2,026 research outputs found
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
Orbital selective and tunable Kondo effect of magnetic adatoms on graphene: Correlated electronic structure calculations
We have studied the effect of dynamical correlations on the electronic
structure of single Co adatoms on graphene monolayers with a recently developed
novel method for nanoscopic materials that combines density functional
calculations with a fully dynamical treatment of the strongly interacting
3d-electrons. The coupling of the Co 3d-shell to the graphene substrate and
hence the dynamic correlations are strongly dependent on the orbital symmetry
and the system parameters (temperature, distance of the adatom from the
graphene sheet, gate voltage). When the Kondo effect takes place, we find that
the dynamical correlations give rise to strongly temperature-dependent peaks in
the Co 3d-spectra near the Fermi level. Moreover, we find that the Kondo effect
can be tuned by the application of a gate voltage. It turns out that the
position of the Kondo peaks is pinned to the Dirac points of graphene rather
than to the chemical potential.Comment: 12 pages, 7 figure
Spatially dependent Kondo effect in Quantum Corrals
We study the Kondo screening of a single magnetic impurity inside a
non-magnetic quantum corral located on the surface of a metallic host system.
We show that the spatial structure of the corral's eigenmodes lead to a
spatially dependent Kondo effect whose signatures are spatial variations of the
Kondo temperature, . Moreover, we predict that the Kondo screening is
accompanied by the formation of multiple Kondo resonances with characteristic
spatial patterns. Our results open new possibilities to manipulate and explore
the Kondo effect by using quantum corrals.Comment: 4 pages 5 figure
Magnetoconductance through a vibrating molecule in the Kondo regime
The effect of a magnetic field on the equilibrium spectral and transport
properties of a single-molecule junction is studied using the numerical
renormalization group method. The molecule is described by the
Anderson-Holstein model in which a single vibrational mode is coupled to the
electron density. The effect of an applied magnetic field on the conductance in
the Kondo regime is qualitatively different in the weak and strong
electron-phonon coupling regimes. In the former case, the Kondo resonance is
split and the conductance is strongly suppressed by a magnetic field , with the Kondo temperature. In the strong
electron-phonon coupling regime a charge analog of the Kondo effect develops.
In this case the Kondo resonance is not split by the field and the conductance
in the Kondo regime is enhanced in a broad range of values of .Comment: 6 pages, 4 figure
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