907 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
Spectral properties of a two-orbital Anderson impurity model across a non-Fermi liquid fixed point
We study by NRG the spectral properties of a two-orbital Anderson impurity
model in the presence of an exchange splitting which follows either regular or
inverted Hund's rules. The phase diagram contains a non-Fermi liquid fixed
point separating a screened phase, where conventional Kondo effect occurs, from
an unscreened one, where the exchange-splitting takes care of quenching the
impurity degrees of freedom. On the Kondo screened side close to this fixed
point the impurity density of states shows a narrow Kondo-peak on top of a
broader resonance. This narrow peak transforms in the unscreened phase into a
narrow pseudo-gap inside the broad resonance. Right at the fixed point only the
latter survives. The fixed point is therefore identified by a jump of the
density of states at the chemical potential. We also show that particle-hole
perturbations which simply shift the orbital energies do not wash out the fixed
point, unlike those perturbations which hybridize the two orbitals.
Consequently the density-of-state jump at the chemical potential remains finite
even away from particle-hole symmetry, and the pseudo-gap stays pinned at the
chemical potential, although it is partially filled in. We also discuss the
relevance of these results for lattice models which map onto this Anderson
impurity model in the limit of large lattice-coordination. Upon approaching the
Mott metal-insulator transition, these lattice models necessarily enter a
region with a local criticality which reflects the impurity non-Fermi liquid
fixed point. However, unlike the impurity, the lattice can get rid of the
single-impurity fixed-point instability by spontaneously developing
bulk-coherent symmetry-broken phases, which we identify for different lattice
models.Comment: 43 pages, 11 figures. Minor corrections in the Appendi
Numerical renormalization group calculation of impurity internal energy and specific heat of quantum impurity models
We introduce a method to obtain the specific heat of quantum impurity models
via a direct calculation of the impurity internal energy requiring only the
evaluation of local quantities within a single numerical renormalization group
(NRG) calculation for the total system. For the Anderson impurity model, we
show that the impurity internal energy can be expressed as a sum of purely
local static correlation functions and a term that involves also the impurity
Green function. The temperature dependence of the latter can be neglected in
many cases, thereby allowing the impurity specific heat, , to be
calculated accurately from local static correlation functions; specifically via
, where and are the
energies of the (embedded) impurity and the hybridization energy, respectively.
The term involving the Green function can also be evaluated in cases where its
temperature dependence is non-negligible, adding an extra term to . For the non-degenerate Anderson impurity model, we show by comparison
with exact Bethe ansatz calculations that the results recover accurately both
the Kondo induced peak in the specific heat at low temperatures as well as the
high temperature peak due to the resonant level. The approach applies to
multiorbital and multichannel Anderson impurity models with arbitrary local
Coulomb interactions. An application to the Ohmic two state system and the
anisotropic Kondo model is also given, with comparisons to Bethe ansatz
calculations. The new approach could also be of interest within other impurity
solvers, e.g., within quantum Monte Carlo techniques.Comment: 16 pages, 15 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
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
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
Evaluating the expression of urokinase and tissue leukocyte being in benign and malignant breast disease
Introduction: Our objectives is to show that the expression of uPA leukocyte could be considered, in the future, as a marker of
the expression of uPA in the malignant tissue and therefore a potential indicator of prognosis.
Methods: We examined the expression of uPa in leukocytes and tissues of three groups of women: with breast cancer; with
benign breast lesion and healthy women (control group). We used RT Real Time PCR assay. The expression of urokinase is
significantly higher in malignant breast lumps compared to benign lesions. However, in women with carcinoma of the breast,
malignant tissue expresses higher amounts of uPA than the healthy counterpart. There are no statistically significant differences in
the expression of uPA, between tissues taken from women with benign lesions. The lymphocytes taken from healthy volunteers
show a level of expression of uPA significantly lower than the other tested samples Lymphocytes extracted from cancer patients
express higher amounts of uPA compared to lymphocytes belonging to women with benign breast lesions. The expression of uPA
was compared with the clinical and biological parameters commonly used in clinical practice for the definition of the prognosis.
The only exception found, concerns those tumors characterized by the simultaneous negativity for estrogen receptors,
progesterone and HER2 (state of triple negative), in which the expression of uPA is very high.
Results and conclusions: Our data show that uPA expressed by leukocytes of each individual patient is the mirror image of the
one expressed by malignant nodular uPA.Introduction: Our objectives is to show that the expression of uPA leukocyte could be considered, in the future, as a marker of
the expression of uPA in the malignant tissue and therefore a potential indicator of prognosis.
Methods: We examined the expression of uPa in leukocytes and tissues of three groups of women: with breast cancer; with
benign breast lesion and healthy women (control group). We used RT Real Time PCR assay. The expression of urokinase is
significantly higher in malignant breast lumps compared to benign lesions. However, in women with carcinoma of the breast,
malignant tissue expresses higher amounts of uPA than the healthy counterpart. There are no statistically significant differences in
the expression of uPA, between tissues taken from women with benign lesions. The lymphocytes taken from healthy volunteers
show a level of expression of uPA significantly lower than the other tested samples Lymphocytes extracted from cancer patients
express higher amounts of uPA compared to lymphocytes belonging to women with benign breast lesions. The expression of uPA
was compared with the clinical and biological parameters commonly used in clinical practice for the definition of the prognosis.
The only exception found, concerns those tumors characterized by the simultaneous negativity for estrogen receptors,
progesterone and HER2 (state of triple negative), in which the expression of uPA is very high.
Results and conclusions: Our data show that uPA expressed by leukocytes of each individual patient is the mirror image of the
one expressed by malignant nodular uPA
Interplay of Coulomb interaction and spin-orbit effects in multi-level quantum dots
We study electron transport through a multi-level quantum dot with Rashba
spin-orbit interaction in the presence of local Coulomb repulsion. Motivated by
recent experiments, we compute the level splitting induced by the spin-orbit
interaction at finite Zeeman fields , which provides a measure of the
renormalized spin-orbit energy. This level splitting is responsible for the
suppression of the Kondo ridges at finite characteristic for the
multi-level structure. In addition, the dependence of renormalized -factors
on the relative orientation of the applied field and the spin-orbit
direction following two different protocols used in experiments is
investigated.Comment: 11 pages, 13 figure
Quantum Magnetic Impurities in Magnetically Ordered Systems
We discuss the problem of a spin 1/2 impurity immersed in a spin S
magnetically ordered background. We show that the problem maps onto a
generalization of the dissipative two level system (DTLS) with two independent
heat baths, associated with the Goldstone modes of the magnet, that couple to
different components of the impurity spin operator. Using analytical
perturbative renormalization group (RG) methods and accurate numerical
renormalization group (NRG) we show that contrary to other dissipative models
there is quantum frustration of decoherence and quasi-scaling even in the
strong coupling regime. We make predictions for the behavior of the impurity
magnetic susceptibility that can be measured in nuclear magnetic resonance
(NMR) experiments. Our results may also have relevance to quantum computation.Comment: 4 pages, 3 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
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