1,688 research outputs found
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
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
Variational approach to transport in quantum dots
We have derived a variational principle that defines the nonequilibrium
steady-state transport across a correlated impurity mimicking, e.g., a quantum
dot coupled to biased leads. This variational principle has been specialized to
a Gutzwiller's variational space, and applied to the study of the simple
single-orbital Anderson impurity model at half filling, finding a good
qualitative accord with the observed behavior in quantum dots for the expected
regime of values of the bias. Beyond the purely theoretical interest in the
formal definition of a variational principle in a nonequilibrium problem, the
particular methods proposed have the important advantage to be simple and
flexible enough to deal with more complicated systems and variational spaces.Comment: 15 pages, 4 figure
Localized states due to expulsion of resonant impurity levels from the continuum in bilayer graphene
Anderson impurity problem is considered for a graphene bilayer subject to a
gap-opening bias. In-gap localized states are produced even when the impurity
level overlaps with the continuum of band electrons. The effect depends
strongly on the polarity of the applied bias as long as hybridization with the
impurity occurs within a single layer. For an impurity level inside the
conduction band a positive bias creates the new localized in-gap state. A
negative bias does not produce the same result and leads to a simple broadening
of the impurity level. The implications for transport are discussed including a
possibility of gate-controlled Kondo effect.Comment: 5 pages, 2 figure
Friedel sum rule for an interacting multiorbital quantum dot
A generalized Friedel sum rule is derived for a quantum dot with internal
orbital and spin degrees of freedom. The result is valid when all many-body
correlations are taken into account and it links the phase shift of the
scattered electron to the displacement of its SPECTRAL density into the dot.Comment: RevTeX 4.0, 5 page
Phase diagram, energy scales and nonlocal correlations in the Anderson lattice model
We study the Anderson lattice model with one f-orbital per lattice site as
the simplest model which describes generic features of heavy fermion materials.
The resistivity and magnetic susceptibility results obtained within dynamical
mean field theory (DMFT) for a nearly half-filled conduction band show the
existence of a single energy scale which is similar to the single ion
Kondo temperature . To determine the importance of inter-site
correlations, we have also solved the model within cellular DMFT (CDMFT) with
two sites in a unit cell. The antiferromagnetic region on the phase diagram is
much narrower than in the single-site solution, having a smaller critical
hybridization and N\'eel temperature . At temperatures above
the nonlocal correlations are small, and the DMFT paramagnetic solution is in
this case practically exact, which justifies the ab initio LDA+DMFT approach in
theoretical studies of heavy fermions. Strong inter-site correlations in the
CDMFT solution for , however, indicate that they have to be properly
treated in order to unravel the physical properties near the quantum critical
point.Comment: 10 page
Slave-boson approach to the infinite-U Anderson-Holstein impurity model
The infinite- Anderson-Holstein impurity model is studied with a focus on
the interplay between the strong electron correlation and the weak
electron-phonon interaction. The slave boson method has been employed in
combination with the large degeneracy expansion (1/N) technique. The charge and
spin susceptibilities and the phonon propagator are obtained in the
approximation scheme where the saddle point configuration and the Gaussian 1/N
fluctuations are taken into account. The spin susceptibility is found not to be
renormalized by electron-phonon interaction, while the charge susceptibility is
renormalized.
From the renormalized charge susceptibility the Kondo temperature is found to
increase by the electron-phonon interaction. It turns out that the bosonic 1/N
Gaussian fluctuations play a very crucial role, in particular, for the phonon
propagator.Comment: 12pages, 3 figures. Published in Physical Review
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