176,627 research outputs found
Derivation of effective spin models from a three band model for CuO_2-planes
The derivation of effective spin models describing the low energy magnetic
properties of undoped CuO_2-planes is reinvestigated. Our study aims at a
quantitative determination of the parameters of effective spin models from
those of a multi-band model and is supposed to be relevant to the analysis of
recent improved experimental data on the spin wave spectrum of La_2CuO_4.
Starting from a conventional three-band model we determine the exchange
couplings for the nearest and next-nearest neighbor Heisenberg exchange as well
as for 4- and 6-spin exchange terms via a direct perturbation expansion up to
12th (14th for the 4-spin term) order with respect to the copper-oxygen hopping
t_pd. Our results demonstrate that this perturbation expansion does not
converge for hopping parameters of the relevant size. Well behaved
extrapolations of the couplings are derived, however, in terms of Pade
approximants. In order to check the significance of these results from the
direct perturbation expansion we employ the Zhang-Rice reformulation of the
three band model in terms of hybridizing oxygen Wannier orbitals centered at
copper ion sites. In the Wannier notation the perturbation expansion is
reorganized by an exact treatment of the strong site-diagonal hybridization.
The perturbation expansion with respect to the weak intersite hybridizations is
calculated up to 4th order for the Heisenberg coupling and up to 6th order for
the 4-spin coupling. It shows excellent convergence and the results are in
agreement with the Pade approximants of the direct expansion. The relevance of
the 4-spin coupling as the leading correction to the nearest neighbor
Heisenberg model is emphasized.Comment: 27 pages, 10 figures. Changed from particle to hole notation, right
value for the charge transfer gap used; this results in some changes in the
figures and a higher value of the ring exchang
Modulational instability in asymmetric coupled wave functions
The evolution of the amplitude of two nonlinearly interacting waves is
considered, via a set of coupled nonlinear Schroedinger-type equations. The
dynamical profile is determined by the wave dispersion laws (i.e. the group
velocities and the GVD terms) and the nonlinearity and coupling coefficients,
on which no assumption is made. A generalized dispersion relation is obtained,
relating the frequency and wave-number of a small perturbation around a coupled
monochromatic (Stokes') wave solution. Explicitly stability criteria are
obtained. The analysis reveals a number of possibilities. Two (individually)
stable systems may be destabilized due to coupling. Unstable systems may, when
coupled, present an enhanced instability growth rate, for an extended wave
number range of values. Distinct unstable wavenumber windows may arise
simultaneously.Comment: NEXT Sigma-Phi Statistical Physics Conference (2005, Kolymbari,
Greece) Proceedings, submitted; v.2 is a shorter version of the text in v.1
(more detailed and somehow more explanatory, yet abbreviated due to
submission regulations); some typos corrected as wel
Behaviour of the energy gap in a model of Josephson coupled Bose-Einstein condensates
In this work we investigate the energy gap between the ground state and the
first excited state in a model of two single-mode Bose-Einstein condensates
coupled via Josephson tunneling. The energy gap is never zero when the
tunneling interaction is non-zero. The gap exhibits no local minimum below a
threshold coupling which separates a delocalised phase from a self-trapping
phase which occurs in the absence of the external potential. Above this
threshold point one minimum occurs close to the Josephson regime, and a set of
minima and maxima appear in the Fock regime. Analytic expressions for the
position of these minima and maxima are obtained. The connection between these
minima and maxima and the dynamics for the expectation value of the relative
number of particles is analysed in detail. We find that the dynamics of the
system changes as the coupling crosses these points.Comment: 12 pages, 5 .eps figures + 4 figs, classical analysis, perturbation
theor
Macrospin approximation and quantum effects in models for magnetization reversal
The thermal activation of magnetization reversal in magnetic nanoparticles is
controlled by the anisotropy-energy barrier. Using perturbation theory, exact
diagonalization and stability analysis of the ferromagnetic spin-s Heisenberg
model with coupling or single-site anisotropy, we study the effects of quantum
fluctuations on the height of the energy barrier. Opposed to the classical
case, there is no critical anisotropy strength discriminating between reversal
via coherent rotation and via nucleation/domain-wall propagation. Quantum
fluctuations are seen to lower the barrier depending on the anisotropy
strength, dimensionality and system size and shape. In the weak-anisotropy
limit, a macrospin model is shown to emerge as the effective low-energy theory
where the microscopic spins are tightly aligned due to the ferromagnetic
exchange. The calculation provides explicit expressions for the anisotropy
parameter of the effective macrospin. We find a reduction of the
anisotropy-energy barrier as compared to the classical high spin-s limit.Comment: 10 pages, 11 figure
Nonperturbative Light-Front QCD
In this work the determination of low-energy bound states in Quantum
Chromodynamics is recast so that it is linked to a weak-coupling problem. This
allows one to approach the solution with the same techniques which solve
Quantum Electrodynamics: namely, a combination of weak-coupling diagrams and
many-body quantum mechanics. The key to eliminating necessarily nonperturbative
effects is the use of a bare Hamiltonian in which quarks and gluons have
nonzero constituent masses rather than the zero masses of the current picture.
The use of constituent masses cuts off the growth of the running coupling
constant and makes it possible that the running coupling never leaves the
perturbative domain. For stabilization purposes an artificial potential is
added to the Hamiltonian, but with a coefficient that vanishes at the physical
value of the coupling constant. The weak-coupling approach potentially
reconciles the simplicity of the Constituent Quark Model with the complexities
of Quantum Chromodynamics. The penalty for achieving this perturbative picture
is the necessity of formulating the dynamics of QCD in light-front coordinates
and of dealing with the complexities of renormalization which such a
formulation entails. We describe the renormalization process first using a
qualitative phase space cell analysis, and we then set up a precise similarity
renormalization scheme with cutoffs on constituent momenta and exhibit
calculations to second order. We outline further computations that remain to be
carried out. There is an initial nonperturbative but nonrelativistic
calculation of the hadronic masses that determines the artificial potential,
with binding energies required to be fourth order in the coupling as in QED.
Next there is a calculation of the leading radiative corrections to these
masses, which requires our renormalization program. Then the real struggle of
finding the right extensions to perturbation theory to study the
strong-coupling behavior of bound states can begin.Comment: 56 pages (REVTEX), Report OSU-NT-94-28. (figures not included,
available via anaonymous ftp from pacific.mps.ohio-state.edu in subdirectory
pub/infolight/qcd
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