2,014 research outputs found
Double ionization of a three-electron atom: Spin correlation effects
We study the effects of spin degrees of freedom and wave function symmetries
on double ionization in three-electron systems. Each electron is assigned one
spatial degree of freedom. The resulting three-dimensional Schr\"odinger
equation is integrated numerically using grid-based Fourier transforms. We
reveal three-electron effects on the double ionization yield by comparing
signals for different ionization channels. We explain our findings by the
existence of fundamental differences between three-electronic and truly
two-electronic spin-resolved ionization schemes. We find, for instance, that
double ionization from a three-electron system is dominated by electrons that
have the opposite spin
Elastic properties of mono- and polydisperse two-dimensional crystals of hard--core repulsive Yukawa particles
Monte Carlo simulations of mono-- and polydisperse two--dimensional crystals
are reported. The particles in the studied system, interacting through
hard--core repulsive Yukawa potential, form a solid phase of hexagonal lattice.
The elastic properties of crystalline Yukawa systems are determined in the
ensemble with variable shape of the periodic box. Effects of the Debye
screening length (), contact value of the potential (),
and the size polydispersity of particles on elastic properties of the system
are studied. The simulations show that the polydispersity of particles strongly
influences the elastic properties of the studied system, especially on the
shear modulus. It is also found that the elastic moduli increase with density
and their growth rate depends on the screening length. Shorter screening length
leads to faster increase of elastic moduli with density and decrease of the
Poisson's ratio. In contrast to its three-dimensional version, the studied
system is non-auxetic, i.e. shows positive Poisson's ratio
Ground-state phase diagrams of the generalized Falicov-Kimball model with Hund coupling
Charge and spin orderings are studied on the simplest 1D and the 2D square
lattice within the generalized Falicov-Kimball model with Hund coupling between
localized and itinerant electrons. Using the restricted phase diagrams method
(RPDM) a number of simple rules of formation of various sorts of ground state
phases have been detected. In particular, relationships between density of
current carriers (electrons or holes) and type of charge and magnetic
arrangement has been determined. In 2D in the mixed valence regime only axial
stripes (vertical or horizontal) have been found for intermediate values of the
coupling constants. They are composed of ferromagnetic or antiferromagnetic
chains interchanged with non-magnetic ones. For band fillings close to the half
filling stripe phases oriented along one of the main diagonal direction are
formed. The results suggest a possibility of tuning modulations of charge and
magnetic superstructures with a change of doping.Comment: 10 pages, 6 figures; Fig. 2 slightly modified and the text adjusted
accordingly, references adde
Restricted space ab initio models for double ionization by strong laser pulses
Double electron ionisation process occurs when an intense laser pulse
interacts with atoms or molecules. Exact {\it ab initio} numerical simulation
of such a situation is extremely computer resources demanding, thus often one
is forced to apply reduced dimensionality models to get insight into the
physics of the process. The performance of several algorithms for simulating
double electron ionization by strong femtosecond laser pulses are studied. The
obtained ionization yields and the momentum distributions of the released
electrons are compared, and the effects of the model dimensionality on the
ionization dynamics discussed
Strong-field ionization of atoms with valence shell: Two versus three active electrons
For a model atom with the valence shell we construct consistent three-
and two-active electrons models enabling their direct comparison. Within these
models, we study the influence of the third active electron on the double
ionization yield in strong femtosecond laser fields. We reveal proportionality
between double ionization signals obtained with both models in the field
intensity region where non-sequential ionization dominates. We derive
analytically a correspondence rule connecting the double ionization yields
obtained within the three- and two-active electrons models.Comment: version accepted for Phys. Rev.
Average rate of phenological changes in Poland according to climatic changes : evaluation and mapping
The main objective of this study was evaluation and mapping of an average rate of phenological
changes for example special plant s indicators as a result of climatic changes in Poland.
Multi-year analysis clearly showed a tendency to earlier onset of spring events. The average advance of
flowering/leafing was -1.4 days/decade and -2.4 days/1^{\circ}C. Whereas the response of autumn phenophases
was ambiguous. Phenological and climate data come from archives of the Institute of Meteorology and Water
Management.
Analysis covered the period of 1951-1992. The relation between temperature and date of phenophases was
described with Pearson's linear regression model. Statistical significance of the model parameters was checked
with Student's t-test at the following levels: 0.05, 0.01, and 0.001.
The results were visualised on maps. ArcGIS 9.2 Geostatistical Analyst was used to examine the data and
create prediction maps. Numerous tests were performed in order to find an appropriate method of spatial
interpolation. Finally kriging was chosen as the most precise
G-Compactness and Groups
Lascar described E_KP as a composition of E_L and the topological closure of
EL. We generalize this result to some other pairs of equivalence relations.
Motivated by an attempt to construct a new example of a non-G-compact theory,
we consider the following example. Assume G is a group definable in a structure
M. We define a structure M_0 consisting of M and X as two sorts, where X is an
affine copy of G and in M_0 we have the structure of M and the action of G on
X. We prove that the Lascar group of M_0 is a semi-direct product of the Lascar
group of M and G/G_L. We discuss the relationship between G-compactness of M
and M_0. This example may yield new examples of non-G-compact theories.Comment: 18 page
Strong-Field Double Ionization in a Three-Electron System: Momentum Distribution Analysis
We study strong-field double ionization in a three-electron system by
applying a simplified, reduced-dimensionality model with three active
electrons. The influence of the spin-induced symmetry of the spatial part of
the wavefunction on the final two-photoectron momentum distribution is
discussed. We identify partial momentum distributions originating from
different sets of spins of outgoing electrons providing in this way a quantum
support connection between V-structure and direct ionization typically
explained classically. Changes in the momentum distribution with increasing
field amplitude obtained in our simplified model are shown to be
well-correlated with experimental data known from the literature. The possible
relation between the observed dependencies and different ionization mechanisms
is discussed.Comment: 9 pages, 4 figure
k-Approximate Quasiperiodicity under Hamming and Edit Distance
Quasiperiodicity in strings was introduced almost 30 years ago as an extension of string periodicity. The basic notions of quasiperiodicity are cover and seed. A cover of a text T is a string whose occurrences in T cover all positions of T. A seed of text T is a cover of a superstring of T. In various applications exact quasiperiodicity is still not sufficient due to the presence of errors. We consider approximate notions of quasiperiodicity, for which we allow approximate occurrences in T with a small Hamming, Levenshtein or weighted edit distance.
In previous work Sip et al. (2002) and Christodoulakis et al. (2005) showed that computing approximate covers and seeds, respectively, under weighted edit distance is NP-hard. They, therefore, considered restricted approximate covers and seeds which need to be factors of the original string T and presented polynomial-time algorithms for computing them. Further algorithms, considering approximate occurrences with Hamming distance bounded by k, were given in several contributions by Guth et al. They also studied relaxed approximate quasiperiods that do not need to cover all positions of T.
In case of large data the exponents in polynomial time complexity play a crucial role. We present more efficient algorithms for computing restricted approximate covers and seeds. In particular, we improve upon the complexities of many of the aforementioned algorithms, also for relaxed quasiperiods. Our solutions are especially efficient if the number (or total cost) of allowed errors is bounded. We also show NP-hardness of computing non-restricted approximate covers and seeds under Hamming distance.
Approximate covers were studied in three recent contributions at CPM over the last three years. However, these works consider a different definition of an approximate cover of T, that is, the shortest exact cover of a string T\u27 with the smallest Hamming distance from T
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