879 research outputs found
Impact of climate induced glacial melting on coastal marine systems in the Western Antarctic Peninsula region
IMCOAST is an international research program that features a multidisciplinary approach involving geo and biological sciences, field investigations, remote sensing and modeling and knowledge into the hydrographical and biological history of the marine coastal ecosystems of the Western Antarctic Peninsula region
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
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
Kondo effect in a magnetic field and the magnetoresistivity of Kondo alloys
The effect of a magnetic field on the spectral density of a
Kondo impurity is investigated at zero and finite temperatures by using
Wilson's numerical renormalization group method. A splitting of the total
spectral density is found for fields larger than a critical value
, where is the Kondo scale. The splitting
correlates with a peak in the magnetoresistivity of dilute magnetic alloys
which we calculate and compare with the experiments on
. The linear magnetoconductance of quantum
dots exhibiting the Kondo effect is also calculated.Comment: 4 pages, 4 eps figure
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
Transport Coefficients of the Anderson Model via the Numerical Renormalization Group
The transport coefficients of the Anderson model are calculated by extending
Wilson's NRG method to finite temperature Green's functions. Accurate results
for the frequency and temperature dependence of the single--particle spectral
densities and transport time are obtained and used to extract
the temperature dependence of the transport coefficients in the strong
correlation limit. The low temperature anomalies in the resistivity, ,
thermopower, , thermal conductivity and Hall coefficient,
, are discussed. All quantities exhibit the expected Fermi liquid
behaviour at low temperature with power law dependecies on in very
good agreement with analytic results based on Fermi liquid theory. Scattering
of conduction electrons in higher, , angular momentum channels is also
considered and an expression is derived for the corresponding transport time
and used to discuss the influence of non--resonant scattering on the transport
properties.Comment: 45 pages, RevTeX, 28 figures, available on reques
Mechanism for large thermoelectric power in negative-U molecular quantum dots
We investigate with the aid of numerical renormalization group techniques the
thermoelectric properties of a molecular quantum dot described by the
negative-U Anderson model. We show that the charge Kondo effect provides a
mechanism for enhanced thermoelectric power via a correlation induced asymmetry
in the spectral function close to the Fermi level. We show that this effect
results in a dramatic enhancement of the Kondo induced peak in the thermopower
of negative-U systems with Seebeck coefficients exceeding 50 over a
wide range of gate voltages.Comment: 4 pages, 4 figures; published versio
The numerical renormalization group method for quantum impurity systems
In the beginning of the 1970's, Wilson developed the concept of a fully
non-perturbative renormalization group transformation. Applied to the Kondo
problem, this numerical renormalization group method (NRG) gave for the first
time the full crossover from the high-temperature phase of a free spin to the
low-temperature phase of a completely screened spin. The NRG has been later
generalized to a variety of quantum impurity problems. The purpose of this
review is to give a brief introduction to the NRG method including some
guidelines of how to calculate physical quantities, and to survey the
development of the NRG method and its various applications over the last 30
years. These applications include variants of the original Kondo problem such
as the non-Fermi liquid behavior in the two-channel Kondo model, dissipative
quantum systems such as the spin-boson model, and lattice systems in the
framework of the dynamical mean field theory.Comment: 55 pages, 27 figures, submitted to Rev. Mod. Phy
Thermoelectric effects in Kondo correlated quantum dots
In this Letter we study thermoelectric effects in ultra small quantum dots.
We study the behaviour of the thermopower, Peltier coefficient and thermal
conductance both in the sequencial tunneling regime and in the regime where
Kondo correlations develope. Both cases of linear response and non-equilibrium
induced by strong temperature gradients are considered. The thermopower is a
very sensitive tool to detect Kondo correlations. It changes sign both as a
function of temperature and temperature gradient. We also discuss violations of
the Wiedemann-Franz law.Comment: 7 pages; 5 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
- …