3,323 research outputs found
Theory of strong localization effects of light in disordered loss or gain media
We present a systematical theory for the interplay of strong localization
effects and absorption or gain of classical waves in 3-dimensional, disordered
dielectrics. The theory is based on the selfconsistent Cooperon resummation,
implementing the effects of energy conservation and its absorptive or emissive
corrections by an exact, generalized Ward identity. Substantial
renormalizations are found, depending on whether the absorption/gain occurs in
the scatterers or in the background medium. We find a finite, gain-induced
correlation volume which may be significantly smaller than the scale set by the
scattering mean free path, even if there are no truly localized modes. Possible
consequences for coherent feedback in random lasers as well as the possibility
of oscillatory in time behavior induced by sufficiently strong gain are
discussed.Comment: 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
Auxiliary particle theory of threshold singularities in photoemission and X-ray absorption spectra: Test of a conserving T-matrix approximation
We calculate the exponents of the threshold singularities in the
photoemission spectrum of a deep core hole and its X-ray absorption spectrum in
the framework of a systematic many-body theory of slave bosons and
pseudofermions (for the empty and occupied core level). In this representation,
photoemission and X-ray absorption can be understood on the same footing; no
distinction between orthogonality catastrophe and excitonic effects is
necessary. We apply the conserving slave particle T-matrix approximation
(CTMA), recently developed to describe both Fermi and non-Fermi liquid behavior
systems with strong local correlations, to the X-ray problem as a test case.
The numerical results for both photoemission and X-ray absorption are found to
be in agreement with the exact infrared powerlaw behavior in the weak as well
as in the strong coupling regions. We point out a close relation of the CTMA
with the parquet equation approach of Nozi{\`e}res et al.Comment: 10 pages, 9 figures, published versio
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
Fermi and non-Fermi liquid behavior in quantum impurity systems: Conserving slave boson theory
The question of Fermi liquid vs. non-Fermi liquid behavior induced by strong
correlations is one of the prominent problems in metallic local moment systems.
As standard models for such systems, the SU(N)xSU(M) Anderson impurity models
exhibit both Fermi liquid and non-Fermi liquid behavior, depending on their
symmetry. Taking the Anderson model as an example, these lectures first give an
introduction to the auxiliary boson method to describe correlated systems
governed by a strong, short-range electronic repulsion. It is then shown how to
include the relevant low-lying excitations (coherent spin flip and charge
fluctuation processes), while preserving the local gauge symmetry of the model.
This amounts to a conserving T-matrix approximation (CTMA). We prove a
cancellation theorem showing that the CTMA incorporates all leading and
subleading infrared singularities at any given order in a self-consistent loop
expansion of the free energy. As a result, the CTMA recovers the correct
infrared behavior of the auxiliary particle propagators, indicating that it
correctly describes both the Fermi and the non-Fermi regimes of the Anderson
model.Comment: 37 pages, LaTeX, style file included, 10 postscript figures; to
appear in Proceedings of the XXXVIII Cracow School of Theoretical Physics,
Zakopane, Poland, June 1-10, 199
Construction and Test of MDT Chambers for the ATLAS Muon Spectrometer
The Monitored Drift Tube (MDT) chambers for the muon spectrometer of the AT-
LAS detector at the Large Hadron Collider (LHC) consist of 3-4 layers of
pressurized drift tubes on either side of a space frame carrying an optical
monitoring system to correct for deformations. The full-scale prototype of a
large MDT chamber has been constructed with methods suitable for large-scale
production. X-ray measurements at CERN showed a positioning accuracy of the
sense wires in the chamber of better than the required 20 ?microns (rms). The
performance of the chamber was studied in a muon beam at CERN. Chamber
production for ATLAS now has started
Conserving Diagrammatic Approximations for Quantum Impurity Models: NCA and CTMA
Self-consistent diagrammatic approximations to the Anderson or Kondo impurity
model, using an exact pseudoparticle representation of the impurity states, are
reviewed. We first discuss the infrared exponents of the pseudoparticle
propagators as indicators of Fermi liquid behavior through their dependence on
the impurity occupation and on magnetic field. Then we discuss the Non-Crossing
Approximation (NCA), identifying its strengths, but also its fundamental
shortcomings. Physical arguments as well as a perturbative renormalization
group analysis suggest that an infinite parquet-type resummation of
two-particle vertex diagrams, the Conserving T-Matrix Approximation (CTMA) will
cure the deficiencies of NCA. We review results on the pseudoparticle spectral
functions, the spin susceptibility and the impurity electron spectral function,
supporting that the CTMA provides qualitatively correct results, both in the
high-temperature regime and in the strong coupling Fermi liquid regime at low
temperatures.Comment: 10 pages, invited article, to appear in a special edition of JPSJ
"Kondo Effect - 40 Years after the Discovery", published version, some minor
typos correcte
Construction and Test of the Precision Drift Chambers for the ATLAS Muon Spectrometer
The Monitored Drift Tube (MDT) chambers for the muon spectrometer of the
ATLAS detector at the Large Hadron Collider (LHC) consist of 3-4 layers of
pressurised drift tubes on either side of a space frame carrying an optical
deformation monitoring system. The chambers have to provide a track position
resolution of 40 microns with a single-tube resolution of at least 80 microns
and a sense wire positioning accu- racy of 20 ?microns (rms). The feasibility
was demonstrated with the full-scale prototype of one of the largest MDT
chambers with 432 drift tubes of 3.8 m length. For the ATLAS muon spectrometer,
88 chambers of this type have to be built. The first chamber has been completed
with a wire positioning accuracy of 14 microns (rms)
Astrophysical S factor for the radiative capture 12N(p,gamma)13O determined from the 14N(12N,13O)13C proton transfer reaction
The cross section of the radiative proton capture reaction on the drip line
nucleus 12N was investigated using the Asymptotic Normalization Coefficient
(ANC) method. We have used the 14N(12N,13O)13C proton transfer reaction at 12
MeV/nucleon to extract the ANC for 13O -> 12N + p and calculate from it the
direct component of the astrophysical S factor of the 12N(p,gamma)13O reaction.
The optical potentials used and the DWBA analysis of the proton transfer
reaction are discussed. For the entrance channel, the optical potential was
inferred from an elastic scattering measurement carried out at the same time
with the transfer measurement. From the transfer, we determined the square of
the ANC, C^2(13Og.s.) = 2.53 +/- 0.30 fm-1, and hence a value of 0.33(4) keVb
was obtained for the direct astrophysical S factor at zero energy. Constructive
interference at low energies between the direct and resonant captures leads to
an enhancement of Stotal(0) = 0.42(5) keVb. The 12N(p,gamma)13O reaction was
investigated in relation to the evolution of hydrogen-rich massive Population
III stars, for the role that it may play in the hot pp-chain nuclear burning
processes, possibly occurring in such objects.Comment: 15 pages, 10 figures, 3 tables submitted to Phys. Rev.
Charge gaps and quasiparticle bands of the ionic Hubbard model
The ionic Hubbard model on a cubic lattice is investigated using analytical
approximations and Wilson's renormalization group for the charge excitation
spectrum. Near the Mott insulating regime, where the Hubbard repulsion starts
to dominate all energies, the formation of correlated bands is described. The
corresponding partial spectral weights and local densities of states show
characteristic features, which compare well with a hybridized-band picture
appropriate for the regime at small , which at half-filling is known as a
band insulator. In particular, a narrow charge gap is obtained at half-filling,
and the distribution of spectral quasi-particle weight reflects the fundamental
hybridization mechanism of the model
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