3,214 research outputs found
Butterfly Hysteresis and Slow Relaxation of the Magnetization in (Et4N)3Fe2F9: Manifestations of a Single-Molecule Magnet
(Et4N)3Fe2F9 exhibits a butterfly--shaped hysteresis below 5 K when the
magnetic field is parallel to the threefold axis, in accordance with a very
slow magnetization relaxation in the timescale of minutes. This is attributed
to an energy barrier Delta=2.40 K resulting from the S=5 dimer ground state of
[Fe2F9]^{3-} and a negative axial anisotropy. The relaxation partly occurs via
thermally assisted quantum tunneling. These features of a single-molecule
magnet are observable at temperatures comparable to the barrier height, due to
an extremely inefficient energy exchange between the spin system and the
phonons. The butterfly shape of the hysteresis arises from a phonon avalanche
effect.Comment: 18 pages, 5 eps figures, latex (elsart
Structure and evolution of strange attractors in non-elastic triangular billiards
We study pinball billiard dynamics in an equilateral triangular table. In
such dynamics, collisions with the walls are non-elastic: the outgoing angle
with the normal vector to the boundary is a uniform factor
smaller than the incoming angle. This leads to contraction in phase space for
the discrete-time dynamics between consecutive collisions, and hence to
attractors of zero Lebesgue measure, which are almost always fractal strange
attractors with chaotic dynamics, due to the presence of an expansion
mechanism. We study the structure of these strange attractors and their
evolution as the contraction parameter is varied. For in
the interval (0, 1/3), we prove rigorously that the attractor has the structure
of a Cantor set times an interval, whereas for larger values of the
billiard dynamics gives rise to nonaccessible regions in phase space. For
close to 1, the attractor splits into three transitive components,
the basins of attraction of which have fractal basin boundaries.Comment: 12 pages, 10 figures; submitted for publication. One video file
available at http://sistemas.fciencias.unam.mx/~dsanders
Fixed points of dynamic processes of set-valued F-contractions and application to functional equations
The article is a continuation of the investigations concerning F-contractions which have been recently introduced in [Wardowski in Fixed Point Theory Appl. 2012:94,2012]. The authors extend the concept of F-contractive mappings to the case of nonlinear F-contractions and prove a fixed point theorem via the dynamic processes. The paper includes a non-trivial example which shows the motivation for such investigations. The work is summarized by the application of the introduced nonlinear F-contractions to functional equations
Quantum tunneling in a three dimensional network of exchange coupled single-molecule magnets
A Mn4 single-molecule magnet (SMM) is used to show that quantum tunneling of
magnetization (QTM) is not suppressed by moderate three dimensional exchange
coupling between molecules. Instead, it leads to an exchange bias of the
quantum resonances which allows precise measurements of the effective exchange
coupling that is mainly due to weak intermolecular hydrogen bounds. The
magnetization versus applied field was recorded on single crystals of [Mn4]2
using an array of micro-SQUIDs. The step fine structure was studied via minor
hysteresis loops.Comment: 4 pages, 4 figure
Lattice Gauge Fixing as Quenching and the Violation of Spectral Positivity
Lattice Landau gauge and other related lattice gauge fixing schemes are known
to violate spectral positivity. The most direct sign of the violation is the
rise of the effective mass as a function of distance. The origin of this
phenomenon lies in the quenched character of the auxiliary field used to
implement lattice gauge fixing, and is similar to quenched QCD in this respect.
This is best studied using the PJLZ formalism, leading to a class of covariant
gauges similar to the one-parameter class of covariant gauges commonly used in
continuum gauge theories. Soluble models are used to illustrate the origin of
the violation of spectral positivity. The phase diagram of the lattice theory,
as a function of the gauge coupling and the gauge-fixing parameter
, is similar to that of the unquenched theory, a Higgs model of a type
first studied by Fradkin and Shenker. The gluon propagator is interpreted as
yielding bound states in the confined phase, and a mixture of fundamental
particles in the Higgs phase, but lattice simulation shows the two phases are
connected. Gauge field propagators from the simulation of an SU(2) lattice
gauge theory on a lattice are well described by a quenched mass-mixing
model. The mass of the lightest state, which we interpret as the gluon mass,
appears to be independent of for sufficiently large .Comment: 28 pages, 14 figures, RevTeX
Chiral perturbation theory for lattice QCD including O(a^2)
The O(a^2) contributions to the chiral effective Lagrangian for lattice QCD
with Wilson fermions are constructed. The results are generalized to partially
quenched QCD with Wilson fermions as well as to the "mixed'' lattice theory
with Wilson sea quarks and Ginsparg-Wilson valence quarks.Comment: 3 pages, Lattice2003 (spectrum
PĂȘches expĂ©rimentales de l'Ćil de bĆuf (Etelis oculatus) aux filets profonds en Guadeloupe (F.W.I.); Experimental deep sea gill net fishery of queen snapper (Etelis oculatus) in Guadeloupe (F.W.I.); Pescas experimentales del pargo cachucho (Etelis oculatus) con redes profundas en Guadeloupe (F.W.I.)
The constraint equations for the Einstein-scalar field system on compact manifolds
We study the constraint equations for the Einstein-scalar field system on
compact manifolds. Using the conformal method we reformulate these equations as
a determined system of nonlinear partial differential equations. By introducing
a new conformal invariant, which is sensitive to the presence of the initial
data for the scalar field, we are able to divide the set of free conformal data
into subclasses depending on the possible signs for the coefficients of terms
in the resulting Einstein-scalar field Lichnerowicz equation. For many of these
subclasses we determine whether or not a solution exists. In contrast to other
well studied field theories, there are certain cases, depending on the mean
curvature and the potential of the scalar field, for which we are unable to
resolve the question of existence of a solution. We consider this system in
such generality so as to include the vacuum constraint equations with an
arbitrary cosmological constant, the Yamabe equation and even (all cases of)
the prescribed scalar curvature problem as special cases.Comment: Minor changes, final version. To appear: Classical and Quantum
Gravit
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