization. The
comparison with numerically exact data from the Density-Matrix Renormalization
Group shows that the ground-state energy is quantitatively reliable for Coulomb
parameters as large as the band width. The single-particle gap can almost
triple from its bare Peierls value before substantial deviations appear. For
the calculation of the dominant optical excitations, we follow two approaches.
In Wannier theory, we perturb the Wannier exciton states to second order. In
two-step perturbation theory, similar in spirit to the GW-BSE approach, we form
excitons from dressed electron-hole excitations. We find the Wannier approach
to be superior to the two-step perturbation theory. For singlet excitons,
Wannier theory is applicable up to Coulomb parameters as large as half band
width. For triplet excitons, second-order perturbation theory quickly fails
completely.Comment: 32 pages, 12 figures, submtted to JSTA
Gutzwiller variational theory for the Hubbard model with attractive interaction
We investigate the electronic and superconducting properties of a negative-U
Hubbard model. For this purpose we evaluate a recently introduced variational
theory based on Gutzwiller-correlated BCS wave functions. We find significant
differences between our approach and standard BCS theory, especially for the
superconducting gap. For small values of , we derive analytical
expressions for the order parameter and the superconducting gap which we
compare to exact results from perturbation theory.Comment: 10 pages, 2 figure
Antiferromagnetic order in multi-band Hubbard models for iron-pnictides
We investigate multi-band Hubbard models for the three iron 3-
bands and the two iron 3- bands in by means of the
Gutzwiller variational theory. Our analysis of the paramagnetic ground state
shows that neither Hartree--Fock mean-field theories nor effective spin models
describe these systems adequately. In contrast to Hartree--Fock-type
approaches, the Gutzwiller theory predicts that antiferromagnetic order
requires substantial values of the local Hund's-rule exchange interaction. For
the three-band model, the antiferromagnetic moment fits experimental data for a
broad range of interaction parameters. However, for the more appropriate
five-band model, the iron electrons polarize the electrons and
they substantially contribute to the ordered moment.Comment: 4 pages, 4 figure
Comparison of Variational Approaches for the Exactly Solvable 1/r-Hubbard Chain
We study Hartree-Fock, Gutzwiller, Baeriswyl, and combined
Gutzwiller-Baeriswyl wave functions for the exactly solvable one-dimensional
-Hubbard model. We find that none of these variational wave functions is
able to correctly reproduce the physics of the metal-to-insulator transition
which occurs in the model for half-filled bands when the interaction strength
equals the bandwidth. The many-particle problem to calculate the variational
ground state energy for the Baeriswyl and combined Gutzwiller-Baeriswyl wave
function is exactly solved for the~-Hubbard model. The latter wave
function becomes exact both for small and large interaction strength, but it
incorrectly predicts the metal-to-insulator transition to happen at infinitely
strong interactions. We conclude that neither Hartree-Fock nor Jastrow-type
wave functions yield reliable predictions on zero temperature phase transitions
in low-dimensional, i.e., charge-spin separated systems.Comment: 23 pages + 3 figures available on request; LaTeX under REVTeX 3.
Brueckner-Goldstone perturbation theory for the half-filled Hubbard model in infinite dimensions
We use Brueckner-Goldstone perturbation theory to calculate the ground-state
energy of the half-filled Hubbard model in infinite dimensions up to fourth
order in the Hubbard interaction. We obtain the momentum distribution as a
functional derivative of the ground-state energy with respect to the bare
dispersion relation. The resulting expressions agree with those from
Rayleigh-Schroedinger perturbation theory. Our results for the momentum
distribution and the quasi-particle weight agree very well with those obtained
earlier from Feynman-Dyson perturbation theory for the single-particle
self-energy. We give the correct fourth-order coefficient in the ground-state
energy which was not calculated accurately enough from Feynman-Dyson theory due
to the insufficient accuracy of the data for the self-energy, and find a good
agreement with recent estimates from Quantum Monte-Carlo calculations.Comment: 15 pages, 8 fugures, submitted to JSTA
Spectral function of the one-dimensional Hubbard model away from half filling
We calculate the photoemission spectral function of the one-dimensional
Hubbard model away from half filling using the dynamical density matrix
renormalization group method. An approach for calculating momentum-dependent
quantities in finite open chains is presented. Comparison with exact Bethe
Ansatz results demonstrates the unprecedented accuracy of our method. Our
results show that the photoemission spectrum of the quasi-one-dimensional
conductor TTF-TCNQ provides evidence for spin-charge separation on the scale of
the conduction band width.Comment: REVTEX, 4 pages including 4 EPS figures (changed); correct chemical
potential used to define excitation energies in figures and tex
Strong-coupling approach to the Mott--Hubbard insulator on a Bethe lattice in Dynamical Mean-Field Theory
We calculate the Hubbard bands for the half-filled Hubbard model on a Bethe
lattice with infinite coordination number up to and including third order in
the inverse Hubbard interaction. We employ the Kato--Takahashi perturbation
theory to solve the self-consistency equation of the Dynamical Mean-Field
Theory analytically for the single-impurity Anderson model in multi-chain
geometry. The weight of the secondary Hubbard sub-bands is of fourth order so
that the two-chain geometry is sufficient for our study. Even close to the
Mott--Hubbard transition, our results for the Mott--Hubbard gap agree very well
with those from numerical Dynamical Density-Matrix Renormalization Group
(DDMRG) calculations. The density of states of the lower Hubbard band also
agrees very well with DDMRG data, apart from a resonance contribution at the
upper band edge which cannot be reproduced in low-order perturbation theory.Comment: 40 pages, 7 figure
Optical excitations of Peierls-Mott insulators with bond disorder
The density-matrix renormalization group (DMRG) is employed to calculate
optical properties of the half-filled Hubbard model with nearest-neighbor
interactions. In order to model the optical excitations of oligoenes, a Peierls
dimerization is included whose strength for the single bonds may fluctuate.
Systems with up to 100 electrons are investigated, their wave functions are
analyzed, and relevant length-scales for the low-lying optical excitations are
identified. The presented approach provides a concise picture for the size
dependence of the optical absorption in oligoenes.Comment: 12 pages, 13 figures, submitted to Phys. Rev.
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