52 research outputs found
Entanglement between a qubit and the environment in the spin-boson model
The quantitative description of the quantum entanglement between a qubit and
its environment is considered. Specifically, for the ground state of the
spin-boson model, the entropy of entanglement of the spin is calculated as a
function of , the strength of the ohmic coupling to the environment,
and , the level asymmetry. This is done by a numerical
renormalization group treatment of the related anisotropic Kondo model. For
, the entanglement increases monotonically with , until it
becomes maximal for . For fixed , the entanglement
is a maximum as a function of for a value, .Comment: 4 pages, 3 figures. Shortened version restricted to groundstate
entanglemen
Real-Time-RG Analysis of the Dynamics of the Spin-Boson Model
Using a real-time renormalization group method we determine the complete
dynamics of the spin-boson model with ohmic dissipation for coupling strengths
. We calculate the relaxation and dephasing time, the
static susceptibility and correlation functions. Our results are consistent
with quantum Monte Carlo simulations and the Shiba relation. We present for the
first time reliable results for finite cutoff and finite bias in a regime where
perturbation theory in or in tunneling breaks down. Furthermore, an
unambigious comparism to results from the Kondo model is achieved.Comment: 4 pages, 5 figures, 1 tabl
Magnetotransport through a strongly interacting quantum dot
We study the effect of a magnetic field on the conductance through a strongly
interacting quantum dot by using the finite temperature extension of Wilson's
numerical renormalization group method to dynamical quantities. The quantum dot
has one active level for transport and is modelled by an Anderson impurity
attached to left and right electron reservoirs. Detailed predictions are made
for the linear conductance and the spin-resolved conductance as a function of
gate voltage, temperature and magnetic field strength. A strongly coupled
quantum dot in a magnetic field acts as a spin filter which can be tuned by
varying the gate voltage. The largest spin-filtering effect is found in the
range of gate voltages corresponding to the mixed valence regime of the
Anderson impurity model.Comment: Revised version, to appear in PRB, 4 pages, 4 figure
Anderson impurity model at finite Coulomb interaction U: generalized Non-crossing Approximation
We present an extension of the non-crossing approximation (NCA), which is
widely used to calculate properties of Anderson impurity models in the limit of
infinite Coulomb repulsion , to the case of finite . A
self-consistent conserving pseudo-particle representation is derived by
symmetrizing the usual NCA diagrams with respect to empty and doubly occupied
local states. This requires an infinite summation of skeleton diagrams in the
generating functional thus defining the ``Symmetrized finite-U NCA'' (SUNCA).
We show that within SUNCA the low energy scale (Kondo temperature) is
correctly obtained, in contrast to other simpler approximations discussed in
the literature.Comment: 7 pages, 6 figure
Fano resonance in electronic transport through a quantum wire with a side-coupled quantum dot: X-boson treatment
The transport through a quantum wire with a side coupled quantum dot is
studied. We use the X-boson treatment for the Anderson single impurity model in
the limit of . The conductance presents a minimum for values of T=0
in the crossover from mixed-valence to Kondo regime due to a destructive
interference between the ballistic channel associated with the quantum wire and
the quantum dot channel. We obtain the experimentally studied Fano behavior of
the resonance. The conductance as a function of temperature exhibits a
logarithmic and universal behavior, that agrees with recent experimental
results.Comment: 6 pages, 10 eps figs., revtex
Many-body theory of the quantum mirage
In recent scanning tunneling microscopy experiments, confinement in an
elliptical corral has been used to project the Kondo effect from one focus to
the other one. I solve the Anderson model at arbitrary temperatures, for an
impurity hybridized with eigenstates of an elliptical corral, each of which has
a resonant level width delta. This width is crucial. If delta < 20 meV, the
Kondo peak disappears, while if delta > 80 meV, the mirage disappears. For
particular conditions, a stronger mirage with the impurity out of the foci is
predicted.Comment: 5 pages, 5 figures. Some clarifications of the method added, and a
reference included to show that the hybridization of the impurity with bulk
states can be neglecte
Thermodynamic properties of the periodic Anderson model:X-boson treatment
We study the specific dependence of the periodic Anderson Model (PAM) in the
limit of employing the X-boson treatment in two fifferent regimes of
the PAM: the heavy fermion Kondo (HF-K) and the heavy fermion local magnetic
regime (HF-LMM). We obtain a multiple peak structure for the specific heat in
agreement with experimental results as well as the increase of the electronic
effective mass at low temperatures associated with the HF-K regime. The entropy
per site at low T tends to zero in the HF-K regime, corresponding to a singlet
ground state, and it tends to in the HF-LMM, corresponding to a
doublet ground state at each site. The linear coefficient
of the specific heat qualitatively agrees with the experimental results
obtained for differents materials in the two regimes considered here.Comment: 9 pages, 14 figure
Wilson chains are not thermal reservoirs
Wilson chains, based on a logarithmic discretization of a continuous
spectrum, are widely used to model an electronic (or bosonic) bath for Kondo
spins and other quantum impurities within the numerical renormalization group
method and other numerical approaches. In this short note we point out that
Wilson chains can not serve as thermal reservoirs as their temperature changes
by a number of order Delta E when a finite amount of energy Delta E is added.
This proves that for a large class of non-equilibrium problems they cannot be
used to predict the long-time behavior.Comment: 2 page
Kondo effect induced by a magnetic field
We study peculiarities of transport through a Coulomb blockade system tuned
to the vicinity of the spin transition in its ground state. Such transitions
can be induced in practice by application of a magnetic field. Tunneling of
electrons between the dot and leads mixes the states belonging to the ground
state manifold of the dot. Remarkably, both the orbital and spin degrees of
freedom of the electrons are engaged in the mixing at the singlet-triplet
transition point. We present a model which provides an adequate theoretical
description of recent experiments with semiconductor quantum dots and carbon
nanotubes
Oscillations of the magnetic polarization in a Kondo impurity at finite magnetic fields
The electronic properties of a Kondo impurity are investigated in a magnetic
field using linear response theory. The distribution of electrical charge and
magnetic polarization are calculated in real space. The (small) magnetic field
does not change the charge distribution. However, it unmasks the Kondo cloud.
The (equal) weight of the d-electron components with their magnetic moment up
and down is shifted and the compensating s-electron clouds don't cancel any
longer (a requirement for an experimental detection of the Kondo cloud). In
addition to the net magnetic polarization of the conduction electrons an
oscillating magnetic polarization with a period of half the Fermi wave length
is observed. However, this oscillating magnetic polarization does not show the
long range behavior of Rudermann-Kittel-Kasuya-Yosida oscillations because the
oscillations don't extend beyond the Kondo radius. They represent an internal
electronic structure of the Kondo impurity in a magnetic field. PACS: 75.20.Hr,
71.23.An, 71.27.+
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