19,542 research outputs found
Statistical evaporation of rotating clusters. II. Angular momentum distribution
The change in the angular momentum of an atomic cluster following evaporation
is investigated using rigorous phase space theory and molecular dynamics
simulations, with an aim at the possible rotational cooling and heating
effects. Influences of the shape of the interaction potential, anharmonicity of
the vibrational density of states (DOS), and the initial distribution of
excitation energies are systematically studied on the example of the
Lennard-Jones cluster LJ_14. For this system, the predictions of PST are in
quantitative agreement with the results of the simulations, provided that the
correct forms for the vibrational density of states and the interaction
potential are used. The harmonic approximation to the DOS is used to obtain
explicit forms for the angular momentum distribution in larger clusters. These
are seen to undergo preferential cooling when thermally excited, and
preferential heating when subject to a strong vibrational excitation.Comment: 10 pages, 7 figure
Antiferromagnetic spin chain behavior and a transition to 3D magnetic order in Cu(D,L-alanine)2: Roles of H-bonds
We study the spin chain behavior, a transition to 3D magnetic order and the
magnitudes of the exchange interactions for the metal-amino acid complex
Cu(D,L-alanine)2.H2O, a model compound to investigate exchange couplings
supported by chemical paths characteristic of biomolecules. Thermal and
magnetic data were obtained as a function of temperature (T) and magnetic field
(B0). The magnetic contribution to the specific heat, measured between 0.48 and
30 K, displays above 1.8 K a 1D spin-chain behavior that can be fitted with an
intrachain antiferromagnetic (AFM) exchange coupling constant 2J0 = (-2.12
0.08) cm, between neighbor coppers at 4.49 {\AA} along chains
connected by non-covalent and H-bonds. We also observe a narrow specific heat
peak at 0.89 K indicating a phase transition to a 3D magnetically ordered
phase. Magnetization curves at fixed T = 2, 4 and 7 K with B0 between 0 and 9
T, and at T between 2 and 300 K with several fixed values of B0 were globally
fitted by an intrachain AFM exchange coupling constant 2J0 = (-2.27 0.02)
cm and g = 2.091 0.005. Interchain interactions J1 between coppers
in neighbor chains connected through long chemical paths with total length of
9.51 {\AA} are estimated within the range 0.1 < |2J1| < 0.4 cm, covering
the predictions of various approximations. We analyze the magnitudes of 2J0 and
2J1 in terms of the structure of the corresponding chemical paths. The main
contribution in supporting the intrachain interaction is assigned to H-bonds
while the interchain interactions are supported by paths containing H-bonds and
carboxylate bridges, with the role of the H-bonds being predominant. We compare
the obtained intrachain coupling with studies of compounds showing similar
behavior and discuss the validity of the approximations allowing to calculate
the interchain interactions.Comment: 10 pages, 4 figure
Sampling along reaction coordinates with the Wang-Landau method
The multiple range random walk algorithm recently proposed by Wang and Landau
[Phys. Rev. Lett. 86, 2050 (2001)] is adapted to the computation of free energy
profiles for molecular systems along reaction coordinates. More generally, we
show how to extract partial averages in various statistical ensembles without
invoking simulations with constraints, biasing potentials or unknown
parameters. The method is illustrated on a model 10-dimensional potential
energy surface, for which analytical results are obtained. It is then applied
to the potential of mean force associated with the dihedral angle of the butane
molecule in gas phase and in carbon tetrachloride solvent. Finally,
isomerization in a small rocksalt cluster, Na4F4, is investigated in the
microcanonical ensemble, and the results are compared to those of parallel
tempering Monte Carlo.Comment: 6 pages, 5 figure
Statistical evaporation of rotating clusters. IV. Alignment effects in the dissociation of nonspherical clusters
Unimolecular evaporation in rotating, non-spherical atomic clusters is
investigated using Phase Space Theory in its orbiting transition state version.
The distributions of the total kinetic energy release epsilon_tr and the
rotational angular momentum J_r are calculated for oblate top and prolate top
main products with an arbitrary degree of deformation. The orientation of the
angular momentum of the product cluster with respect to the cluster symmetry
axis has also been obtained. This statistical approach is tested in the case of
the small 8-atom Lennard-Jones cluster, for which comparison with extensive
molecular dynamics simulations is presented. The role of the cluster shape has
been systematically studied for larger, model clusters in the harmonic
approximation for the vibrational densities of states. We find that the type of
deformation (prolate vs. oblate) plays little role on the distributions and
averages of epsilon_tr and J_r except at low initial angular momentum. However,
alignment effects between the product angular momentum and the symmetry axis
are found to be significant, and maximum at some degree of oblateness. The
effects of deformation on the rotational cooling and heating effects are also
illustrated.Comment: 15 pages, 9 figure
Dynamics of a 1-D model for the emergence of the plasma edge shear flow layer with momentum conserving Reynolds stress
A one-dimensional version of the second-order transition model based on the
sheared flow amplification by Reynolds stress and turbulence supression by
shearing is presented. The model discussed in this paper includes a form of the
Reynolds stress which explicitly conserves momentum. A linear stability
analysis of the critical point is performed. Then, it is shown that the
dynamics of weakly unstable states is determined by a reduced equation for the
shear flow. In the case in which the flow damping term is diffusive, the
stationary solutions are those of the real Ginzburg-Landau equation.Comment: 21 pages, 8 figure
Spatiotemporal instability of a confined capillary jet
Recent experimental studies on the instability appearance of capillary jets
have revealed the capabilities of linear spatiotemporal instability analysis to
predict the parametrical map where steady jetting or dripping takes place. In
this work, we present an extensive analytical, numerical and experimental
analysis of confined capillary jets extending previous studies. We propose an
extended, accurate analytic model in the limit of low Reynolds flows, and
introduce a numerical scheme to predict the system response when the liquid
inertia is not negligible. Theoretical predictions show a remarkable accuracy
with results from the extensive experimental exploration provided.Comment: Submitted to the Physical Review E (20-March-2008
Time Reversal Mirror and Perfect Inverse Filter in a Microscopic Model for Sound Propagation
Time reversal of quantum dynamics can be achieved by a global change of the
Hamiltonian sign (a hasty Loschmidt daemon), as in the Loschmidt Echo
experiments in NMR, or by a local but persistent procedure (a stubborn daemon)
as in the Time Reversal Mirror (TRM) used in ultrasound acoustics. While the
first is limited by chaos and disorder, the last procedure seems to benefit
from it. As a first step to quantify such stability we develop a procedure, the
Perfect Inverse Filter (PIF), that accounts for memory effects, and we apply it
to a system of coupled oscillators. In order to ensure a many-body dynamics
numerically intrinsically reversible, we develop an algorithm, the pair
partitioning, based on the Trotter strategy used for quantum dynamics. We
analyze situations where the PIF gives substantial improvements over the TRM.Comment: Submitted to Physica
Dual branes in topological sigma models over Lie groups. BF-theory and non-factorizable Lie bialgebras
We complete the study of the Poisson-Sigma model over Poisson-Lie groups.
Firstly, we solve the models with targets and (the dual group of the
Poisson-Lie group ) corresponding to a triangular -matrix and show that
the model over is always equivalent to BF-theory. Then, given an
arbitrary -matrix, we address the problem of finding D-branes preserving the
duality between the models. We identify a broad class of dual branes which are
subgroups of and , but not necessarily Poisson-Lie subgroups. In
particular, they are not coisotropic submanifolds in the general case and what
is more, we show that by means of duality transformations one can go from
coisotropic to non-coisotropic branes. This fact makes clear that
non-coisotropic branes are natural boundary conditions for the Poisson-Sigma
model.Comment: 24 pages; JHEP style; Final versio
Phase changes in 38 atom Lennard-Jones clusters. II: A parallel tempering study of equilibrium and dynamic properties in the molecular dynamics and microcanonical
We study the 38-atom Lennard-Jones cluster with parallel tempering Monte
Carlo methods in the microcanonical and molecular dynamics ensembles. A new
Monte Carlo algorithm is presented that samples rigorously the molecular
dynamics ensemble for a system at constant total energy, linear and angular
momenta. By combining the parallel tempering technique with molecular dynamics
methods, we develop a hybrid method to overcome quasi-ergodicity and to extract
both equilibrium and dynamical properties from Monte Carlo and molecular
dynamics simulations. Several thermodynamic, structural and dynamical
properties are investigated for LJ, including the caloric curve, the
diffusion constant and the largest Lyapunov exponent. The importance of
insuring ergodicity in molecular dynamics simulations is illustrated by
comparing the results of ergodic simulations with earlier molecular dynamics
simulations.Comment: Journal of Chemical Physics, accepte
An Enactive Theory of Need Satisfaction
In this paper, based on the predictive processing approach to cognition, an enactive theory of need satisfaction is discussed. The theory can be seen as a first step towards a computational cognitive model of need satisfaction
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