Characterization of Mg low-index surfaces by first-principlesMiha Gunde
, 2017, magistrsko delo
Opis: 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.
Ključne besede: Density Functional Theory, first-principle calculations, Mg surface, Surface Core-Level Shift, surface relaxation, surface reconstruction
Objavljeno: 04.07.2017; Ogledov: 299; Prenosov: 318
Polno besedilo (1,77 MB)
SiPM as light detector in Cherenkov telescopes cameras at CTAAleš Bogovič
, 2017, magistrsko delo
Opis: This master thesis describes characterization of Silicon photomultipliers (SiPM) for the upgrade of large size telescope (LST) camera at Cherenkov telescope array (CTA). Upon the construction, CTA will be the largest and most sensitive facility for detection of very-high energy (VHE, > 30 GeV) gamma-rays ever. It will use three different sizes of telescopes, where small and medium telescopes camera will be SiPM based from the beginning, but LST camera is planned to get SiPM based camera with frst upgrade. Camera upgrade is among others also being developed at University of Padovas Department of Physics and Astronomy (INFN). Due to limitations of their laboratory characterization of prototypes is done at Institute Jooef Stefan in Ljubljana, where I measured breakdown voltage of and determined photon detection effciency depending on angle of photon arrival. This information is important for determining which SiPM is best for CTA telescopes.
Ključne besede: Silicon photomultiplier, SiPM, CTA
Objavljeno: 03.05.2017; Ogledov: 474; Prenosov: 5
Polno besedilo (14,05 MB)
Measurements and modeling of air mass motion in the troposphereMiha Živec
, 2016, diplomsko delo
Opis: 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.
Ključne besede: remote sensing, wind, atmosphere
Objavljeno: 13.10.2016; Ogledov: 1191; Prenosov: 22
Polno besedilo (9,48 MB)
WATER-POLYPEPTIDE INTERACTION IN CLASSICAL MODELS OF HELIX-COIL TRANSITIONKnarik Yeritsyan
, 2016, diplomsko delo
Opis: Zimm-Bragg model is the simplest to describe the conformational transitions in biopolymers and is regularly used for preocessing the experimental data. We review the model and its Hamiltonian definition with the goal to introduce the interaction with water into the picture. We show how modified ZB model with the account of water-polypeptide interactions allows to describe both cold denaturation and helix-coil transition and derive such the formula explicitly. The obtained theoretical expression for the helicity degree contains two independent parameters that can be fitted with the experimental data to determine the parameters of cold denaturation and helix-coil transition from a single fit and for a single set of experimental data.
Ključne besede: Zimm-Bragg model, conformational transitions in biopolymers, cold denaturation
Objavljeno: 28.09.2016; Ogledov: 1026; Prenosov: 49
Polno besedilo (1,70 MB)
Detection of gravitational wavesGregor Mrak
, 2016, diplomsko delo
Opis: This report mainly focuses on the first two detections of gravitational waves
(GW). At the beginning of the thesis the historical background of GW explorations
is discussed by pointing out some arguments and researchers
that were essential for the progress. Continuing with the derivation based
on the theory of general relativity, in order to express where certain properties
derive. In the main part of the report, the subject of interest becomes
Laser interferometer gravitational-wave observatory (LIGO), beginning
with the problems occurring on the way to detection, describing different
noise sources one by one. After pointing out the problems, solutions
are discussed, namely the technological advances made to lower the background
noise as much as possible. The chapter (4) points out different parts
of the detector which are implemented to produce a clearer signal. As only
physical noise reduction methods are not enough, digital signal processing
algorithms are mentioned as well which are used to analyze the data. Subsequently
the required theoretical knowledge is given, so the main part ends
with the analysis of actual data taken from LIGO’s public release for both
GW150914 and GW151226 events (first and second official detections). In
the conclusion of the report a brief descriptions of various detectors besides
Ligo are given and some insight of the future detectors that are planned to
Ključne besede: Gravitational waves, Ligo, Binary black hole merger, gravitational
Objavljeno: 27.09.2016; Ogledov: 1243; Prenosov: 62
Polno besedilo (3,27 MB)
Characterization of Nd2Fe14B sintered magnets and effect of addition TbF and DyF by Grain Boundary Diffusion ProcessJan Ferjančič
, 2016, magistrsko delo
Opis: Sintrani Nd2Fe14B magneti tretje generacije so večinoma uporabljeni v avtomobilski in transportni industriji. Sintranim magnetom s strukturo Nd2Fe14B so dodane težke redke zemlje (TRZ, e.g. Dy in Tb). Ta vrsta magnetov je uporabljena v električnih pomožnih sistemih (EPS), ki vrtijo volanski drog. Zahtevani lastnosti magnetov v EPS motorjih (v MAHLE Letriki d.o.o.) sta remanenca (Br) 1240-1350 mT in koercitivnost (Hcj) ≥1592 kA/m. Dodajanje TRZ omogoča, da magnet ohranja svojo magnetnost tudi pri visokih temperaturah, kar pa občutno poveča ceno magneta. V zadnjih letih so proizvajalci magnetov razvili novo metodo, tako imenovan postopek interkristalne difuzije Tb in Dy. Difuzija prične med termično obdelavo končanega magneta prevlečenim s slojem TRZ. Med termično obdelavo TRZ difundirajo s površine v notranjost magneta prek kristalne strukture Nd in med zrna Fe, rezultat je visoka prostorska razpršenost TRZ. Interkristalna difuzija omogoča, da lahko ima magnet z manjšo vsebnostjo TRZ enako ali boljšo koercitivnost in remanenco v primerjavi s standardnim postopkom.
Cilj naloge je karakterizacija magnetov, z osredotočenostjo na opis mikrostrukturne homogenosti, homogenosti kristalnih območij in homogenost posameznih elementov v magnetni strukturi. S ciljem, da se definira posebnost interkristalnega difuzijskega postopka. Analiza magnetov v prerezu je bila izvedena z vrstičnim elektronskim mikroskopom (SEM) v povezavi z energijsko disperzijsko rentgensko spektroskopijo (EDX), s katero smo določali lokalne elemente.
Z opazovanjem mikrostrukture smo želeli potrditi homogenost celotne magnetne strukture, kljub nekaterim nehomogenostim pridobljenih iz drugih člankov. Opazili smo razlike v homogenosti med različnimi magneti le pri standardnem postopku. Kljub vsemu povprečna vrednost komponent in prostorska razporeditev elementov potrjuje naše predpostavke.
Ključne besede: NdFeB magneti, interkristalna difuzija TRZ, SEM in EDX
Objavljeno: 29.07.2016; Ogledov: 1358; Prenosov: 79
Polno besedilo (4,17 MB)
Properties of Null HypersurfacesHovhannes Demirtshyan
, 2016, magistrsko delo
Opis: The aim of this thesis is to investigate the effects that a lightlike singular hypersurface can have on a congruence of timelike (spacelike) geodesics and to extend the existing theory to the case of null geodesics.
The introduction discusses the applications of singular hypersurfaces for the description of physical phenomena, their major classfications and includes a short discussion of the two theoretical approaches that exist to study singular hypersurfaces. The second chapter contains detailed description of these approaches. The theoretical frameworks for both cases of lightlike and timelike (spacelike) hypersurfaces are developed. This chapter also discusses the application of these theories to the case when the hypersurface contains a plane fronted lightlike signal.
The final chapter starts with a discussion of the effects that a lightlike singular hypersurface can have on a congruence of timelike (spacelike) geodesics. A new approach to these calculations is presented together with an extension of the theory to the case of a congruence of null geodesics. At
the end of the chapter a concrete example and its similarities with the case of timelike geodesics is discussed.
In conclusion, the thesis suggests a new mathematical framework for describing a congruence of null geodesics crossing a singular null hypersurface. The results may be applied in experimental physics to detect impulsive signals which are located in singular null hypersurfaces and to this end there
is a discussion of the properties and possibilities for a detector of impulsive lightlike signals, which include gravitational waves.
Ključne besede: singular hypersurface, impulsive signal, gravitational wave, null geodesic, timelike (spacelike) geodesic
Objavljeno: 16.05.2016; Ogledov: 1230; Prenosov: 56
Polno besedilo (475,54 KB)