3,497 research outputs found
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
Switchable lasing in coupled multimode microcavities
We propose the new concept of a switchable multimode microlaser. As a
generic, realistic model of a multimode microresonator a system of two coupled
defects in a two-dimensional photonic crystal is considered. We demonstrate
theoretically that lasing of the cavity into one selected resonator mode can be
caused by injecting an appropriate optical pulse at the onset of laser action
(injection seeding). Temporal mode-to-mode switching by re-seeding the cavity
after a short cool-down period is demonstrated by direct numerical solution. A
qualitative analytical explanation of the mode switching in terms of the laser
bistability is presented.Comment: Accepted for publication in Physical Review Letters. Published,
somewhat shortened versio
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
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
Astrophysical factor for the reaction from -matrix analysis and asymptotic normalization coefficient for . Is any fit acceptable?
The reaction provides a path from the CN
cycle to the CNO bi-cycle and CNO tri-cycle. The measured astrophysical factor
for this reaction is dominated by resonant capture through two strong
resonances at and 962 keV and direct capture to
the ground state. Recently, a new measurement of the astrophysical factor for
the reaction has been published [P. J.
LeBlanc {\it et al.}, Phys. Rev. {\bf C 82}, 055804 (2010)]. The analysis has
been done using the -matrix approach with unconstrained variation of all
parameters including the asymptotic normalization coefficient (ANC). The best
fit has been obtained for the square of the ANC fm,
which exceeds the previously measured value by a factor of . Here we
present a new -matrix analysis of the Notre Dame-LUNA data with the fixed
within the experimental uncertainties square of the ANC
fm. Rather than varying the ANC we add the contribution from a
background resonance that effectively takes into account contributions from
higher levels. Altogether we present 8 fits, five unconstrained and three
constrained. In all the fits the ANC is fixed at the previously determined
experimental value fm. For the unconstrained fit with
the boundary condition , where is the energy of the
second level, we get keVb and normalized , i.e. the result which is similar to [P. J. LeBlanc {\it et
al.}, Phys. Rev. {\bf C 82}, 055804 (2010)]. From all our fits we get the range
keVb which overlaps with the result of [P. J.
LeBlanc {\it et al.}, Phys. Rev. {\bf C 82}, 055804 (2010)]. We address also
physical interpretation of the fitting parameters.Comment: Submitted to PR
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
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.
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
Antiferromagnetic interlayer exchange coupling across an amorphous metallic spacer layer
By means of magneto-optical Kerr effect we observe for the first time
antiferromagnetic coupling between ferromagnetic layers across an amorphous
metallic spacer layer. Biquadratic coupling occurs at the transition from a
ferromagnetically to an antiferromagnetically coupled region. Scanning
tunneling microscopy images of all involved layers are used to extract
thickness fluctuations and to verify the amorphous state of the spacer. The
observed antiferromagnetic coupling behavior is explained by RKKY interaction
taking into account the amorphous structure of the spacer material.Comment: Typset using RevTex, 4 pages with 4 figures (.eps
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