1,027 research outputs found
Spectral Densities of Response Functions for the O(3) Symmetric Anderson and Two Channel Kondo Models
The O(3) symmetric Anderson model is an example of a system which has a
stable low energy marginal Fermi liquid fixed point for a certain choice of
parameters. It is also exactly equivalent, in the large U limit, to a localized
model which describes the spin degrees of freedom of the linear dispersion two
channel Kondo model. We first use an argument based on conformal field theory
to establish this precise equivalence with the two channel model. We then use
the numerical renormalization group (NRG) approach to calculate both
one-electron and two-electron response functions for a range of values of the
interaction strength U. We compare the behaviours about the marginal Fermi
liquid and Fermi liquid fixed points and interpret the results in terms of a
renormalized Majorana fermion picture of the elementary excitations. In the
marginal Fermi liquid case the spectral densities of all the Majorana fermion
modes display a |omega| dependence on the lowest energy scale, and in addition
the zero Majorana mode has a delta function contribution. The weight of this
delta function is studied as a function of the interaction U and is found to
decrease exponentially with U for large U. Using the equivalence with the two
channel Kondo model in the large U limit, we deduce the dynamical spin
susceptibility of the two channel Kondo model over the full frequency range. We
use renormalized perturbation theory to interpret the results and to calculate
the coefficient of the ln omega divergence found in the low frequency behaviour
of the T=0 dynamic susceptibility.Comment: 26 pages, 18 figures, to be published in Eur. Phys. J.
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
Singular dynamics and pseudogap formation in the underscreened Kondo impurity and Kondo lattice models
We study a generalization of the Kondo model in which the impurity spin is
represented by Abrikosov fermions in a rotation group SU(P) larger than the
SU(N) group associated to the spin of the conduction electrons, thereby forcing
the single electronic bath to underscreen the localized moment. We demonstrate
how to formulate a controlled large N limit preserving the property of
underscreening, and which can be seen as a ``dual'' theory of the multichannel
large N equations usually associated to overscreening. Due to the anomalous
scattering on the uncompensated degrees of freedom, the Fermi liquid
description of the electronic fluid is invalidated, with the logarithmic
singularities known to occur in the S=1 SU(2) Kondo impurity model being
replaced by continuous power laws at N=\infty. The present technique can be
extended to tackle the related underscreened Kondo lattice model in the large N
limit. We discover the occurence of an insulating pseudogap regime in place of
the expected renormalized metallic phase of the fully screened case, preventing
the establishement of coherence over the lattice. This work and the recent
observation of a similar weakly insulating behavior on transport in CeCuAs_2
should give momentum for further studies of underscreened impurity models on
the lattice.Comment: 9 pages, 3 figures. Several modifications in published version,
including new title, further details on the interpretation of the formalism
and possible experimental connection
Spectral properties of locally correlated electrons in a BCS superconductor
We present a detailed study of the spectral properties of a locally
correlated site embedded in a BCS superconducting medium. To this end the
Anderson impurity model with superconducting bath is analysed by numerical
renormalisation group (NRG) calculations. We calculate one and two-particle
dynamic response function to elucidate the spectral excitation and the nature
of the ground state for different parameter regimes with and without
particle-hole symmetry. The position and weight of the Andreev bound states is
given for all relevant parameters. We also present phase diagrams for the
different ground state parameter regimes. This work is also relevant for
dynamical mean field theory extensions with superconducting symmetry breaking.Comment: 22 pages, 12 figure
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
Testing for redshift evolution of Type Ia supernovae using the strongly lensed PS1-10afx at
The light from distant supernovae (SNe) can be magnified through
gravitational lensing when a foreground galaxy is located along the line of
sight. This line-up allows for detailed studies of SNe at high redshift that
otherwise would not be possible. Spectroscopic observations of lensed
high-redshift Type Ia supernovae (SNe Ia) are of particular interest since they
can be used to test for evolution of their intrinsic properties. The use of SNe
Ia for probing the cosmic expansion history has proven to be an extremely
powerful method for measuring cosmological parameters. However, if systematic
redshift-dependent properties are found, their usefulness for future surveys
could be challenged. We investigate whether the spectroscopic properties of the
strongly lensed and very distant SN Ia PS1-10afx at deviates from the
well-studied populations of normal SNe Ia at nearby or intermediate distance.
We created median spectra from nearby and intermediate-redshift
spectroscopically normal SNe Ia from the literature at -5 and +1 days from
light-curve maximum. We then compared these median spectra to those of
PS1-10afx. We do not find signs of spectral evolution in PS1-10afx. The
observed deviation between PS1-10afx and the median templates are within what
is found for SNe at low- and intermediate-redshift. There is a noticeable broad
feature centred at ~\AA{}, which is present only to a
lesser extent in individual low and intermediate redshift SN Ia spectra. From a
comparison with a recently developed explosion model, we find this feature to
be dominated by iron peak elements, in particular, singly ionized cobalt and
chromium.Comment: accepted for publication in section 4. Extragalactic astronomy of
Astronomy and Astrophysic
iPTF16abc and the population of Type Ia supernovae: Comparing the photospheric, transitional and nebular phases
Key information about the progenitor system and the explosion mechanism of
Type Ia supernovae (SNe~Ia) can be obtained from early observations, within a
few days from explosion. iPTF16abc was discovered as a young SN~Ia with
excellent early time data. Here, we present photometry and spectroscopy of the
SN in the nebular phase. A comparison of the early time data with a sample of
SNe~Ia shows distinct features, differing from normal SNe~Ia at early phases
but similar to normal SNe~Ia at a few weeks after maximum light (i.e. the
transitional phase) and well into the nebular phase. The transparency
timescales () for this sample of SNe~Ia range between 25 and 41
days indicating a diversity in the ejecta masses. also weakly correlates
with the peak bolometric luminosity, consistent with the interpretation that
SNe with higher ejecta masses would produce more Ni. Comparing the
and the maximum luminosity, L\, distribution of a sample of SNe~Ia to
predictions from a wide range of explosion models we find an indication that
the sub-Chandrasekhar mass models span the range of observed values. However,
the bright end of the distribution can be better explained by Chandrasekhar
mass delayed detonation models, hinting at multiple progenitor channels to
explain the observed bolometric properties of SNe~Ia. iPTF16abc appears to be
consistent with the predictions from the M models.Comment: 13 pages, 8 figures, accepted for publication in MNRA
Electron Transfer in Donor-Acceptor Systems: Many-Particle Effects and Influence of Electronic Correlations
We investigate electron transfer processes in donor-acceptor systems with a
coupling of the electronic degrees of freedom to a common bosonic bath. The
model allows to study many-particle effects and the influence of the local
Coulomb interaction U between electrons on donor and acceptor sites. Using the
non-perturbative numerical renormalization group approach we find distinct
differences between the electron transfer characteristics in the single- and
two-particle subspaces. We calculate the critical electron-boson coupling
alpha_c as a function of and show results for density-density correlation
functions in the whole parameter space. The possibility of many-particle
(bipolaronic) and Coulomb-assisted transfer is discussed.Comment: 4 pages, 4 figure
Nonresonant inelastic light scattering in the Hubbard model
Inelastic light scattering from electrons is a symmetry-selective probe of
the charge dynamics within correlated materials. Many measurements have been
made on correlated insulators, and recent exact solutions in large dimensions
explain a number of anomalous features found in experiments. Here we focus on
the correlated metal, as described by the Hubbard model away from half filling.
We can determine the B1g Raman response and the inelastic X-ray scattering
along the Brillouin zone diagonal exactly in the large dimensional limit. We
find a number of interesting features in the light scattering response which
should be able to be seen in correlated metals such as the heavy fermions.Comment: 9 pages, 7 figures, typeset with ReVTe
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