78 research outputs found
Spin and Charge Structure Factor of the 2-d Hubbard Model
The spin and charge structure factors are calculated for the Hubbard model on
the square lattice near half-filling using a spin-rotation invariant six-slave
boson representation. The charge structure factor shows a broad maximum at the
zone corner and is found to decrease monotonically with increasing interaction
strength and electron density and increasing temperature. The spin structure
factor develops with increasing interaction two incommensurate peaks at the
zone boundary and along the zone diagonal. Comparison with results of Quantum
Monte Carlo and variational calculations is carried out and the agreement is
found to be good. The limitations of an RPA-type approach are pointed out.Comment: 18 pages, revtex, 13 postscript figures, submitted to Phys. Rev.
Pseudogap and photoemission spectra in the attractive Hubbard model
Angle-resolved photoemission spectra are calculated microscopically for the
two-dimensional attractive Hubbard model. A system of self-consistent T-matrix
equations are solved numerically in the real-time domain. The single-particle
spectral function has a two-peak structure resulting from the presense of bound
states. The spectral function is suppressed at the chemical potential, leading
to a pseudogap-like behavior. At high temperatures and densities the pseudogap
diminishes and finally disappears; these findings are similar to experimental
observations for the cuprates.Comment: 5 pages, 4 figures, published versio
On the correct continuum limit of the functional-integral representation for the four-slave-boson approach to the Hubbard model: Paramagnetic phase
The Hubbard model with finite on-site repulsion U is studied via the
functional-integral formulation of the four-slave-boson approach by Kotliar and
Ruckenstein. It is shown that a correct treatment of the continuum imaginary
time limit (which is required by the very definition of the functional
integral) modifies the free energy when fluctuation (1/N) corrections beyond
mean-field are considered. Our analysis requires us to suitably interpret the
Kotliar and Ruckenstein choice for the bosonic hopping operator and to abandon
the commonly used normal-ordering prescription, in order to obtain meaningful
fluctuation corrections. In this way we recover the exact solution at U=0 not
only at the mean-field level but also at the next order in 1/N. In addition, we
consider alternative choices for the bosonic hopping operator and test them
numerically for a simple two-site model for which the exact solution is readily
available for any U. We also discuss how the 1/N expansion can be formally
generalized to the four-slave-boson approach, and provide a simplified
prescription to obtain the additional terms in the free energy which result at
the order 1/N from the correct continuum limit.Comment: Changes: Printing problems (due to non-standard macros) have been
removed, 44 page
Superconductivity in the Hubbard model with correlated hopping: Slave-boson study
The slave boson mean-field studies of the ground state of the Hubbard model
with correlated hopping were performed. The approach qualitatively recovers the
exact results for the case of the hopping integral t equal to the correlated
hopping integral X. The phase diagram for the strongly correlated state with
only singly occupied sites, the weakly correlated state, where single and
double occupation is allowed, and for the superconducting state, was determined
for any values of X and any electron concentration n. At the half-filled band
(n=1) a direct transition from the superconductor to the Mott insulator was
found. In the region of strong correlations the superconducting solution is
stable for n close to 1, in contrast to the case of weak correlations, in which
superconductivity occurs at n close to 0 and n close to 2. We found also that
strong correlations change characteristics of the superconducting phase, e.g.
the gap in the excitation spectrum has a nonexponential dependence close to the
point of the phase transition.Comment: 13 pages, 24 Postscript figures (in 12 files
Slave boson model for two-dimensional trapped Bose-Einstein condensate
A system of N bosons in a two-dimensional harmonic trap is considered. The
system is treated in term of the slave boson representation for hard-core
bosons which is valid in the arbitrary density regimes. I discuss the
consequences of higher order interactions on the density profiles by mapping
the slave boson equation to the known Kohn-Sham type equation within the
density functional scheme.Comment: 12 pages, 3 figures. Submitted to J. Phys. B : At. mol. opt. phy
Static overscreening and nonlinear response in the Hubbard Model
We investigate the static charge response for the Hubbard model. Using the
Slave-Boson method in the saddle-point approximation we calculate the charge
susceptibility. We find that RPA works quite well close to half-filling,
breaking, of course, down close to the Mott transition. Away from half filling
RPA is much less reliable: Already for very small values of the Hubbard
interaction U, the linear response becomes much more efficient than RPA,
eventually leading to overscreening already beyond quite moderate values of U.
To understand this behavior we give a simple argument, which implies that the
response to an external perturbation at large U should actually be strongly
non-linear. This prediction is confirmed by the results of exact
diagonalization.Comment: 10 pages, 7 figures, RevTe
Friedel oscillations induced by non-magnetic impurities in the two-dimensional Hubbard model
We study the interplay of correlations and disorder using an unrestricted
Slave-Boson technique in real space. Within the saddle-point approximation, we
find Friedel oscillations of the charge density in the vicinity of a
nonmagnetic impurity, in agreement with numerical simulations. The
corresponding amplitudes are suppressed by repulsive interactions, while
attractive correlations lead to a charge-density-wave enhancement. In addition,
we investigate the spatial dependence of the local magnetic moment and the
formation of a magnetic state at the impurity site.Comment: 9 pages, RevTeX, includes 8 figure
Density-Induced Breaking of Pairs in the Attractive Hubbard Model
A conserving T-matrix approximation is applied to the two-dimensional
attractive Hubbard model in the low-density regime. A set of self-consistent
equations is solved in the real-frequency domain to avoid the analytic
continuation procedure. By tuning the chemical potential the particle density
was varied in the limits 0.01 < n < 0.18. For the value of the attractive
potential U=8t the binding energy of pairs monotonically decreases with
increasing n, from its zero-density limit 2.3t and vanishes at a critical
density n=0.19. A pairing-induced pseudogap in the single-particle density of
states is found at low densities and temperatures.Comment: 5 pages, 4 figures, accepted for publication in Phys. Rev. Let
Time-dependent Gutzwiller theory of pairing fluctuations in the Hubbard model
We present a method to compute pairing fluctuations on top of the Gutzwiller
approximation (GA). Our investigations are based on a charge-rotational
invariant GA energy functional which is expanded up to second order in the pair
fluctuations. Equations of motion for the fluctuations lead to a renormalized
ladder type approximation. Both spectral functions and corrections to static
quantities, like the ground-state energy, are computed. The quality of the
method is examined for the single-band Hubbard model where we compare the
dynamical pairing correlations for s- and d-wave symmetries with exact
diagonalizations and find a significant improvement with respect to analogous
calculations done within the standard Hartree-Fock ladder approximation. The
technique has potential applications in the theory of Auger spectroscopy,
superconductivity, and cold atom physics.Comment: 17 pages, 7 figure
Effective three-band model for double perovskites
We start from a six-band model describing the transition-metal t2g orbitals
of half-metallic double perovskite systems, such as Sr2FeMoO6, in which only
one of the transition metal ions (Fe) contains important intratomic repulsion
Ufe. By eliminating the Mo orbitals using a low-energy reduction similar to
that used in the cuprates, we construct a Hamiltonian which contains only
effective t2g Fe orbitals. This allows to treat exactly Ufe, and most of the
Fe-Mo hopping. As an application, we treat the effective Hamiltonian in the
slave-boson mean-field approximation and calculate the position of the
metal-insulator transition and other quantities as a function of pressure or
on-site energy difference.Comment: 8 pages, 3 figure
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