754 research outputs found
Anderson impurity in a correlated conduction band
We investigate the physics of a magnetic impurity with spin 1/2 in a
correlated metallic host. Describing the band by a Hubbard Hamiltonian, the
problem is analyzed using dynamical mean-field-theory in combination with
Wilson's nonperturbative numerical renormalization group. We present results
for the single-particle density of states and the dynamical spin susceptibility
at zero temperature. New spectral features (side peaks) are found which should
be observable experimentally. In addition, we find a general enhancement of the
Kondo scale due to correlations. Nevertheless, in the metallic phase, the Kondo
scale always vanishes exponentially in the limit of small hybridization.Comment: Final version, 4 pages RevTeX, 8 eps figures include
Numerical Renormalization Group for Impurity Quantum Phase Transitions: Structure of Critical Fixed Points
The numerical renormalization group method is used to investigate zero
temperature phase transitions in quantum impurity systems, in particular in the
particle-hole symmetric soft-gap Anderson model. The model displays two stable
phases whose fixed points can be built up of non-interacting single-particle
states. In contrast, the quantum phase transitions turn out to be described by
interacting fixed points, and their excitations cannot be described in terms of
free particles. We show that the structure of the many-body spectrum of these
critical fixed points can be understood using renormalized perturbation theory
close to certain values of the bath exponents which play the role of critical
dimensions. Contact is made with perturbative renormalization group
calculations for the soft-gap Anderson and Kondo models. A complete description
of the quantum critical many-particle spectra is achieved using suitable
marginal operators; technically this can be understood as epsilon-expansion for
full many-body spectra.Comment: 14 pages, 12 figure
Gap formation and soft phonon mode in the Holstein model
We investigate electron-phonon coupling in many-electron systems using
dynamical mean-field theory in combination with the numerical renormalization
group. This non-perturbative method reveals significant precursor effects to
the gap formation at intermediate coupling strengths. The emergence of a soft
phonon mode and very strong lattice fluctuations can be understood in terms of
Kondo-like physics due to the development of a double-well structure in the
effective potential for the ions
Spectral scaling and quantum critical behaviour in the pseudogap Anderson model
The pseudogap Anderson impurity model provides a classic example of an
essentially local quantum phase transition. Here we study its single-particle
dynamics in the vicinity of the symmetric quantum critical point (QCP)
separating generalized Fermi liquid and local moment phases, via the local
moment approach. Both phases are shown to be characterized by a low-energy
scale that vanishes at the QCP; and the universal scaling spectra, on all
energy scales, are obtained analytically. The spectrum precisely at the QCP is
also obtained; its form showing clearly the non-Fermi liquid, interacting
nature of the fixed point.Comment: 7 pages, 2 figure
Freely and fearlessly : The 1863 New York editors\u27 resolutions
The eighth of June in 1863 was a cool late spring day in New York. In the Astor House Hotel, at one o\u27clock in the afternoon, sixteen journalists representing approximately two million readers assembled for what one New York paper called one of the most remarkable and important meetings which has occurred since the war started. New York journalists had met in 1848 to respond to their depiction in a play titled New York in Slices. Reporters had gathered at the Capitol in Washington, DC, at the start of the war to protest Winfield Scott\u27s decision to limit newspaper transmissions on the telegraph lines without his approval. The editors at the Astor House were standing up for their profession. They drew up nonpartisan resolutions that declared freedom of the press to be a bedrock principle of democratic society, even in wartime—even in time of civil war
Multiple-charge transfer and trapping in DNA dimers
We investigate the charge transfer characteristics of one and two excess
charges in a DNA base-pair dimer using a model Hamiltonian approach. The
electron part comprises diagonal and off-diagonal Coulomb matrix elements such
a correlated hopping and the bond-bond interaction, which were recently
calculated by Starikov [E. B. Starikov, Phil. Mag. Lett. {\bf 83}, 699 (2003)]
for different DNA dimers. The electronic degrees of freedom are coupled to an
ohmic or a super-ohmic bath serving as dissipative environment. We employ the
numerical renormalization group method in the nuclear tunneling regime and
compare the results to Marcus theory for the thermal activation regime. For
realistic parameters, the rate that at least one charge is transferred from the
donor to the acceptor in the subspace of two excess electrons significantly
exceeds the rate in the single charge sector. Moreover, the dynamics is
strongly influenced by the Coulomb matrix elements. We find sequential and pair
transfer as well as a regime where both charges remain self-trapped. The
transfer rate reaches its maximum when the difference of the on-site and
inter-site Coulomb matrix element is equal to the reorganization energy which
is the case in a GC-GC dimer. Charge transfer is completely suppressed for two
excess electrons in AT-AT in an ohmic bath and replaced by damped coherent
electron-pair oscillations in a super-ohmic bath. A finite bond-bond
interaction alters the transfer rate: it increases as function of when
the effective Coulomb repulsion exceeds the reorganization energy (inverted
regime) and decreases for smaller Coulomb repulsion
Access to diagnosis and treatment of Chagas disease/infection in endemic and non-endemic countries in the XXI century.
In this article, Médicos Sin Fronteras (MSF) Spain faces the challenge of selecting, piecing together, and conveying in the clearest possible way, the main lessons learnt over the course of the last seven years in the world of medical care for Chagas disease. More than two thousand children under the age of 14 have been treated; the majority of whom come from rural Latin American areas with difficult access. It is based on these lessons learnt, through mistakes and successes, that MSF advocates that medical care for patients with Chagas disease be a reality, in a manner which is inclusive (not exclusive), integrated (with medical, psychological, social, and educational components), and in which the patient is actively followed. This must be a multi-disease approach with permanent quality controls in place based on primary health care (PHC). Rapid diagnostic tests and new medications should be available, as well as therapeutic plans and patient management (including side effects) with standardised flows for medical care for patients within PHC in relation to secondary and tertiary level, inclusive of epidemiological surveillance systems
Numerical renormalization group study of the symmetric Anderson-Holstein model: phonon and electron spectral functions
We study the symmetric Anderson-Holstein (AH) model at zero temperature with
Wilson's numerical renormalization group (NRG) technique to study the interplay
between the electron-electron and electron-phonon interactions. An improved
method for calculating the phonon propagator using the NRG technique is
presented, which turns out to be more accurate and reliable than the previous
works in that it calculates the phonon renormalization explicitly and satisfies
the boson sum rule better. The method is applied to calculate the renormalized
phonon propagators along with the electron propagators as the onsite Coulomb
repulsion and electron-phonon coupling constant are varied. As is
increased, the phonon mode is successively renormalized, and for crosses over to the regime where the mode splits into two components,
one of which approaches back to the bare frequency and the other develops into
a soft mode. The initial renormalization of the phonon mode, as is
increased from 0, depends on and the hybridization ; it gets
softened (hardened) for . Correlated with
the emergence of the soft mode is the central peak of the electron spectral
function severely suppressed. These NRG calculations will be compared with the
standard Green's function results for the weak coupling regime to understand
the phonon renormalization and soft mode.Comment: 18 pages, 4 figures. Submitted to Phys. Rev.
A spin-dependent local moment approach to the Anderson impurity model
We present an extension of the local moment approach to the Anderson impurity
model with spin-dependent hybridization. By employing the two-self-energy
description, as originally proposed by Logan and co-workers, we applied the
symmetry restoration condition for the case with spin-dependent hybridization.
Self-consistent ground states were determined through variational minimization
of the ground state energy. The results obtained with our spin-dependent local
moment approach applied to a quantum dot system coupled to ferromagnetic leads
are in good agreement with those obtained from previous work using numerical
renormalization group calculations
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