19,327 research outputs found
Parameters of the crystalline undulator and its radiation for particular experimental conditions
We report the results of theoretical and numerical analysis of the
crystalline undulators planned to be used in the experiments which are the part
of the ongoing PECU project [1]. The goal of such an analysis was to define the
parameters (different from those pre-set by the experimental setup) of the
undulators which ensure the highest yield of photons of specified energies. The
calculations were performed for 0.6 and 10 GeV positrons channeling through
periodically bent Si and SiGe crystals.Comment: 13 pages, 8 figures, submitted to SPI
Two-Dimensional Dynamics of Ultracold Atoms in Optical Lattices
We analyze the dynamics of ultracold atoms in optical lattices induced by a
sudden shift of the underlying harmonic trapping potential. In order to study
the effect of strong interactions, dimensionality and lattice topology on
transport properties, we consider bosonic atoms with arbitrarily strong
repulsive interactions, on a two-dimensional square lattice and a hexagonal
lattice. On the square lattice we find insulating behavior for weakly
interacting atoms and slow relaxation for strong interactions, even when a Mott
plateau is present, which in one dimension blocks the dynamics. On the
hexagonal lattice the center of mass relaxes to the new equilibrium for any
interaction strength.Comment: 4 pages, 6 figures; references added; improved figure
Conformational changes in glycine tri- and hexapeptide
We have investigated the potential energy surfaces for glycine chains
consisting of three and six amino acids. For these molecules we have calculated
potential energy surfaces as a function of the Ramachandran angles phi and psi,
which are widely used for the characterization of the polypeptide chains. These
particular degrees of freedom are essential for the characterization of
proteins folding process. Calculations have been carried out within ab initio
theoretical framework based on the density functional theory and accounting for
all the electrons in the system. We have determined stable conformations and
calculated the energy barriers for transitions between them. Using a
thermodynamic approach, we have estimated the times of the characteristic
transitions between these conformations. The results of our calculations have
been compared with those obtained by other theoretical methods and with the
available experimental data extracted from the Protein Data Base. This
comparison demonstrates a reasonable correspondence of the most prominent
minima on the calculated potential energy surfaces to the experimentally
measured angles phi and psi for the glycine chains appearing in native
proteins. We have also investigated the influence of the secondary structure of
polypeptide chains on the formation of the potential energy landscape. This
analysis has been performed for the sheet and the helix conformations of chains
of six amino acids.Comment: 23 pages, 9 figure
Fusion process of Lennard-Jones clusters: global minima and magic numbers formation
We present a new theoretical framework for modelling the fusion process of
Lennard-Jones (LJ) clusters. Starting from the initial tetrahedral cluster
configuration, adding new atoms to the system and absorbing its energy at each
step, we find cluster growing paths up to the cluster sizes of up to 150 atoms.
We demonstrate that in this way all known global minima structures of the
LJ-clusters can be found. Our method provides an efficient tool for the
calculation and analysis of atomic cluster structure. With its use we justify
the magic number sequence for the clusters of noble gas atoms and compare it
with experimental observations. We report the striking correspondence of the
peaks in the dependence on cluster size of the second derivative of the binding
energy per atom calculated for the chain of LJ-clusters based on the
icosahedral symmetry with the peaks in the abundance mass spectra
experimentally measured for the clusters of noble gas atoms. Our method serves
an efficient alternative to the global optimization techniques based on the
Monte-Carlo simulations and it can be applied for the solution of a broad
variety of problems in which atomic cluster structure is important.Comment: 47 pages, MikTeX, 17 figure
Superheavies: Theoretical incitements and predictions
It is well known that in fusion reactions one may get only neutron deficient
superheavy nuclei located far from the island of stability. The multi-nucleon
transfer reactions allow one to produce more neutron enriched new heavy nuclei
but the corresponding cross sections are rather low. Neutron capture process is
considered here as alternative method for production of long-lived neutron rich
superheavy nuclei. Strong neutron fluxes might be provided by nuclear reactors
and nuclear explosions in laboratory frame and by supernova explosions in
nature. All these cases are discussed in the paper.Comment: 7 FIGURE
True ternary fission of superheavy nuclei
We found that a true ternary fission with formation of a heavy third fragment
(a new type of radioactivity) is quite possible for superheavy nuclei due to
the strong shell effects leading to a three-body clusterization with the two
doubly magic tin-like cores. The simplest way to discover this phenomenon in
the decay of excited superheavy nuclei is a detection of two tin-like clusters
with appropriate kinematics in low-energy collisions of medium mass nuclei with
actinide targets. The three-body quasi-fission process could be even more
pronounced for giant nuclear systems formed in collisions of heavy actinide
nuclei. In this case a three-body clusterization might be proved experimentally
by detection of two coincident lead-like fragments in low-energy U+U
collisions.Comment: 4 pages, 7 figure
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