31. Tidal Disruption Events seen through the eyes of Vera C. Rubin ObservatoryKatja Bučar Bricman, 2021, doktorska disertacija Opis: Tidal Disruption Events (TDEs) are rare transients, which are considered to be promising tools in probing supermassive black holes (SMBHs) and their environments in quiescent galaxies, accretion physics, and jet formation mechanisms. The majority of $\approx$ 60 detected TDEs has been discovered with large field of view time-domain surveys in the last two decades. Currently, about 10 TDEs are discovered per year, and we expect this number will increase largely once the Legacy Survey of Space and Time (LSST) at Vera C. Rubin Observatory begins its observations.
In this work we demonstrate and explore the capabilities of the LSST to study TDEs. To begin with, we simulate LSST observations of TDEs over $10$ years of survey duration by including realistic SED models from MOSFiT into the simulation framework of the LSST. SEDs are then converted into observed fluxes and light curves are simulated with the LSST observing strategy minion_1016. Simulated observations are used to estimate the number of TDEs the LSST is expected to observe and to assess the possibility of probing the SMBH mass distribution in the Universe with the observed TDE sample. We find that the LSST has a potential of observing ~1000 TDEs per year, the exact number depending on the SMBH mass distribution and the adopted observing strategy. In spite of this large number, we find that probing the SMBH mass distribution with LSST observed TDEs will not be straightforward, especially at the low-mass end. This is largely attributed to the fact that TDEs caused by low-mass black holes ($\le 10^6 M_\odot$) are less luminous and shorter than TDEs by heavier SMBHs ($> 10^6 M_\odot$), and the probability of observationally missing them with LSST is higher.
Second, we built a MAF TDE metric for photometric identification of TDEs based on LSST data. We use the metric to evaluate the performance of different proposed survey strategies in identifying TDEs with pre-defined identification requirements. Since TDEs are blue in color for months after peak light, which separates them well from SNe and AGN, we include u-band observations as one of the criteria for a positive identification. We find that the number of identified TDEs strongly depends of the observing strategy and the number of u-band visits to a given field in the sky. Observing strategies with a larger number of u-band observations perform significantly better. For these strategies up to 10% of LSST observed TDEs satisfy the identification requirements.
Ključne besede: Ground-based ultraviolet, optical and infrared telescopes
Astronomical catalogs, atlases, sky surveys, databases, retrieval systems, archives, Black holes, Galactic nuclei (including black holes), circumnuclear matter, and bulges, Infall, accretion, and accretion disks
Objavljeno v RUNG: 03.01.2022; Ogledov: 2634; Prenosov: 60
 Celotno besedilo (124,61 MB) |
32. Spectroscopic investigation of oxygen vacancies in CeO[sub]2 : dissertationThanveer Thajudheen, 2021, doktorska disertacija Opis: A unique material, ceria (CeO2), which is widely applied in automobile exhaust catalysts, is functional due to presence of defects in its crystal structure. Furthermore, the structural defects dictate electrical and chemical properties of ceria. The creation of intrinsic oxygen vacancies in ceria is responsible for oxygen-ion conductivity in solid oxide fuel cells. This unfolds the keen interest in ceria defects. Using the analytical technique cathodoluminescence spectroscopy (CLS) we can characterize ceria for its band gap and the defect states within the band gap. Since CLS has a high spatial resolution, high sensitivity to low concentration of defects and ability to obtain depth resolved information it is an obvious technique of choice.
The first part of the thesis is an introduction to the topic and description of the experimental techniques. Importance of ceria as a multifaceted material finding applications in areas spanning from energy production and conversion to biomedical applications is detailed. CLS as a tool to understand defect-related optical properties and advancement in the CL detection systems are discussed. To study the relationship between local structure and its impact on CL emission spectra, an X-ray absorption spectroscopy techniques were used. The X-ray absorption near edge structure (XANES) and the Extended x-ray absorption fine structure (EXAFS) techniques are summarized.
The second part discusses CL emission from ceria. Initially, CL emission from reduced ceria and its dependence on oxygen vacancy concentration are presented. The origin of emission was attributed to different configurations of the oxygen vacancies and polarons. The recent F center description in ceria was adopted here. The intriguing observation of CL emission quenching as a function of oxygen vacancy concentration was explained on the basis of a relative change in population of F centers in ceria. This demonstrated the relevance of local structure for the CL emission in ceria. In order to have a better understanding of the system, La-doped ceria was proposed as a model system. A precise control over the stoichiometry helped to achieve a desired oxygen vacancy concentration. The CL emission behavior, as observed in reduced ceria, was replicated in the case of La-doped ceria and the analysis revealed that F+ centers favor CL emission whereas F0 centers are disadvantageous. The local structure investigation using EXAFS analysis of both cations Ce and La (K-Edge) showed distortion from the fluorite symmetry and corroborated the F center description of oxygen vacancies in ceria. Our results provide an experimental evidence for F center description involving oxygen vacancies and polarons.
Ključne besede: ceria, cathodoluminescence spectroscopy, local structure distortion, EXAFS analysis, La doped ceria, luminescence quenching, F centers, dissertations
Objavljeno v RUNG: 25.11.2021; Ogledov: 2444; Prenosov: 104
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35. Application of machine learning techniques for cosmic ray event classification and implementation of a real-time ultra-high energy photon search with the surface detector of the Pierre Auger Observatory : dissertationLukas Zehrer, 2021, doktorska disertacija Opis: Despite their discovery already more than a century ago, Cosmic Rays (CRs) still did not divulge all their properties yet. Theories about the origin of ultra-high energy (UHE, > 10^18 eV) CRs predict accompanying primary photons. The existence of UHE photons can be investigated with the world’s largest ground-based experiment for detection of CR-induced extensive air showers (EAS), the Pierre Auger Observatory, which offers an unprecedented exposure to rare UHE cosmic particles.
The discovery of photons in the UHE regime would open a new observational window to the Universe, improve our understanding of the origin of CRs, and potentially uncloak new physics beyond the standard model.
The novelty of the presented work is the development of a "real-time" photon candidate event stream to a global network of observatories, the Astrophysical Multimessenger Observatory Network (AMON). The stream classifies CR events observed by the Auger surface detector (SD) array as regards their probability to be photon nominees, by feeding to advanced machine learning (ML) methods observational air shower parameters of individual CR events combined in a multivariate analysis (MVA).
The described straightforward classification procedure further increases the Pierre Auger Observatory’s endeavour to contribute to the global effort of multi-messenger (MM) studies of the highest energy astrophysical phenomena, by supplying AMON partner observatories the possibility to follow-up detected UHE events, live or in their archival data.
Ključne besede: astroparticle physics, ultra-high energy cosmic rays, ultra-high energy photons, extensive air showers, Pierre Auger Observatory, multi-messenger, AMON, machine learning, multivariate analysis, dissertations
Objavljeno v RUNG: 27.10.2021; Ogledov: 2797; Prenosov: 149
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36. Geotourism in Arcos-Pains Karst Region, Minas Gerais, Brazil : dissertationMariana Barbosa Timo, 2021, doktorska disertacija Opis: Knowing the geological evolution of Earth surface allows identifying various natural attractions (biotic and abiotic). It makes possible the use of the most differentiated resources for the practice of tourism. The Arcos-Pains Karst Region has excellent potential for geotourism use. Examples of geosites include the Santuario and Brega Caves, the Dry Valley in the Corumbá region, and the Mastodonte Cave, where fossils of the Haplomastodon waringi mastodon species were found. However, public investment policies aiming at the environmental preservation of this heritage are still minimal or absent. The region also suffers tremendous anthropic pressure due to the extraction and processing of limestone, agribusiness, urban expansion and informal tourism. The process of inventorying and quantifying geosites is an essential tool for defining geoconservation strategies since it is not possible to protect all the geodiversity of the planet. The attributes analysed should integrate several criteria that take into account the intrinsic characteristics of each geosite, its potential use and the level of protection required. Regarding this research, the objective is approaching karst and speleological heritage, intending to define geosites for scientific, educational and tourist use at the Arcos-Pains Karst Region. The purpose of the inventory is to develop a geotourism project that provides essential information for better territorial management. It is also intended to assist environmental agencies in the enterprises’ environmental licensing processes installed in the region. Among the thirty-five (35) sites evaluated, eleven (11) presented above-average use-values in all categories analysed. Also, they presented national relevance and were classified as geosites. In addition to these, another sixteen (16) sites scored sufficiently to be categorised with national relevance. Within the geosites, one (1) was of international relevance: the São Francisco River Canyon. Considering the results, it is essential to deepen the research regarding karst dynamics to equalise geodiversity preservation while allowing its use to benefit society.
Ključne besede: Geodiversity, Geotourism, Geosites, Arcos-Pains Karst Region, Karst, Speleology
Objavljeno v RUNG: 22.10.2021; Ogledov: 2391; Prenosov: 164
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39. Theoretical and experimental aspects of numerosity and quantification in Lebanese Arabic : dissertationAli Al Moussaoui, 2021, doktorska disertacija Opis: A question that is receiving an increasing attention in linguistics research concerns the language components of counting. This topic is approached from syntactic, semantic and pragmatic points of view. While some accounts advocate for purely syntactic or semantic approaches of countability, other adopt hybrid accounts in which labor is divided between syntax and semantics. At the same time, research finds that there is a pragmatic component enriching the interpretation of countability and numerosity in language. This dissertation attempts to contribute into the lines of research concerned in the syntactic, semantic and pragmatic aspects of countability with a focus on Lebanese Arabic (Henceforth LA), a language that has received little attention in the literature of counting.
In this dissertation, a broken agreement pattern in LA between the cardinal numerals and the post-numeral counted noun phrases is examined on the basis of a syntactic-semantic model of countability developed by Stepanov and Stateva (2018). We argue that the current existing model can be used to account for the countability broken agreement pattern in LA when necessary modifications are applied to it. The
success of this model in accounting for the numeral-noun-phrase agreement in LA is added to its success in similar missions in countability in Russian (Stepanov and Stateva 2018) and previously in Japanese (Watanabe 2010).
The pragmatic component of countability in LA is also examined from the angle of pragmatic strengthening. The traditional difference between singular nominals and plural nominals which makes a distinction between domains of atoms
and domains of sums is seriously challenged by current semantic and pragmatic research. The plural is found to be more complex than a simplistic view of more than one, and its interpretation has a pragmatic component which involves enriching the meaning of plurality against singularity. In our dissertation, we intend to examine an enriching pragmatic process of plural against duality in LA, a language that still preserves special morphological marking for duality. So, our research investigates the pragmatic strengthening of the plural morphology in LA against the dual morphology which results in at-least-three meaning of the plural.
On the assumption that pragmatic reasoning contributes to the interpretation of plural nominals, we extend our research to the area of bilingual LA speakers who can be an ideal environment to look into the cognitive processes involved in the interaction between two linguistic systems that have diverging features pertaining to a given linguistic phenomenon, which is countability in our study. We predict crosslinguistic variation in the interpretation of plural morphology. If languages like English associate plural with an at-least-two meaning as a result of pragmatic enrichment with an anti-singularity inference, then, in languages that
morphologically differentiate among singular, plural and dual number, the morphological plural is predicted to correspond to an at-least-three meaning. In this study, we ask whether the predicted variation in the interpretation of plural
morphology among the non-dual and dual-featuring languages is a locus of negative pragmatic transfer of features from LA as a mother tongue and English/ French as foreign languages.
Ključne besede: countability, numerosity, Lebanese Arabic, countability model, negative pragmatic transfer, and foreign language exposure
Objavljeno v RUNG: 16.07.2021; Ogledov: 2923; Prenosov: 124
Celotno besedilo (3,00 MB)
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40. Structural, morphological and chemical properties of metal/topological insulator interfaces : dissertationKatja Ferfolja, 2021, doktorska disertacija Opis: Topological insulators (TIs) represent a new state of matter that possess a different band structure than regular insulators or conductors. They are characterized with a band gap in the bulk and conductive topological states on the surface, which are spin polarized and robust toward contamination or deformation of the surface. Since the intriguing properties of the TIs are localized at the surface, it is important to obtain knowledge of the possible phenomena happening at the interface between TIs and other materials. This is especially true in the case of metals, due to the fact that such interfaces will be present in the majority of foreseen TI applications.
The presented study combines microscopy and spectroscopy techniques for characterization of morphology, stability and chemical interaction at the interface between TI and metals deposited by means of physical vapor deposition. Our research is based on the interface of Bi2Se3 topological insulator with Ag, Ti and Pt – metals that can be encountered in devices or applications predicted to utilize the special properties of topological insulators.
STM and SEM imaging of Ag/Bi2Se3 interface showed that Ag atoms arrange on the surface in the form of islands, whereas significantly bigger agglomerates are found at the surface steps. The interface was found to be unstable in time and resulted in the absorption of the metal into the crystal at room temperature. Evidences of a chemical reaction at the Ag/Bi2Se3 interface are presented, showing that new phases (Ag2Se, AgBiSe2 and metallic Bi) are formed.
Deposition of Ti on Bi2Se3 resulted in different morphologies depending on the film thickness. At a very low coverage (<1 Å) islands are formed. However, the islands growth is hindered before the completion of a full layer due to the occurrence of a chemical reaction. No surface features could be detected by SEM for Ti coverage up to 20 nm. In contrary, when Ti thickness reached 40 nm, compressive stress triggered buckling of the deposited film. XPS analysis revealed that a redox solid-state reaction occurs at the Ti/Bi2Se3 interface at room temperature forming titanium selenides and metallic Bi. The reaction has significant kinetics even at cryogenic temperature of 130 K.
Pt forms a homogenous film over the whole substrate surface, which is stable in time at room temperature. Although the interface of Pt with Bi2Se3 was found to be
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less reactive compared to Ag and Ti, an interfacial phase formed upon annealing to ∼90 °C was detected by TEM cross section experiment.
A model for prediction of interfacial reactions between a metal and Bi2Se3 based on the standard reduction potential of the metals and Gibbs free energy for a model reaction is presented. Based on these two values the reaction can be expected to result in the formation of binary and/or ternary selenides and Bi.
Presented work shows on the importance of metal/topological insulator interfaces characterization taking into account the possibility of a chemical reaction with all of its consequences. Results should be considered for future theoretical and applicative studies involving such interfaces as well as for the possible engineering of 2D TI heterostructures.
Ključne besede: topological insulators, topological surface states, Bi2Se3, thin films, Ag, Ti, Pt, morphology, interfaces, solid-state reaction, metal selenides, reactivity, stability, electron microscopy, dissertations
Objavljeno v RUNG: 09.06.2021; Ogledov: 3676; Prenosov: 173
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