976 research outputs found
Thermoelectric transport through strongly correlated quantum dots
The thermoelectric properties of strongly correlated quantum dots, described
by a single level Anderson model coupled to conduction electron leads, is
investigated using Wilson's numerical renormalization group method. We
calculate the electronic contribution, , to the thermal conductance,
the thermopower, , and the electrical conductance, , of a quantum dot as
a function of both temperature, , and gate voltage, , for strong,
intermediate and weak Coulomb correlations, , on the dot. For strong
correlations and in the Kondo regime, we find that the thermopower exhibits two
sign changes, at temperatures and with
. Such sign changes in are particularly sensitive
signatures of strong correlations and Kondo physics. The relevance of this to
recent thermopower measurements of Kondo correlated quantum dots is discussed.
We discuss the figure of merit, power factor and the degree of violation of the
Wiedemann-Franz law in quantum dots. The extent of temperature scaling in the
thermopower and thermal conductance of quantum dots in the Kondo regime is also
assessed.Comment: 21 pages, 12 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
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
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
Out of equilibrium transport through an Anderson impurity: Probing scaling laws within the equation of motion approach
We study non-equilibrium electron transport through a quantum impurity
coupled to metallic leads using the equation of motion technique at finite
temperature T. Assuming that the interactions are taking place solely in the
impurity and focusing in the infinite Hubbard limit, we compute the out of
equilibrium density of states and the differential conductance G_2(T,V) to test
several scaling laws. We find that G_2(T,V)/G_2(T,0) is a universal function of
both eV/T_K and T/T_K, being T_K the Kondo temperature. The effect of an in
plane magnetic field on the splitting of the zero bias anomaly in the
differential conductance is also analyzed. For a Zeeman splitting \Delta, the
computed differential conductance peak splitting depends only on \Delta/T_K,
and for large fields approaches the value of 2\Delta . Besides the traditional
two leads setup, we also consider other configurations that mimics recent
experiments, namely, an impurity embedded in a mesoscopic wire and the presence
of a third weakly coupled lead. In these cases, a double peak structure of the
Kondo resonance is clearly obtained in the differential conductance while the
amplitude of the highest peak is shown to decrease as \ln(eV/T_K). Several
features of these results are in qualitative agreement with recent experimental
observations reported on quantum dots.Comment: 9 pages, 7 figure
Renormalization Group Approach to Non-equilibrium Green Functions in Correlated Impurity Systems
We present a technique for calculating non-equilibrium Green functions for
impurity systems with local interactions. We use an analogy to the calculation
of response functions in the x-ray problem.The initial state and the final
state problems, which correspond to the situations before and after the
disturbance (an electric or magnetic field, for example) is suddenly switched
on, are solved with the aid of Wilson's momentum shell renormalization group.
The method is illustrated by calculating the non-equilibrium dynamics of the
ohmic two-state problem.Comment: 7 pages, 2 figure
Inhibitor Specificity via Protein Dynamics Insights from the Design of Antibacterial Agents Targeted Against Thymidylate Synthase
AbstractStructure-based drug design of species-specific inhibitors generally exploits structural differences in proteins from different organisms. Here, we demonstrate how achieving specificity can be aided by targeting differences in the dynamics of proteins. Thymidylate synthase (TS) is a good target for anticancer agents and a potential target for antibacterial agents. Most inhibitors are folate-analogs that bind at the folate binding site and are not species specific. In contrast, α156 is not a folate-analog and is specific for bacterial TS; it has been shown crystallographically to bind in a nonconserved binding site. Docking calculations and crystal structure-based estimation of the essential dynamics of TSs from five different species show that differences in the dynamics of TSs make the active site more accessible to α156 in the prokaryotic than in the eukaryotic TSs and thereby enhance the specificity of α156
Rehabilitation of COPD patients: which training modality?
Non pharmacological therapy has been gaining more interest and has been evolving rapidly over the last decade as an essential part of therapy for COPD patients. Pulmonary Rehabilitation (PR), the most important non pharmacological treatment in patients with COPD, has a primary goal: to achieve the highest possible level of individual exercise tolerance, thus reducing the primary and/or secondary health care utilisation. The aim of the present review is to focus the role of exercise training in these patients as well as to address the question on which training methods are the most beneficial. We have therefore undertaken a MEDLINE-based search including the terms: pulmonary rehabilitation, exercise, lung disease/obstructive. Several strategies based on endurance or strength training are nowadays implemented during PR programmes in order to maximise the benefits for each patient. The impaired function of ambulation muscles causing breathlessness as one of the more frequent symptoms in many COPD, suggests that training the lower extremities is the most important goal to achieve during pulmonary rehabilitation of these patients. On the other hand, as muscle strength appears to be an independent contributor to survival and utilisation of health care resources, it seems largely justified also to include this further modality in the PR program of these patients. In conclusion, both modalities are effective and useful for COPD patients. However, whether resistance training should be administered to all COPD and which is the optimal length of strength training still needs to be elucidated
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
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