1,986 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 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
Quantum Monte Carlo method for models of molecular nanodevices
We introduce a quantum Monte Carlo technique to calculate exactly at finite
temperatures the Green function of a fermionic quantum impurity coupled to a
bosonic field. While the algorithm is general, we focus on the single impurity
Anderson model coupled to a Holstein phonon as a schematic model for a
molecular transistor. We compute the density of states at the impurity in a
large range of parameters, to demonstrate the accuracy and efficiency of the
method. We also obtain the conductance of the impurity model and analyze
different regimes. The results show that even in the case when the effective
attractive phonon interaction is larger than the Coulomb repulsion, a
Kondo-like conductance behavior might be observed.Comment: 5 pages, 4 figure
Kondo resonance line-shape of magnetic adatoms on decoupling layers
The zero-bias resonance in the dI/dV tunneling spectrum recorded using a
scanning tunneling microscope above a spin-1/2 magnetic adatom (such as Ti)
adsorbed on a decoupling layer on metal surface can be accurately fitted using
the universal spectral function of the Kondo impurity model both at zero field
and at finite external magnetic field. Excellent agreement is found both for
the asymptotic low-energy part and for the high-energy logarithmic tails of the
Kondo resonance. For finite magnetic field, the nonlinear fitting procedure
consists in repeatedly solving the impurity model for different Zeeman energies
in order to obtain accurate spectral functions which are compared with the
experimental dI/dV curves. The experimental results at zero field are
sufficiently restraining to enable an unprecedented reliability in the
determination of the Kondo temperature, while at finite fields the results are
more ambiguous and two different interpretations are proposed
Kondo effect of a Co atom on Cu(111) in contact with an Fe tip
Single Co atoms, which exhibit a Kondo effect on Cu(111), are contacted with
Cu and Fe tips in a low-temperature scanning tunneling microscope. With Fe
tips, the Kondo effect persists with the Abrikosov-Suhl resonance significantly
broadened. In contrast, for Cu-covered W tips, the resonance width remains
almost constant throughout the tunneling and contact ranges. The distinct
changes of the line width are interpreted in terms of modifications of the Co d
state occupation owing to hybridization with the tip apex atoms.Comment: 4 pages, 3 figure
Topological Kondo effect with Majorana fermions
The Kondo effect is a striking consequence of the coupling of itinerant
electrons to a quantum spin with degenerate energy levels. While degeneracies
are commonly thought to arise from symmetries or fine-tuning of parameters, the
recent emergence of Majorana fermions has brought to the fore an entirely
different possibility: a "topological degeneracy" which arises from the
nonlocal character of Majorana fermions. Here we show that nonlocal quantum
spins formed from these degrees of freedom give rise to a novel "topological
Kondo effect". This leads to a robust non-Fermi liquid behavior, known to be
difficult to achieve in the conventional Kondo context. Focusing on mesoscopic
superconductor devices, we predict several unique transport signatures of this
Kondo effect, which would demonstrate the non-local quantum dynamics of
Majorana fermions, and validate their potential for topological quantum
computation
Phonon-assisted Kondo Effect in a Single-Molecule Transistor out of Equilibrium
The joint effect of the electron-phonon interaction and Kondo effect on the
nonequilibrium transport through the single molecule transistor is investigated
by using the improved canonical transformation scheme and extended equation of
motion approach. Two types of Kondo phonon-satellites with different asymmetric
shapes are fully confirmed in the spectral function, and are related to the
electron spin singlet or hole spin singlet, respectively. Moreover, when a
moderate Zeeman splitting is caused by a local magnetic field, the Kondo
satellites in the spin resolved spectral function are found disappeared on one
side of the main peak, which is opposite for different spin component. All
these peculiar signatures that manifest themselves in the nonlinear
differential conductance, are explained with a clear physics picture.Comment: 12 pages, 6 figure
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