446 research outputs found
Comparative analysis of different preparation methods of chalcogenide glasses: Molecular dynamics structure simulations
Two different preparation methods (liquid-quenching and evaporation) of
chalcogenide glasses have been investigated by molecular dynamics simulations.
Our particular aim was to determine how the structural changes occur due to the
different preparation methods. We applied a classical empirical three-body
potential of selenium to describe the interactions between atoms.
Our simulation shows that a significant difference can be observed in the
homogeneities
Numerical Methods I.
A jegyzet az ELTE Informatikai Kar 2015. évi Jegyzetpályázatának támogatásával készült
Solutions to Problems of Numerical Methods I.
A jegyzet az ELTE Informatikai Kar 2015. évi Jegyzetpályázatának támogatásával készült
Photo-induced volume changes in selenium. Tight-binding molecular dynamics study
Tight-binding molecular dynamics simulations of photo-excitations in small Se
clusters (isolated Se ring and helical Se chain) and glassy Se networks
(containing 162 atoms) were carried out in order to analyse the photo induced
instability inside the amorphous selenium. In the cluster systems after taking
an electron from the highest occupied molecular orbital to the lowest
unoccupied molecular orbital a bond breaking occurs. In the glassy networks
photoinduced volume expansion was observed and at the same time the number of
coordination defects changed significantly due to illumination
The contact binary VW Cephei revisited: surface activity and period variation
Context. Despite the fact that VW Cephei is one of the well-studied contact
binaries in the literature, there is no fully consistent model available that
can explain every observed property of this system.
Aims. Our motivation is to obtain new spectra along with photometric
measurements, to analyze what kind of changes may have happened in the system
in the past two decades, and to propose new ideas for explaining them.
Methods. For the period analysis we determined 10 new times of minima from
our light curves, and constructed a new OC diagram of the system. Radial
velocities of the components were determined using the cross-correlation
technique. The light curves and radial velocities were modelled simultaneously
with the PHOEBE code. All observed spectra were compared to synthetic spectra
and equivalent widths of the H line were measured on their differences.
Results. We have re-determined the physical parameters of the system
according to our new light curve and spectral models. We confirm that the
primary component is more active than the secondary, and there is a correlation
between spottedness and the chromospheric activity. We propose that flip-flop
phenomenon occurring on the primary component could be a possible explanation
of the observed nature of the activity. To explain the period variation of VW
Cep, we test two previously suggested scenarios: presence of a fourth body in
the system, and the Applegate-mechanism caused by periodic magnetic activity.
We conclude that although none of these mechanisms can be ruled out entirely,
the available data suggest that mass transfer with a slowly decreasing rate
gives the most likely explanation for the period variation of VW Cep.Comment: 13 pages, 18 figures, 9 tables, accepted for publication in Astronomy
and Astrophysic
Structure and photo-induced volume changes of obliquely deposited amorphous selenium
Atomic scale computer simulations on structures and photo induced volume
changes of flatly and obliquely deposited amorphous selenium films have been
carried out in order to understand how the properties of chalcogenide glasses
are influenced by their preparation method. Obliquely deposited a-Se thin films
contain more coordination defects, larger voids than the flatly deposited ones.
To model the photo induced volume changes the electron excitation and hole
creation were treated independently within the framework of tight-binding
formalism. Covalent and interchain bond breakings and formations were found.
The obliquely deposited samples containing voids showed a wide spectrum of
photo induced structural changes in microscopic and volume changes in
macroscopic levels.Comment: 14 page
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