35,765 research outputs found
Reaction Dynamics with Exotic Beams
We review the new possibilities offered by the reaction dynamics of
asymmetric heavy ion collisions, using stable and unstable beams. We show that
it represents a rather unique tool to probe regions of highly Asymmetric
Nuclear Matter () in compressed as well as dilute phases, and to test the
in-medium isovector interaction for high momentum nucleons. The focus is on a
detailed study of the symmetry term of the nuclear Equation of State () in
regions far away from saturation conditions but always under laboratory
controlled conditions.
Thermodynamic properties of are surveyed starting from nonrelativistic
and relativistic effective interactions. In the relativistic case the role of
the isovector scalar -meson is stressed. The qualitative new features
of the liquid-gas phase transition, "diffusive" instability and isospin
distillation, are discussed. The results of ab-initio simulations of n-rich,
n-poor, heavy ion collisions, using stochastic isospin dependent transport
equations, are analysed as a function of beam energy and centrality. The
isospin dynamics plays an important role in all steps of the reaction, from
prompt nucleon emissions to the final fragments. The isospin diffusion is also
of large interest, due to the interplay of asymmetry and density gradients. In
relativistic collisions, the possibility of a direct study of the covariant
structure of the effective nucleon interaction is shown. Results are discussed
for particle production, collective flows and iso-transparency.
Perspectives of further developments of the field, in theory as well as in
experiment, are presented.Comment: 167+5 pages, 77 figures, general revie
Modelling of a dynamic multiphase flash: the positive flash. Application to the calculation of ternary diagrams
A general and polyvalent model for the dynamic simulation of a vapor, liquid, liquid-liquid, vapor-liquid or vapor-liquid-liquid stage is proposed. This model is based on the -method introduced as a minimization problem by Han & Rangaiah (1998) for steady-state simulation. They suggested modifying the mole fraction summation such that the same set of governing equations becomes valid for all phase regions. Thanks to judicious additional switch equations, the -formulation is extended to dynamic simulation and the minimization problem is transformed into a set of differential algebraic equations (DAE). Validation of the model consists in testing its capacity to overcome phase number changes and to be able to solve several problems with the same set of equations: calculation of heterogeneous residue curves, azeotropic points and distillation boundaries in ternary diagrams
Helium nanodroplet isolation ro-vibrational spectroscopy: methods and recent results
In this article, recent developments in HElium NanoDroplet Isolation (HENDI)
spectroscopy are reviewed, with an emphasis on the infrared region of the
spectrum. Topics discussed include experimental details, comparison of
radiation sources, symmetry issues of the helium solvation structure, sources
of line broadening, changes in spectroscopic constants upon solvation, and
applications including formation of novel chemical structures.Comment: 24 pages, 8 figures, 3 tables; to be published in the Journal of
Chemical Physic
Exploring the Kondo model in and out of equilibrium with alkaline-earth atoms
We propose a scheme to realize the Kondo model with tunable anisotropy using
alkaline-earth atoms in an optical lattice. The new feature of our setup is
Floquet engineering of interactions using time-dependent Zeeman shifts, that
can be realized either using state-dependent optical Stark shifts or magnetic
fields. The properties of the resulting Kondo model strongly depend on the
anisotropy of the ferromagnetic interactions. In particular, easy-plane
couplings give rise to Kondo singlet formation even though microscopic
interactions are all ferromagnetic. We discuss both equilibrium and dynamical
properties of the system that can be measured with ultracold atoms, including
the impurity spin susceptibility, the impurity spin relaxation rate, as well as
the equilibrium and dynamical spin correlations between the impurity and the
ferromagnetic bath atoms. We analyze the non-equilibrium time evolution of the
system using a variational non-Gaussian approach, which allows us to explore
coherent dynamics over both short and long timescales, as set by the bandwidth
and the Kondo singlet formation, respectively. In the quench-type experiments,
when the Kondo interaction is suddenly switched on, we find that real-time
dynamics shows crossovers reminiscent of poor man's renormalization group flow
used to describe equilibrium systems. For bare easy-plane ferromagnetic
couplings, this allows us to follow the formation of the Kondo screening cloud
as the dynamics crosses over from ferromagnetic to antiferromagnetic behavior.
On the other side of the phase diagram, our scheme makes it possible to measure
quantum corrections to the well-known Korringa law describing the temperature
dependence of the impurity spin relaxation rate. Theoretical results discussed
in our paper can be measured using currently available experimental techniques.Comment: 22 pages, 12 figure
Dynamics of clusters and fragments in heavy-ion collisions
A review is given on the studies of formation of light clusters and heavier
fragments in heavy-ion collisions at incident energies from several tens of
MeV/nucleon to several hundred MeV/nucleon, focusing on dynamical aspects and
on microscopic theoretical descriptions. Existing experimental data already
clarify basic characteristics of expanding and fragmenting systems typically in
central collisions, where cluster correlations cannot be ignored. Cluster
correlations appear almost everywhere in excited low-density nuclear many-body
systems and nuclear matter in statistical equilibrium where the properties of a
cluster may be influenced by the medium. On the other hand, transport models to
solve the time evolution have been developed based on the single-nucleon
distribution function. Different types of transport models are reviewed putting
emphasis both on theoretical features and practical performances in the
description of fragmentation. A key concept to distinguish different models is
how to consistently handle single-nucleon motions in the mean field,
fluctuation or branching induced by two-nucleon collisions, and localization of
nucleons to form fragments and clusters. Some transport codes have been
extended to treat light clusters explicitly. Results indicate that cluster
correlations can have strong impacts on global collision dynamics and
correlations between light clusters should also be taken into account.Comment: review article, 64 pages, 27 figure
Solution of the general dynamic equation along approximate fluid trajectories generated by the method of moments
We consider condensing flow with droplets that nucleate and grow, but do not slip with respect to the surrounding gas phase. To compute the local droplet size distribution, one could solve the general dynamic equation and the fluid dynamics equations simultaneously. To reduce the overall computational effort of this procedure by roughly an order of magnitude, we propose an alternative procedure, in which the general dynamic equation is initially replaced by moment equations complemented with a closure assumption. The key notion is that the flow field obtained from this so-called method of moments, i.e., solving the moment equations and the fluid dynamics equations simultaneously, approximately accommodates the thermodynamic effects of condensation. Instead of estimating the droplet size distribution from the obtained moments by making assumptions about its shape, we subsequently solve the exact general dynamic equation along a number of selected fluid trajectories, keeping the flow field fixed. This alternative procedure leads to fairly accurate size distribution estimates at low cost, and it eliminates the need for assumptions on the distribution shape. Furthermore, it leads to the exact size distribution whenever the closure of the moment equations is exact
Oxygen in the Earth's core: a first principles study
First principles electronic structure calculations based on density
functional theory have been used to study the thermodynamic, structural and
transport properties of solid solutions and liquid alloys of iron and oxygen at
Earth's core conditions. Aims of the work are to determine the oxygen
concentration needed to account for the inferred density in the outer core, to
probe the stability of the liquid against phase separation, to interpret the
bonding in the liquid, and to find out whether the viscosity differs
significantly from that of pure liquid iron at the same conditions. It is shown
that the required concentration of oxygen is in the region 25-30 mol percent,
and evidence is presented for phase stability at these conditions. The Fe-O
bonding is partly ionic, but with a strong covalent component. The viscosity is
lower than that of pure liquid iron at Earth's core conditions. It is shown
that earlier first-principles calculations indicating very large enthalpies of
formation of solid solutions may need reinterpretation, since the assumed
crystal structures are not the most stable at the oxygen concentration of
interest.Comment: 21 pages, 12 figure
RETRASO, a code for modeling reactive transport in saturated and unsaturated porous media
The code RETRASO (REactive TRAnsport of SOlutes) simulates reactive transport of dissolved and gaseous species in non-isothermal saturated or unsaturated problems. Possible chemical reactions include aqueous complexation (including redox reactions), sorption, precipitation-dissolution of minerals and gas dissolution. Various models for sorption of solutes on solids are available, from experimental relationships (linear KD, Freundlich and Langmuir isotherms) to cation exchange and surface complexation models (constant capacitance, diffuse layer and triple layer models). Precipitation-dissolution and aqueous complexation can be modelled in equilibrium or according to kinetic laws. For the numerical solution of the reactive transport equations it uses the Direct Substitution Approach. The use of the code is demonstrated by three examples. The first example models various sorption processes in a smectite barrier. The second example models a complex chemical system in a two dimensional cross-section. The last example models pyrite weathering in an unsaturated medium
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