12,892 research outputs found
Reactive Force Field for Proton Diffusion in BaZrO3 using an empirical valence bond approach
A new reactive force field to describe proton diffusion within the
solid-oxide fuel cell material BaZrO3 has been derived. Using a quantum
mechanical potential energy surface, the parameters of an interatomic potential
model to describe hydroxyl groups within both pure and yttrium-doped BaZrO3
have been determined. Reactivity is then incorporated through the use of the
empirical valence bond model. Molecular dynamics simulations (EVB-MD) have been
performed to explore the diffusion of hydrogen using a stochastic thermostat
and barostat whose equations are extended to the isostress-isothermal ensemble.
In the low concentration limit, the presence of yttrium is found not to
significantly influence the diffusivity of hydrogen, despite the proton having
a longer residence time at oxygen adjacent to the dopant. This lack of
influence is due to the fact that trapping occurs infrequently, even when the
proton diffuses through octahedra adjacent to the dopant. The activation energy
for diffusion is found to be 0.42 eV, in good agreement with experimental
values, though the prefactor is slightly underestimated.Comment: Corrected titl
Trivalent scandium, yttrium and lanthanide complexes with thia-oxa and selena-oxa macrocycles and crown ether coordination
Complexes of the oxa-thia macrocycles [18]aneO4S2, [15]aneO3S2 and the oxa-selena macrocycle [18]aneO4Se2 (L) of types [MCl2(L)]FeCl4 (M = Sc or Y) were prepared from [ScCl3(thf)3] or [YCl2(THF)5][YCl4(THF)2] and the ligand in anhydrous MeCN, using FeCl3 as a chloride abstractor. The [MI2(L)]I, [LaI3(L)] and [LuI2(L)]I have been prepared from the ligands and the appropriate anhydrous metal triiodide in MeCN. Complexes of type [LaI3(crown)] and [LuI2(crown)]I (crown = 18-crown-6, 15-crown-5) were made for comparison. Use of the metal iodide results in complexes with high solubility compared to the corresponding chlorides, although also with increased sensitivity to moisture. All complexes were characterised by microanalysis, IR, (1)H, (45)Sc and (77)Se NMR spectroscopy as appropriate. X-ray crystal structures are reported for [ScCl2([18]aneO4S2)][FeCl4], [ScI2([18]aneO4S2)]I, [YCl2(18-crown-6)]3[Y2Cl9], [YCl2([18]aneO4S2)][FeCl4], [LaI3(15-crown-5)], [LaI2(18-crown-6)(MeCN)]I, [LuI(18-crown-6)(MeCN)2]I2, [Lu(15-crown-5)(MeCN)2(OH2)]I3, [LaI3([18]aneO4S2)], [LaI([18]aneO4S2)(OH2)]I2, [LaI3([18]aneO4Se2)] and [LuI2([18]aneO4Se2)]I. In each complex all the neutral donor atoms of the macrocycles are coordinated to the metal centre, showing very rare examples of these oxophilic metal centres coordinated to thioether groups, and the first examples of coordinated selenoether donors. In some cases MeCN or adventitious water displaces halide ligands, but not the S/Se donors from La or Lu complexes. A complex of the oxa-tellura macrocycle [18]aneO4Te2, [ScCl2([18]aneO4Te2)][FeCl4] was isolated, but is unstable in MeCN solution, depositing elemental Te. YCl3 and 18-crown-6 produced [YCl2(18-crown-6)]3[Y2Cl9], the asymmetric unit of which contains two cations with a trans-YCl2 arrangement and a third with a cis-YCl2 group
Exploring the S-Matrix of Massless Particles
We use the recently proposed generalised on-shell representation for
scattering amplitudes and a consistency test to explore the space of tree-level
consistent couplings in four-dimensional Minkowski spacetime. The extension of
the constructible notion implied by the generalised on-shell representation,
i.e. the possibility to reconstruct at tree level all the scattering amplitudes
from the three-particle ones, together with the imposition of the consistency
conditions at four-particle level, allow to rediscover all the known theories
and their algebra structure, if any. Interestingly, this analysis seems to
leave room for high-spin couplings, provided that at least the requirement of
locality is weakened. We do not claim to have found tree-level consistent
high-spin theories, but rather that our methods show signatures of them and
very likely, with a suitable modification, they can be a good framework to
perform a systematic search.Comment: 44 pages, 1 figur
Non dissipative decoherence of Rabi oscillations
We present a simple theoretical description of two recent experiments where
damping of Rabi oscillations, which cannot be attributed to dissipative
decoherence, has been observed. This is obtained considering the evolution time
or the Hamiltonian as random variables and then averaging the usual unitary
evolution on a properly defined, model-independent, probability distribution.Comment: 4 pages, RevTe
Discord and non-classicality in probabilistic theories
Quantum discord quantifies non-classical correlations in quantum states. We
introduce discord for states in causal probabilistic theories, inspired by the
original definition proposed in Ref. [17]. We show that the only probabilistic
theory in which all states have null discord is classical probability theory.
Non-null discord is then not just a quantum feature, but a generic signature of
non-classicality.Comment: 5 pages, revtex styl
Coarsening in surface growth models without slope selection
We study conserved models of crystal growth in one dimension [] which are linearly unstable and develop a mound
structure whose typical size L increases in time (). If the local
slope () increases indefinitely, depends on the exponent
characterizing the large behaviour of the surface current (): for and for
.Comment: 7 pages, 2 EPS figures. To be published in J. Phys. A (Letter to the
Editor
Nonlinear dynamics in one dimension: On a criterion for coarsening and its temporal law
We develop a general criterion about coarsening for a class of nonlinear
evolution equations describing one dimensional pattern-forming systems. This
criterion allows one to discriminate between the situation where a coarsening
process takes place and the one where the wavelength is fixed in the course of
time. An intermediate scenario may occur, namely `interrupted coarsening'. The
power of the criterion lies in the fact that the statement about the occurrence
of coarsening, or selection of a length scale, can be made by only inspecting
the behavior of the branch of steady state periodic solutions. The criterion
states that coarsening occurs if lambda'(A)>0 while a length scale selection
prevails if lambda'(A)<0, where is the wavelength of the pattern and A
is the amplitude of the profile. This criterion is established thanks to the
analysis of the phase diffusion equation of the pattern. We connect the phase
diffusion coefficient D(lambda) (which carries a kinetic information) to
lambda'(A), which refers to a pure steady state property. The relationship
between kinetics and the behavior of the branch of steady state solutions is
established fully analytically for several classes of equations. Another
important and new result which emerges here is that the exploitation of the
phase diffusion coefficient enables us to determine in a rather straightforward
manner the dynamical coarsening exponent. Our calculation, based on the idea
that |D(lambda)|=lambda^2/t, is exemplified on several nonlinear equations,
showing that the exact exponent is captured. Some speculations about the
extension of the present results to higher dimension are outlined.Comment: 16 pages. Only a few minor changes. Accepted for publication in
Physical Review
Deterministic reaction models with power-law forces
We study a one-dimensional particles system, in the overdamped limit, where
nearest particles attract with a force inversely proportional to a power of
their distance and coalesce upon encounter. The detailed shape of the
distribution function for the gap between neighbouring particles serves to
discriminate between different laws of attraction. We develop an exact
Fokker-Planck approach for the infinite hierarchy of distribution functions for
multiple adjacent gaps and solve it exactly, at the mean-field level, where
correlations are ignored. The crucial role of correlations and their effect on
the gap distribution function is explored both numerically and analytically.
Finally, we analyse a random input of particles, which results in a stationary
state where the effect of correlations is largely diminished
Exact solution of a model DNA-inversion genetic switch with orientational control
DNA inversion is an important mechanism by which bacteria and bacteriophage
switch reversibly between phenotypic states. In such switches, the orientation
of a short DNA element is flipped by a site-specific recombinase enzyme. We
propose a simple model for a DNA inversion switch in which recombinase
production is dependent on the switch state (orientational control). Our model
is inspired by the fim switch in Escherichia coli. We present an exact
analytical solution of the chemical master equation for the model switch, as
well as stochastic simulations. Orientational control causes the switch to
deviate from Poissonian behaviour: the distribution of times in the on state
shows a peak and successive flip times are correlated.Comment: Revised version, accepted for publicatio
Electric field control of the magnetic chiralities in ferroaxial multiferroic RbFe(MoO4)2
The coupling of magnetic chiralities to the ferroelectric polarisation in
multiferroic RbFe(MoO) is investigated by neutron spherical
polarimetry. Because of the axiality of the crystal structure below
= 190 K, helicity and triangular chirality are
symmetric-exchange coupled, explaining the onset of the ferroelectricity in
this proper-screw magnetic structure - a mechanism that can be generalised to
other systems with "ferroaxial" distortions in the crystal structure. With an
applied electric field we demonstrate control of the chiralities in both
structural domains simultaneously.Comment: 5 pages, 4 figure
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