7,573 research outputs found
Comparison of models and lattice-gas simulations for Liesegang patterns
For more than a century Liesegang patterns -- self-organized, quasi-periodic
structures occurring in diffusion-limited chemical reactions with two
components -- have been attracting scientists. The pattern formation can be
described by four basic empirical laws. In addition to many experiments,
several models have been devised to understand the formation of the bands and
rings. Here we review the most important models and complement them with
detailed three-dimensional lattice-gas simulations. We show how the mean-field
predictions can be reconciled with experimental data by a redefinition of the
distances suggested by our lattice-gas simulations.Comment: 21 pages, 9 figures, accepted for publication in EPJ Special Topic
Tuning the magnetic anisotropy of single molecules
The magnetism of single atoms and molecules is governed by the atomic scale
environment. In general, the reduced symmetry of the surrounding splits the
states and aligns the magnetic moment along certain favorable directions. Here,
we show that we can reversibly modify the magnetocrystalline anisotropy by
manipulating the environment of single iron(II) porphyrin molecules adsorbed on
Pb(111) with the tip of a scanning tunneling microscope. When we decrease the
tip--molecule distance, we first observe a small increase followed by an
exponential decrease of the axial anisotropy on the molecules. This is in
contrast to the monotonous increase observed earlier for the same molecule with
an additional axial Cl ligand. We ascribe the changes in the anisotropy of both
species to a deformation of the molecules in the presence of the attractive
force of the tip, which leads to a change in the level alignment. These
experiments demonstrate the feasibility of a precise tuning of the magnetic
anisotropy of an individual molecule by mechanical control.Comment: 16 pages, 5 figures; online at Nano Letters (2015
Astrophysical neutrinos flavored with Beyond the Standard Model physics
We systematically study the allowed parameter space for the flavor
composition of astrophysical neutrinos measured at Earth, including beyond the
Standard Model theories at production, during propagation, and at detection.
One motivation is to illustrate the discrimination power of the next-generation
neutrino telescopes such as IceCube-Gen2. We identify several examples that
lead to potential deviations from the standard neutrino mixing expectation such
as significant sterile neutrino production at the source, effective operators
modifying the neutrino propagation at high energies, dark matter interactions
in neutrino propagation, or non-standard interactions in Earth matter.
IceCube-Gen2 can exclude about 90% of the allowed parameter space in these
cases, and hence will allow to efficiently test and discriminate models. More
detailed information can be obtained from additional observables such as the
energy-dependence of the effect, fraction of electron antineutrinos at the
Glashow resonance, or number of tau neutrino events.Comment: 21 pages, 9 figures, 3 tables, v2: references added, typos corrected,
conclusion unchanged, matches final version in PR
Consistent actions for massive particles interacting with electromagnetism and gravity
Consistent interactions with electromagnetism and gravity for mass
particles of any spin are obtained. This is done by finding interactions which
preserve the covariantized massive gauge symmetry present in recently
constructed massive particle actions. This gauge principle is sufficient for
finding consistent completions of minimal as well as non-minimal couplings of
any type. For spins , consistency requires infinitely many
interaction terms in the action, including arbitrarily high order derivatives
of electromagnetic and gravitational curvatures, with correspondingly high
powers of . These interactions may be formally resummed and expressed in
terms of non-local operators. The inherent non-locality is a manifestation of
the known causality problems present in interacting massive particles with spin
.Comment: v1, 29 pages; v2, 30 pages: interactions linear in matter fields
adde
Covariant actions and propagators for all spins, masses, and dimensions
The explicit covariant actions and propagators are given for fields
describing particles of all spins and masses, in any spacetime dimension.
Massive particles are realized as "dimensionally reduced" massless particles.
To obtain compact expressions for the propagators, it was useful to introduce
an auxiliary vector coordinate and consider "hyperfields" that are
functions of space and . The actions and propagators serve
as a basic starting point for concrete high spin computations amenable to
dimensional regularization, provided that gauge invariant interactions are
introduced.Comment: 37 page
Moving Five-Branes in Low-Energy Heterotic M-Theory
We construct cosmological solutions of four-dimensional effective heterotic
M-theory with a moving five-brane and evolving dilaton and T modulus. It is
shown that the five-brane generates a transition between two asymptotic
rolling-radii solutions. Moreover, the five-brane motion always drives the
solutions towards strong coupling asymptotically. We present an explicit
example of a negative-time branch solution which ends in a brane collision
accompanied by a small-instanton transition. The five-dimensional origin of
some of our solutions is also discussed.Comment: 16 pages, Latex, 3 eps figure
On the Friedmann Equation in Brane-World Scenarios
The Friedmann law on the brane generically depends quadratically on the brane
energy density and involves a ``dark radiation'' term due to the bulk Weyl
tensor. Despite its unfamiliar form, we show how it can be derived from a
standard four-dimensional Brans-Dicke theory at low energy. In particular, the
dark radiation term is found to depend linearly on the brane energy densities.
For any equation of state on the branes, the radion evolves such as to generate
radiation-dominated cosmology. The radiation-dominated era is conventional and
consistent with nucleosynthesis.Comment: 4 pages. v2,v3: discussion on BBN extended, minor correction
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