4,537 research outputs found
Recent Developments in the Lorentz Integral Transform (LIT) Method
Recent results on electromagnetic reactions into the continuum of systems
with A from 3 to 7 are presented. They have been obtained using the Lorentz
Integral Transform (LIT) method. The method is shortly reviewed, emphasizing
how all the results, though obtained with the sole ingredient of the N-N
potential, contain the full complicated dynamics of the A-body system, both in
the initial and in the final states.Comment: 7 pages, 6 figures, Proc. XIX European Few-Body Conf., Groningen,
Aug. 23-27, 200
Physics of Electroweak Interactions with Nuclei
In this series of lectures it is illustrated how one can study the strong
dynamics of nuclei by means of the electroweak probe. In particular, the most
important steps to derive the cross sections in first order perturbation theory
are reviewed. In the derivation the focus is put on the main ingredients
entering the hadronic part (response functions), i.e. the initial and final
states of the system and the operators relevant for the reaction. Emphasis is
put on the electromagnetic interaction with few-nucleon systems. The Lorentz
integral transform method to calculate the response functions ab initio is
described. A few examples of the comparison between theoretical and
experimental results are shown. The dependence of the response functions on the
nuclear interaction and in particular on three-body forces is emphasized.Comment: Lectures given at the International Scientific Meeting on Nuclear
Physics, La Rabida, Huelva (Spain), July 4-10, 200
Collapse transitions of a periodic hydrophilic hydrophobic chain
We study a single self avoiding hydrophilic hydrophobic polymer chain,
through Monte Carlo lattice simulations. The affinity of monomer for water
is characterized by a (scalar) charge , and the monomer-water
interaction is short-ranged. Assuming incompressibility yields an effective
short ranged interaction between monomer pairs , proportional to
. In this article, we take (resp.
()) for hydrophilic (resp. hydrophobic) monomers and consider a
chain with (i) an equal number of hydro-philic and -phobic monomers (ii) a
periodic distribution of the along the chain, with periodicity
. The simulations are done for various chain lengths , in (square
lattice) and (cubic lattice). There is a critical value of the
periodicity, which distinguishes between different low temperature structures.
For , the ground state corresponds to a macroscopic phase separation
between a dense hydrophobic core and hydrophilic loops. For (but not
too small), one gets a microscopic (finite scale) phase separation, and the
ground state corresponds to a chain or network of hydrophobic droplets, coated
by hydrophilic monomers. We restrict our study to two extreme cases, and to illustrate the physics of the various phase
transitions. A tentative variational approach is also presented.Comment: 21 pages, 17 eps figures, accepted for publication in Eur. Phys. J.
Monte Carlo Results for Projected Self-Avoiding Polygons: A Two-dimensional Model for Knotted Polymers
We introduce a two-dimensional lattice model for the description of knotted
polymer rings. A polymer configuration is modeled by a closed polygon drawn on
the square diagonal lattice, with possible crossings describing pairs of
strands of polymer passing on top of each other. Each polygon configuration can
be viewed as the two- dimensional projection of a particular knot. We study
numerically the statistics of large polygons with a fixed knot type, using a
generalization of the BFACF algorithm for self-avoiding walks. This new
algorithm incorporates both the displacement of crossings and the three types
of Reidemeister transformations preserving the knot topology. Its ergodicity
within a fixed knot type is not proven here rigorously but strong arguments in
favor of this ergodicity are given together with a tentative sketch of proof.
Assuming this ergodicity, we obtain numerically the following results for the
statistics of knotted polygons: In the limit of a low crossing fugacity, we
find a localization along the polygon of all the primary factors forming the
knot. Increasing the crossing fugacity gives rise to a transition from a
self-avoiding walk to a branched polymer behavior.Comment: 36 pages, 30 figures, latex, epsf. to appear in J.Phys.A: Math. Ge
Interacting Elastic Lattice Polymers: a Study of the Free-Energy of Globular Rings
We introduce and implement a Monte Carlo scheme to study the equilibrium
statistics of polymers in the globular phase. It is based on a model of
"interacting elastic lattice polymers" and allows a sufficiently good sampling
of long and compact configurations, an essential prerequisite to study the
scaling behaviour of free energies. By simulating interacting self-avoiding
rings at several temperatures in the collapsed phase, we estimate both the bulk
and the surface free energy. Moreover from the corresponding estimate of the
entropic exponent we provide evidence that, unlike for swollen and
-point rings, the hyperscaling relation is not satisfied for globular
rings.Comment: 8 pages; v2: typos removed, published versio
Separation of traces of metal ions from sodium matrices
Method for isolating metal ion traces from sodium matrices consists of two extractions and an ion exchange step. Extraction is accomplished by using 2-thenoyltrifluoracetone and dithizone followed by cation exchange
Combining rare events techniques: phase change in Si nanoparticles
We introduce a combined Restrained MD/Parallel Tempering approach to study
the difference in free energy as a function of a set of collective variables
between two states in presence of unknown slow degrees of freedom.
We applied this method to study the relative stability of the amorphous vs
crystalline nanoparticles of size ranging between 0.8 and 1.8 nm as a function
of the temperature. We found that, at variance with bulk systems, at low T
small nanoparticles are amorphous and undergo an amorphous-to-crystalline phase
transition at higher T. On the contrary, large nanoparticles recover the
bulk-like behavior: crystalline at low and amorphous at high T
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