634 research outputs found
Can universality of the QCD evolution be checked in W boson decays into hadrons?
Hadron multiplicity from boson is calculated in pQCD. The agreement of
our theoretical predictions with the LEP data says in favor of universality of
the QCD evolution in hard processes.Comment: 14 pages, 7 figure
Effects of Frustrated Surface in Heisenberg Thin Films
We study by extensive Monte Carlo (MC) simulations and analytical Green
function (GF) method effects of frustrated surfaces on the properties of thin
films made of stacked triangular layers of atoms bearing Heisenberg spins with
an Ising-like interaction anisotropy. We suppose that the in-plane surface
interaction can be antiferromagnetic or ferromagnetic while all other
interactions are ferromagnetic. We show that the ground-state spin
configuration is non linear when is lower than a critical value .
The film surfaces are then frustrated. In the frustrated case, there are two
phase transitions related to disorderings of surface and interior layers. There
is a good agreement between MC and GF results. In addition, we show from MC
histogram calculation that the value of the ratio of critical exponents
of the observed transitions is deviated from the values of two and
three Ising universality classes. The origin of this deviation is discussed
with general physical arguments.Comment: 9 pages, 16 figure
Nonlinear Bogolyubov-Valatin transformations and quaternions
In introducing second quantization for fermions, Jordan and Wigner
(1927/1928) observed that the algebra of a single pair of fermion creation and
annihilation operators in quantum mechanics is closely related to the algebra
of quaternions H. For the first time, here we exploit this fact to study
nonlinear Bogolyubov-Valatin transformations (canonical transformations for
fermions) for a single fermionic mode. By means of these transformations, a
class of fermionic Hamiltonians in an external field is related to the standard
Fermi oscillator.Comment: 6 pages REVTEX (v3: two paragraphs appended, minor stylistic changes,
eq. (39) corrected, references [10]-[14], [36], [37], [41], [67]-[69] added;
v4: few extensions, references [62], [63] added, final version to be
published in J. Phys. A: Math. Gen.
Anderson Localization of Bogolyubov Quasiparticles in Interacting Bose-Einstein Condensates
We study the Anderson localization of Bogolyubov quasiparticles in an
interacting Bose-Einstein condensate (with healing length \xi) subjected to a
random potential (with finite correlation length \sigma_R). We derive
analytically the Lyapunov exponent as a function of the quasiparticle momentum
k and we study the localization maximum k_{max}. For 1D speckle potentials, we
find that k_{max} is proportional to 1/\xi when \xi is much larger than
\sigma_R while k_{max} is proportional to 1/\sigma_R when \xi is much smaller
than \sigma_R, and that the localization is strongest when \xi is of the order
of \sigma_R. Numerical calculations support our analysis and our estimates
indicate that the localization of the Bogolyubov quasiparticles is accessible
in current experiments with ultracold atoms.Comment: published version (no significant changes compared to last version
Orbital order and ferrimagnetic properties of the new compound
By means of the LSDA+U method and the Green function method, we investigate
the electronic and magnetic properties of the new material of
SrCaReCuO. Our LSDA+U calculation shows that this system is
an insulator with a net magnetic moment of 1.01 /f.u., which is in
good agreement with the experiment. Magnetic moments are mainly located at Cu
atoms, and the magnetic moments of neighboring Cu sites align anti-parallel. It
is the non-magnetic Re atoms that induce an orbital order of electrons of
Cu atoms, which is responsible for the strong exchange interaction and the high
magnetic transition temperature. Based on the LSDA+U results, we introduce an
effective model for the spin degrees of freedom, and investigate the
finite-temperature properties by the Green function method. The obtained
results are consistent with the experimental results, indicating that the
spin-alternating Heisenberg model is suitable for this compound.Comment: 8 pages and 5 figur
Heavy-to-light form factors: sum rules on the light cone and beyond
We report the first systematic analysis of the off-light-cone effects in sum
rules for heavy-to-light form factors. These effects are investigated in a
model based on scalar constituents, which allows a technically rather simple
analysis but has the essential features of the analogous QCD calculation. The
correlator relevant for the extraction of the heavy-to-light form factor is
calculated in two different ways: first, by adopting the full Bethe-Salpeter
amplitude of the light meson and, second, by performing the expansion of this
amplitude near the light cone . We demonstrate that the contributions to
the correlator from the light-cone term and the off-light-cone terms
have the same order in the expansion. The light-cone
correlator, corresponding to , is shown to systematically overestimate
the full correlator, the difference being , with
the continuum subtraction parameter of order 1 GeV. Numerically, this
difference is found to be 10-20%.Comment: revtex 14 pages, version to be published in Phys. Rev. D (discussion
in Sect. 3 extended, example in Sect. 4 added
Localization of Bogoliubov quasiparticles in interacting Bose gases with correlated disorder
We study the Anderson localization of Bogoliubov quasiparticles (elementary
many-body excitations) in a weakly interacting Bose gas of chemical potential
subjected to a disordered potential . We introduce a general mapping
(valid for weak inhomogeneous potentials in any dimension) of the Bogoliubov-de
Gennes equations onto a single-particle Schr\"odinger-like equation with an
effective potential. For disordered potentials, the Schr\"odinger-like equation
accounts for the scattering and localization properties of the Bogoliubov
quasiparticles. We derive analytically the localization lengths for correlated
disordered potentials in the one-dimensional geometry. Our approach relies on a
perturbative expansion in , which we develop up to third order, and we
discuss the impact of the various perturbation orders. Our predictions are
shown to be in very good agreement with direct numerical calculations. We
identify different localization regimes: For low energy, the effective
disordered potential exhibits a strong screening by the quasicondensate density
background, and localization is suppressed. For high-energy excitations, the
effective disordered potential reduces to the bare disordered potential, and
the localization properties of quasiparticles are the same as for free
particles. The maximum of localization is found at intermediate energy when the
quasicondensate healing length is of the order of the disorder correlation
length. Possible extensions of our work to higher dimensions are also
discussed.Comment: Published versio
Solution of reduced equations derived with singular perturbation methods
For singular perturbation problems in dynamical systems, various appropriate
singular perturbation methods have been proposed to eliminate secular terms
appearing in the naive expansion. For example, the method of multiple time
scales, the normal form method, center manifold theory, the renormalization
group method are well known. In this paper, it is shown that all of the
solutions of the reduced equations constructed with those methods are exactly
equal to sum of the most divergent secular terms appearing in the naive
expansion. For the proof, a method to construct a perturbation solution which
differs from the conventional one is presented, where we make use of the theory
of Lie symmetry group.Comment: To be published in Phys. Rev.
A generalised Landau-Lifshitz equation for isotropic SU(3) magnet
In the paper we obtain equations for large-scale fluctuations of the mean
field (the field of magnetization and quadrupole moments) in a magnetic system
realized by a square (cubic) lattice of atoms with spin s >= 1 at each site. We
use the generalized Heisenberg Hamiltonian with biquadratic exchange as a
quantum model. A quantum thermodynamical averaging gives classical effective
models, which are interpreted as Hamiltonian systems on coadjoint orbits of Lie
group SU(3).Comment: 15 pages, 1 figur
The effective action and equations of motion of curved local and global vortices: Role of the field excitations
The effective actions for both local and global curved vortices are derived,
based on the derivative expansion of the corresponding field theoretic actions
of the nonrelativistic Abelian Higgs and Goldstone models. The role of
excitations of the modulus and the phase of the scalar field and of the gauge
field (the Bogolyubov-Anderson mode) emitted and reabsorbed by vortices is
elucidated. In case of the local (gauge) magnetic vortex, they are necessary
for cancellation of the long distance divergence when using the transverse form
of the electric gauge field strength of the background field. In case of global
vortex taking them into account results in the Greiter-Wilczek-Witten form of
the effective action for the Goldstone mode. The expressions for transverse
Magnus-like force and the vortex effective mass for both local and global
vortices are found. The equations of motion of both type of vortices including
the terms due to the field excitations are obtained and solved in cases of
large and small contour displacements.Comment: 16 pages, no figures; accepted for publication in Int. Journ. Mod.
Phys.
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