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Abstract: The present diploma seminar work has been dedicated first to the implementation of a procedure to calculate the vibrational density of states of two structural models of vitreous SiO 2 (v- SiO 2 ) and amorphous Al 2 O 3 (am-Al 2 O 3 ) models, and next to tune a parametric model for the calculation of the infrared (IR) spectra of am-Al 2 O 3 , in particular of the imaginary part of the dielectric function. The ground state of both structural models is obtained by relaxing the atomic structure by using the conjugate gradient method as implemented in the LAMMPS code. Vibrational frequencies and modes are obtained, in the harmonic approximation, by diagonalizing the dynamical matrices calculated for the given v-SiO 2 and am-Al 2 O 3 structural models. Dynamical matrices are obtained through a finite differences approach and vibrational density of states are plotted by applying Gaussian broadening. The calculation of the dielectric function requires the knowledge of the vibrational frequencies as well as the knowledge of the dynamical (or Born) charge tensors related to the atoms of the am-Al 2 O 3 structural model. For the latter model, a parametrization of the ab-initio Born charge tensors has been carried out with the purpose to allow for the fast calculation of the IR spectrum of any other am- Al 2 O 3 model without the need to calculate for it the Born charge tensors using expensive ab- initio methods. The parametrization of Born charge tensors takes into account, for aluminium atoms, only of the isotropic charge which depends on coordination number and average Al- O bond length of aluminium atoms, while for oxygen atoms coordinated to three Al atoms (75%), dynamical charges are parametrized also by the area bounded by aluminium atoms nearest neighbours of the 3-coordinated oxygen atom. Moreover for analyzing the dynamical charge tensors of 3-coordinated oxygen atoms a decomposition in terms of the representations of the spatial rotations was used. The IR spectrum obtained by means of the above described parametrization provides a good approximation to the IR spectrum obtained by using the ab- initio calculated dynamical charges, as it differs from it, on average, by around 5.1% which is much better (12%) than using a average isotropic charge model (i.e. diagonal Born charge tensors where each diagonal element is one third of the average isotropic charge).
Keywords: Vibrational spectra, am-Al 2 O 3, parametric model
Published: 15.09.2021; Views: 1131; Downloads: 0
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Keywords: kvantni Hallov pojav, 2D materiali, Schrödingerjeva enačba, elektroni v električnem in magnetnem polju, grafen, Diracova enačba
Published: 28.06.2021; Views: 1363; Downloads: 0
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Abstract: Modern wide-field-of-view and all-sky satellites (e.g. Gaia) and ground based surveys (e.g. OGLE) repeatedly cover a large part of the sky and are detecting new, transient astrophysical sources on daily basis. In this thesis I analyzed the data from Gaia and OGLE transient surveys, with special focus on transients located near the centres of galaxies to detect possible Tidal Disruption Events. These occur when a star gets too close to a Super-Massive Black Hole, which lurks in the centres of most galaxies, and gets disrupted due to the black hole's gravitational tidal forces. The goal of my research was to detect possible Tidal Disruption Events and eliminate false candidates, such as supernovae. The work involved daily inspection of new alerts, identified with Gaia and OGLE Transient Detection System. I searched for potential transients in galactic nuclei and in case there was such a transient detected, follow-up spectroscopic observations were initiated in order to help classify the object. During the course of my work I analyzed spectra obtained with the largest telescopes in the world (SALT, VLT) and performed the spectral template matching, recognition of spectral features related to known classes of transients, determination of redshift etc.
Keywords: Flares, transients, Gaia, OGLE, supernovae, tidal disruption events, nuclear transients
Published: 14.05.2019; Views: 3145; Downloads: 116
Fulltext (8,11 MB)

Abstract: In this work, three surfaces, namely, Mg(0001), Mg(10 ̄1 0) and Mg(11 ̄2 0) are investigated by means of Density Functional Theory and Generalized Gradient Approximation first-principle calculations. The surfaces have been characterized in terms of their interlayer distances, surface energies, Projected Density of States and Surface Core Level Shifts. Contrary to what was found in previous studies based on Density Functional Theory-Local Density Approximation, the use of Generalized Gradient Approximation exchange correlation functional unravels the oscillatory behavior of both interlayer distances and Surface Core Level Shifts of Mg(10 ̄1 0). Moreover, the Projected Density Of States of un-reconstructed Mg(11 ̄2 0) exhibit the typical quantisation of electronic levels associated to a 2D-confinement of a 3D nearly-free electron gas, as observed in nano-wires, and a Peierls instability along the unconfined direction. Different possible reconstructions on Mg(11 ̄2 0) have then been investigated. In particular, it is found that the surface energy of (2X1) and (3X1) reconstructions is almost degenerate and lower than the surface energy of the unreconstructed Mg(11 ̄2 0) surface. Because of the quasi-degeneracy of surface energies, Mg(11 ̄2 0) surface should be composed by a combination of all the quasi degenerate phases.
Keywords: Density Functional Theory, first-principle calculations, Mg surface, Surface Core-Level Shift, surface relaxation, surface reconstruction
Published: 04.07.2017; Views: 5155; Downloads: 2085
Fulltext (1,77 MB)

Abstract: Throughout the history human race depended on weather, so one of the priorities for its survival was to understand weather patterns and to be able to forecast weather. With the development of powerful computers, atmospheric numerical methods and precision instruments for atmospheric monitoring, it is possible to predict weather with greater accuracy and for a longer period of time ahead. At the same time, we are able to gain improved understanding of physical processes that occur in the atmosphere and represent one of most important features in our world. This diploma thesis focuses on the lowest part of the atmosphere - troposphere only, as all weather occurs in the troposphere. Weather is a complete collection of momentary thermodynamic states in the atmosphere and is defined with thermodynamic variables and relations between them. The goal of this thesis is development and presentation of a new way to determine the direction and speed of air mass movement, based on the combination of passive and active remote sensing techniques. A lidar is being used to determine the range to an object, in our case a cloud, that can be used as a tracer in the air current. Simultaneously with lidar ranging of clouds that same clouds are being visually monitored in a series of optical photographs. Selecting and following the temporal evolution of distinct cloud features and their range allows us to calculate the speed of clouds. The performance of this method was tested on four cases in Feb. and Mar. 2016. Measurements were performed in Ajdovščina in different weather conditions. Along with remote sensing (infra-red lidar and optical cameras), ground measurements of wind at Ajdovščina were performed. Wind speeds and directions obtained from remote sensing were compared to atmospheric sounding data from Ljubljana and Udine at similar heights and performed within as small as possible time window. In all four cases remote sensing results for wind speeds and directions agree relatively well with atmospheric sounding. Deviations are expected to be primarily due to spatial and temporal mismatch between sounding and remote sensing measurements. Another source of uncertainties are the limitations of the present remote sensing method in the determination of the actual direction of the wind, however, theses limitations could be eliminated in the future by using an all-sky camera and vertical lidar configuration.
Keywords: remote sensing, wind, atmosphere
Published: 13.10.2016; Views: 6781; Downloads: 184
Fulltext (9,48 MB)