932 research outputs found
Unitarity Constraints on effective interaction in scattering
Positivity constraints are derived on pion-nucleon scattering amplitudes and
their even-order derivatives inside the Mandelstam Triangle with the help of
dispersion relations. Fairly interesting constraints are obtained on some of
the low energy constants, by a combination of the chiral perturbation theory
for heavy baryons and existing fitting results from available pion-nucleon
phase shifts at intermediate energies.Comment: version to appear in Phys. Lett. B. 6 page
Evidence for d-wave superconductivity in the repulsive Hubbard-model
We perform numerical simulations of the Hubbard model using the projector
Quantum Monte Carlo method. A novel approach for finite size scaling is
discussed. We obtain evidence in favor of d-wave superconductivity in the
repulsive Hubbard model. For U=4, is roughly estimated as .Comment: 13 pages, 3 figures, 2 tables, submitted to Phys. Rev. Letter
Complete analysis of pion-nucleon scattering in chiral perturbation theory to third order
We consider pion-nucleon scattering in heavy-baryon chiral perturbation
theory to third order. All electromagnetic corrections appearing to this order
are included. We thus have a consistent description of strong and
electromagnetic effects, which allows us to isolate the strong part of the
interaction in an unambiguous way. We give pion-nucleon phase shifts up to pion
laboratory momenta of 100 MeV and find sizeable differences for the S-waves of
the elastic channels compared to previous phase shift analyses. The precise
description of the scattering process also allows us to address the question of
isospin violation in the strong interaction. For the usually employed triangle
relation we find an isospin breaking effect of -0.7% in the S-wave, whereas the
P-waves show an effect of -1.5% and -4 to -2.5%, respectively, for pion
laboratory momenta between 25 and 100 MeV.Comment: 17 pp, 7 fig
Unification of the physics of nucleons and nuclei
I outline an ambitious program which aims to achieve a unified description of
nucleon and nuclear properties based on one chiral effective field theory.Comment: 9 pp, 6 figs, plenary talk at 17th International IUPAP Conference on
Few-body Problems in Physics, June 5-10, 2003, Durham, North Carolina, US
Glass Model, Hubbard Model and High-Temperature Superconductivity
In this paper we revisit the glass model describing the macroscopic behavior
of the High-Temperature superconductors. We link the glass model at the
microscopic level to the striped phase phenomenon, recently discussed widely.
The size of the striped phase domains is consistent with earlier predictions of
the glass model when it was introduced for High-Temperature Superconductivity
in 1987. In an additional step we use the Hubbard model to describe the
microscopic mechanism for d-wave pairing within these finite size stripes. We
discuss the implications for superconducting correlations of Hubbard model,
which are much higher for stripes than for squares, for finite size scaling,
and for the new view of the glass model picture.Comment: 7 pages, 7 figures (included), LaTex using Revtex, accepted by Int.
J. Mod. Phys.
Parallelization of the exact diagonalization of the t-t'-Hubbard model
We present a new parallel algorithm for the exact diagonalization of the
-Hubbard model with the Lanczos-method. By invoking a new scheme of
labeling the states we were able to obtain a speedup of up to four on 16 nodes
of an IBM SP2 for the calculation of the ground state energy and an almost
linear speedup for the calculation of the correlation functions. Using this
algorithm we performed an extensive study of the influence of the next-nearest
hopping parameter in the -Hubbard model on ground state energy and
the superconducting correlation functions for both attractive and repulsive
interaction.Comment: 18 Pages, 1 table, 8 figures, Latex uses revtex, submitted to Comp.
Phys. Com
Fixed-Point Analysis of the Low-Energy Constants in the Pion-Nucleon Chiral Lagrangian
In the framework of heavy-baryon chiral perturbation theory, we investigate
the fixed point structure of renormalization group equations (RGE) for the
ratios of the renormalized low energy constants (LECs) that feature in the
pion-nucleon chiral Lagrangian. The ratios of the LECs deduced from our RGE
analysis are found to be in semi-quantitative agreement with those obtained
from direct fit to the experimental data. The naturalness of this agreement is
discussed using a simple dimensional analysis combined with Wilsonian RGEs.Comment: 10 page
Pion-nucleon scattering in chiral perturbation theory II: Fourth order calculation
We analyse elastic-pion nucleon scattering to fourth order in heavy baryon
chiral perturbation theory, extending the third order study published in Nucl.
Phys. A640 (1998) 199. We use various partial wave analyses to pin down the
low-energy constants from data in the physical region. The S-wave scattering
lengths are consistent with recent determinations from pionic hydrogen and
deuterium. We find an improved description of the P-waves. We also discuss the
pion-nucleon sigma term and problems related to the prediction of the
subthreshold parameters.Comment: 28 pp, LaTeX2e, uses epsf, 6 figs, better discussion of the
convergence issues, accepted for publication in Nuclear Physics
Comparison of Calculations for the Hubbard model obtained with Quantum-Monte-Carlo, exact and stochastic Diagonalization
In this paper we compare numerical results for the ground state of the
Hubbard model obtained by Quantum-Monte-Carlo simulations with results from
exact and stochastic diagonalizations. We find good agreement for the ground
state energy and superconducting correlations for both, the repulsive and
attractive Hubbard model. Special emphasis lies on the superconducting
correlations in the repulsive Hubbard model, where the small magnitude of the
values obtained by Monte-Carlo simulations gives rise to the question, whether
these results might be caused by fluctuations or systematic errors of the
method. Although we notice that the Quantum-Monte-Carlo method has convergence
problems for large interactions, coinciding with a minus sign problem, we
confirm the results of the diagonalization techniques for small and moderate
interaction strengths. Additionally we investigate the numerical stability and
the convergence of the Quantum-Monte-Carlo method in the attractive case, to
study the influence of the minus sign problem on convergence. Also here in the
absence of a minus sign problem we encounter convergence problems for strong
interactions.Comment: 24 pages, 9 figure
Nuclear forces with Delta-excitations up to next-to-next-to-leading order I: peripheral nucleon-nucleon waves
We study the two-nucleon force at next-to-next-to-leading order in a chiral
effective field theory with explicit Delta degrees of freedom. Fixing the
appearing low-energy constants from a next-to-leading order calculation of
pion-nucleon threshold parameters, we find an improved convergence of most
peripheral nucleon-nucleon phases compared to the theory with pions and
nucleons only. In the delta-full theory, the next-to-leading order corrections
are dominant in most partial waves considered.Comment: 16 pages, 10 figure
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