839 research outputs found
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
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
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
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
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
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
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
Scaling and universality in the anisotropic Kondo model and the dissipative two-state system
Scaling and universality in the Ohmic two-state system is investigated by
exploiting the equivalence of this model to the anisotropic Kondo model. For
the Ohmic two-state system, we find universal scaling functions for the
specific heat, , static susceptibility, , and
spin relaxation function depending on the reduced
temperature (frequency ), with
the renormalized tunneling frequency, and uniquely specified by the dissipation
strength (). The scaling functions can be used to extract
and in experimental realizations.Comment: 5 pages (LaTeX), 4 EPS figures. Minor changes, typos corrected,
journal reference adde
Unified description of Fermi and non-Fermi liquid behavior in a conserving slave boson approximation for strongly correlated impurity models
We show that the presence of Fermi or non-Fermi liquid behavior in the SU(N)
x SU(M) Anderson impurity models may be read off the infrared threshold
exponents governing the spinon and holon dynamics in a slave boson
representation of these models. We construct a conserving T-matrix
approximation which recovers the exact exponents with good numerical accuracy.
Our approximation includes both coherent spin flip scattering and charge
fluctuation processes. For the single-channel case the tendency to form bound
states drastically modifies the low energy behavior. For the multi-channel case
in the Kondo limit the bound state contributions are unimportant.Comment: 4 pages, Latex, 3 postscript figures included Final version with
minor changes in wording, to appear in Phys.Rev.Let
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