3,221 research outputs found
Field dependent quasiparticles in the infinite dimensional Hubbard model
We present dynamical mean field theory (DMFT) results for the local spectral
densities of the one- and two-particle response functions for the infinite
dimensional Hubbard model in a magnetic field. We look at the different regimes
corresponding to half-filling, near half-filling and well away from
half-filling, for intermediate and strong values of the local interaction .
The low energy results are analyzed in terms of quasiparticles with field
dependent parameters. The renormalized parameters are determined by two
different methods, both based on numerical renormalization group (NRG)
calculations, and we find good agreement. Away from half-filling the
quasiparticle weights, , differ according to the spin type
or . Using the renormalized parameters, we
show that DMFT-NRG results for the local longitudinal and transverse dynamic
spin susceptibilities in an arbitrary field can be understood in terms of
repeated scattering of these quasiparticles. We also check Luttinger's theorem
for the Hubbard model and find it to be satisfied in all parameter regimes and
for all values of the magnetic field.Comment: 14 pages, 21 figure
Magnetic Field Effects on Quasiparticles in Strongly Correlated Local Systems
We show that quasiparticles in a magnetic field of arbitrary strength can
be described by field dependent parameters. We illustrate this approach in the
case of an Anderson impurity model and use the numerical renormalization group
(NRG) to calculate the renormalized parameters for the levels with spin
, , resonance width
and the effective local quasiparticle interaction . In the Kondo or strong correlation limit of the model the progressive
de-renormalization of the quasiparticles can be followed as the magnetic field
is increased. The low temperature behaviour, including the conductivity, in
arbitrary magnetic field can be calculated in terms of the field dependent
parameters using the renormalized perturbation expansion. Using the NRG the
field dependence of the spectral density on higher scales is also calculated.Comment: 15 pages, 17 figure
Renormalized Parameters for Impurity Models
We show that the low energy behaviour of quite diverse impurity systems can
be described by a single renormalized Anderson model, with three parameters, an
effective level , an effective hybridization , and
a quasiparticle interaction . The renormalized parameters are
calculated as a function of the bare parameters for a number of impurity
models, including those with coupling to phonons and a Falikov-Kimball
interaction term. In the model with a coupling to phonons we determine where
the interaction of the quasiparticles changes sign as a function of the
electron-phonon coupling. In the model with a Falikov-Kimball interaction we
show that to a good approximation the low energy behaviour corresponds to that
of a bare Anderson model with a shifted impurity level.Comment: 14 pages, 12 figures; Revised Sec. 2 and
Renormalized parameters and perturbation theory for an n-channel Anderson model with Hund's rule coupling: Asymmetric case
We explore the predictions of the renormalized perturbation theory for an
n-channel Anderson model, both with and without Hund's rule coupling, in the
regime away from particle-hole symmetry. For the model with n=2 we deduce the
renormalized parameters from numerical renormalization group calculations, and
plot them as a function of the occupation at the impurity site, nd. From these
we deduce the spin, orbital and charge susceptibilities, Wilson ratios and
quasiparticle density of states at T=0, in the different parameter regimes,
which gives a comprehensive overview of the low energy behavior of the model.
We compare the difference in Kondo behaviors at the points where nd=1 and nd=2.
One unexpected feature of the results is the suppression of the charge
susceptibility in the strong correlation regime over the occupation number
range 1 <nd <3.Comment: 9 pages, 17 figure
Dynamic response functions for the Holstein-Hubbard model
We present results on the dynamical correlation functions of the
particle-hole symmetric Holstein-Hubbard model at zero temperature, calculated
using the dynamical mean field theory which is solved by the numerical
renormalization group method. We clarify the competing influences of the
electron-electron and electron-phonon interactions particularity at the
different metal to insulator transitions. The Coulomb repulsion is found to
dominate the behaviour in large parts of the metallic regime. By suppressing
charge fluctuations, it effectively decouples electrons from phonons. The
phonon propagator shows a characteristic softening near the metal to
bipolaronic transition but there is very little softening on the approach to
the Mott transition.Comment: 13 pages, 19 figure
High-order current correlation functions in Kondo systems
We examine the statistics of current fluctuations in a junction with a
quantum dot described by Kondo Hamiltonian. With the help of modified Keldish
technique we calculate the third current cumulant. As a function of ratio
the 3rd cumulant was obtained for three different regimes: Fermi
liquid regime (v>1). Unlike
the case of noninteracting dot, 3rd cumulant shows strong non-linear voltage
dependence. Only in the asymptotical limit of large voltages the linear
dependence on is recovered.Comment: 5 pages, 2 figure
Spectral functions for single- and multi-Impurity models using DMRG
This article focuses on the calculation of spectral functions for single- and
multi-impurity models using the density matrix renormalization group (DMRG). To
calculate spectral functions from DMRG, the correction vector method is
presently the most widely used approach. One, however, always obtains
Lorentzian convoluted spectral functions, which in applications like the
dynamical mean-field theory can lead to wrong results. In order to overcome
this restriction we show how to use the Lehmann formula to calculate a peak
spectrum for the spectral function. We show that this peak spectrum is a very
good approximation to a deconvolution of the correction vector spectral
function. Calculating this deconvoluted spectrum directly from the DMRG basis
set and operators is the most natural approach, because it uses only
information from the system itself. Having calculated this excitation spectrum,
one can use an arbitrary broadening to obtain a smooth spectral function, or
directly analyze the excitations. As a nontrivial test we apply this method to
obtain spectral functions for a model of three coupled Anderson impurities.
Although, we are focusing in this article on impurity models, the proposed
method for calculating the peak spectrum can be easily adapted to usual lattice
models.Comment: 11 pages, 14 figure
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