177 research outputs found
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
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
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
Numerical Renormalization Group for Bosonic Systems and Application to the Subohmic Spin-Boson Model
We describe the generalization of Wilson's Numerical Renormalization Group
method to quantum impurity models with a bosonic bath, providing a general
non-perturbative approach to bosonic impurity models which can access
exponentially small energies and temperatures. As an application, we consider
the spin-boson model, describing a two-level system coupled to a bosonic bath
with power-law spectral density, J(omega) ~ omega^s. We find clear evidence for
a line of continuous quantum phase transitions for subohmic bath exponents
0<s<1; the line terminates in the well-known Kosterlitz-Thouless transition at
s=1. Contact is made with results from perturbative renormalization group, and
various other applications are outlined.Comment: 4 pages, 5 figs, (v2) final version as publishe
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
Numerical Renormalization Group for Quantum Impurities in a Bosonic Bath
We present a detailed description of the recently proposed numerical
renormalization group method for models of quantum impurities coupled to a
bosonic bath. Specifically, the method is applied to the spin-boson model, both
in the Ohmic and sub-Ohmic cases. We present various results for static as well
as dynamic quantities and discuss details of the numerical implementation,
e.g., the discretization of a bosonic bath with arbitrary continuous spectral
density, the suitable choice of a finite basis in the bosonic Hilbert space,
and questions of convergence w.r.t. truncation parameters. The method is shown
to provide high-accuracy data over the whole range of model parameters and
temperatures, which are in agreement with exact results and other numerical
data from the literature.Comment: 23 pages, 21 figures; three references and one figure adde
SU(4) Fermi Liquid State and Spin Filtering in a Double Quantum Dot System
We study a symmetrical double quantum dot (DD) system with strong capacitive
inter-dot coupling using renormalization group methods. The dots are attached
to separate leads, and there can be a weak tunneling between them. In the
regime where there is a single electron on the DD the low-energy behavior is
characterized by an SU(4)-symmetric Fermi liquid theory with entangled spin and
charge Kondo correlations and a phase shift . Application of an external
magnetic field gives rise to a large magneto-conductance and a crossover to a
purely charge Kondo state in the charge sector with SU(2) symmetry. In a four
lead setup we find perfectly spin polarized transmission.Comment: 4 pages, 4 figures, ReVTe
Metallic and Insulating Phases of Repulsively Interacting Fermions in a 3D Optical Lattice
The fermionic Hubbard model plays a fundamental role in the description of
strongly correlated materials. Here we report on the realization of this
Hamiltonian using a repulsively interacting spin mixture of ultracold K
atoms in a 3D optical lattice. We have implemented a new method to directly
measure the compressibility of the quantum gas in the trap using in-situ
imaging and independent control of external confinement and lattice depth.
Together with a comparison to ab-initio Dynamical Mean Field Theory
calculations, we show how the system evolves for increasing confinement from a
compressible dilute metal over a strongly-interacting Fermi liquid into a band
insulating state. For strong interactions, we find evidence for an emergent
incompressible Mott insulating phase.Comment: 21 pages, 5 figures and additional supporting materia
A gentle introduction to the functional renormalization group: the Kondo effect in quantum dots
The functional renormalization group provides an efficient description of the
interplay and competition of correlations on different energy scales in
interacting Fermi systems. An exact hierarchy of flow equations yields the
gradual evolution from a microscopic model Hamiltonian to the effective action
as a function of a continuously decreasing energy cutoff. Practical
implementations rely on suitable truncations of the hierarchy, which capture
nonuniversal properties at higher energy scales in addition to the universal
low-energy asymptotics. As a specific example we study transport properties
through a single-level quantum dot coupled to Fermi liquid leads. In
particular, we focus on the temperature T=0 gate voltage dependence of the
linear conductance. A comparison with exact results shows that the functional
renormalization group approach captures the broad resonance plateau as well as
the emergence of the Kondo scale. It can be easily extended to more complex
setups of quantum dots.Comment: contribution to Les Houches proceedings 2006, Springer styl
Frustration of Decoherence in Open Quantum Systems
We study a model of frustration of decoherence in an open quantum system.
Contrary to other dissipative ohmic impurity models, such as the Kondo model or
the dissipative two-level system, the impurity model discussed here never
presents overdamped dynamics even for strong coupling to the environment. We
show that this unusual effect has its origins in the quantum mechanical nature
of the coupling between the quantum impurity and the environment. We study the
problem using analytic and numerical renormalization group methods and obtain
expressions for the frequency and temperature dependence of the impurity
susceptibility in different regimes.Comment: 14 pages, 5 figure
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