2,491 research outputs found
A Brief Introduction to Chiral Perturbation Theory
A brief introduction to the subject of chiral perturbation theory (pt)
is presented, including a discussion of effective field theory and applications
of pt in the arena of purely mesonic interactions as well as in the sector.Comment: 15 pages, talk given at TAPS detector workshop, Rez, Czech Republic,
Sept. 1999, to be published in Czech. J. Phy
Phase diagram of the one dimensional anisotropic Kondo-necklace model
The one dimensional anisotropic Kondo-necklace model has been studied by
several methods. It is shown that a mean field approach fails to gain the
correct phase diagram for the Ising type anisotropy. We then applied the spin
wave theory which is justified for the anisotropic case. We have derived the
phase diagram between the antiferromagnetic long range order and the Kondo
singlet phases. We have found that the exchange interaction (J) between the
itinerant spins and local ones enhances the quantum fluctuations around the
classical long range antiferromagnetic order and finally destroy the ordered
phase at the critical value, J_c. Moreover, our results show that the onset of
anisotropy in the XY term of the itinerant interactions develops the
antiferromagnetic order for J<J_c. This is in agreement with the qualitative
feature which we expect from the symmetry of the anisotropic XY interaction. We
have justified our results by the numerical Lanczos method where the structure
factor at the antiferromagnetic wave vector diverges as the size of system goes
to infinity.Comment: 9 pages and 9 eps figure
Polaron self-trapping in a honeycomb net
Small polaron behavior in a two dimensional honeycomb net is studied by
applying the strong coupling perturbative method to the Holstein molecular
crystal model. We find that small optical polarons can be mobile also if the
electrons are strongly coupled to the lattice. Before the polarons localize and
become very heavy, there is infact a window of {\it e-ph} couplings in which
the polarons are small and have masses of order times the bare
band mass according to the value of the adiabaticity parameter. The 2D
honeycomb net favors the mobility of small optical polarons in comparison with
the square lattice.Comment: 6 pages, 3 figures, to appear in J.Phys.:Condensed Matter {PACS:
63.10.+a, 63.20.Dj, 71.38.+i
Giant enhancement of anisotropy by electron-phonon interaction
Anisotropic electron-phonon interaction is shown to lead to the anisotropic
polaron effect. The resulting anisotropy of the polaron band is an exponential
function of the electron-phonon coupling and might be as big as . This
also makes anisotropy very sensitive to small changes of coupling and implies
wide variations of anisotropy among compounds of similar structure. The isotope
effect on mass anisotropy is predicted. Polaron masses are obtained by an exact
Quantum Monte Carlo method. Implications for high-temperature superconductors
are briefly discussed.Comment: 5 pages, 4 figure
Finite temperature spectral function of Mott insulators and CDW States
We calculate the low temperature spectral function of one-dimensional
incommensurate charge density wave (CDW) states and half-filled Mott insulators
(MI). At there are two dispersing features associated with the spin and
charge degrees of freedom respectively. We show that already at very low
temperatures (compared to the gap) one of these features gets severely damped.
We comment on implications of this result for photoemission experiments.Comment: 4 pages, 2 figures, published versio
Entanglement between atomic condensates in an optical lattice: effects of interaction range
We study the area-dependent entropy and two-site entanglement for two state
Bose-Einstein condensates in a 2D optical lattice. We consider the case where
the array of two component condensates behave like an ensemble of spin-half
particles with the interaction to its nearest neighbors and next nearest
neighbors. We show how the Hamiltonian of their Bose-Einstein condensate
lattice with nearest-neighbor and next-nearest-neighbor interactions can be
mapped into a harmonic lattice. We use this to determine the entropy and
entanglement content of the lattice.Comment: 5 pages, 3 figures, title change
A Review of Rare Pion and Muon Decays
After a decade of no measurements of pion and muon rare decays, PIBETA, a new
experimental program is producing its first results. We report on a new
experimental study of the pion beta decay, Pi(+) -> Pi(0) e(+) Nu, the Pi(e2
gamma) radiative decay, Pi(+) -> e(+) Nu Gamma, and muon radiative decay, Mu ->
e Nu Gamma. The new results represent four- to six-fold improvements in
precision over the previous measurements. Excellent agreement with Standard
Model predictions is observed in all channels except for one kinematic region
of the Pi(e2 gamma) radiative decay involving energetic photons and
lower-energy positrons.Comment: 10 pages, 6 figures, 2 tables, invited talk presented at MESON 2004,
8th Int'l. Workshop on Meson Production, Properties and Interaction, Krakow,
Poland 4-8 June 200
Sum Rules for Magnetic Moments and Polarizabilities in QED and Chiral Effective-Field Theory
We elaborate on a recently proposed extension of the Gerasimov-Drell-Hearn
(GDH) sum rule which is achieved by taking derivatives with respect to the
anomalous magnetic moment. The new sum rule features a {\it linear} relation
between the anomalous magnetic moment and the dispersion integral over a
cross-section quantity. We find some analogy of the linearized form of the GDH
sum rule with the `sideways dispersion relations'. As an example, we apply the
linear sum rule to reproduce the famous Schwinger's correction to the magnetic
moment in QED from a tree-level cross-section calculation and outline the
procedure for computing the two-loop correction from a one-loop cross-section
calculation. The polarizabilities of the electron in QED are considered as well
by using the other forward-Compton-scattering sum rules. We also employ the sum
rules to study the magnetic moment and polarizabilities of the nucleon in a
relativistic chiral EFT framework. In particular we investigate the chiral
extrapolation of these quantities.Comment: 24 pages, 7 figures; several additions, published versio
Pion and Kaon Polarizabilities and Radiative Transitions
CERN COMPASS plans measurements of gamma-pi and gamma-K interactions using
50-280 GeV pion (kaon) beams and a virtual photon target. Pion (kaon)
polarizabilities and radiative transitions will be measured via Primakoff
effect reactions such as pi+gamma->pi'+gamma and pi+gamma->meson. The former
can test a precise prediction of chiral symmetry; the latter for
pi+gamma->a1(1260) is important for understanding the polarizability. The
radiative transition of a pion to a low mass two-pion system, pi+gamma->pi+pi0,
can also be studied to measure the chiral anomaly amplitude F(3pi)
(characterizing gamma->3pi), arising from the effective Chiral Lagrangian. We
review here the motivation for the above physics program. We describe the beam,
target, detector, and trigger requirements for these experiments. We also
describe FNAL SELEX attempts to study related physics via the interaction of
600 GeV pions with target electrons. Data analysis in progress aims to identify
the reactions pi+e->pi'+e'+pi0 related to the chiral anomaly, and
pi+e->pi'+e'+gamma related to pion polarizabilities.Comment: 16 pages, 6 figures, Latex Springer-Verlag style Tel Aviv U. Preprint
TAUP-2469-97, Contribution to the Workshop on Chiral Dynamics Theory and
Experiment, U. of Mainz, Sept. 1-5, 1997, to be published in Springer-Verlag,
Eds. A. Bernstein, Th. Walcher, 199
Polaron features for long-range electron-phonon interaction
The polaron features for long-range electron-phonon interaction are
investigated by extending a variational approach previously proposed for the
study of systems with local coupling. The ground-state spectral weight, the
average kinetic energy, the mean number of phonons, and the electron-lattice
correlation function are discussed for a wide range of model parameters
focusing on the adiabatic regime and comparing the results with the short-range
case (Holstein model). A strong mixing of electronic and phononic degrees of
freedom for small values of the electron-phonon coupling constant is found in
the adiabatic case due to the long-range interaction. Finally a polaron "phase
diagram" is proposed.Comment: 4 figs., to appear in J. Phys.:Condens. Matte
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