Cette thèse est dédiée à l'étude des propriétés de magnéto-transport des gaz d'électrons bidimensionnel, et plus spécifiquement du graphène sur carbure de silicium (G/SiC) ainsi qu’à l'interface entre les oxydes complexes LaAlO3 (LAO) et SrTiO3 (STO). Nous exploitons la génération d’un champ magnétique intense (jusqu'à 80 T) et les très basses températures (jusqu'à 40 mK) pour étudier les propriétés de transport quantique, qui sont évocatrices de la structure de bandes électroniques sous-jacente. Dans G/SiC, à la limite du régime d’effet Hall quantique, nous mesurons un plateau de Hall ultra-large quantifié à R=h/2e² couvrant un champ magnétique de plus de 70 T (de 7 T à 80 T). La résistance longitudinale est proche de zéro mais présente, de manière inattendue, de faibles oscillations périodiques avec l’inverse du champ magnétique. Sur la base d’observations microscopiques, ce gaz d’électrons 2D est modélisé par une matrice de graphène ayant une densité de porteurs de charge faible, parsemée d’ilots de taille micrométrique ayant un dopage plus important. Les simulations numériques des propriétés de transport reproduisent bien le plateau de Hall et la présence des oscillations. Au-delà du substrat de SiC qui agit comme un réservoir de charge et stabilise le facteur de remplissage à ν=2, un transfert de charge dépendant du champ magnétique entre les ilots chargés est responsable de la présence des oscillations de la magnétorésistance. Cette étude originale fournit de nouvelles perspectives pour des applications en métrologie. Les propriétés remarquables des gaz d’électrons 2D à l'interface entre les oxydes complexes LAO et STO sont aujourd'hui envisagées pour le développement de futurs dispositifs multifonctionnels. Toutefois, leurs propriétés électroniques sont encore mal connues et nécessitent des recherches plus approfondies. Dans ces systèmes, la magnétorésistance montre des oscillations de Shubnikov-de Haas (SdH) quasi-périodiques et un effet Hall linéaire jusqu'à 55 T à basse température. Nous observons une différence d’un ordre de grandeur entre la densité de porteurs extraite de la période des oscillations SdH et la pente de la résistance de Hall, impliquant la présence de nombreuses sous-bandes à l'énergie de Fermi. Les oscillations quasi-périodiques de la magnétorésistance sont bien reproduites par des simulations numériques prenant en compte l'effet Rashba à l'interface. De plus, à partir de l'évolution des oscillations SdH avec la tension de grille à très basse température (40mK), nous identifions les sous-bandes électroniques contribuant au transport, les orbitales atomiques dont elles dérivent, ainsi que leur localisation spatiale dans la profondeur de l'interface. / This thesis is devoted to the study of the magneto-transport properties of two dimensional electron gas (2DEG), and more specifically graphene on silicon carbide (G/SiC) as well as the interface between two complex oxides LaAlO3 / SrTiO3 (LAO/STO). We take advantage of very high magnetic field (up to 80 T) and very low temperature (down to 40 mK) to investigate the quantum transport properties, which are evocative of the underlying electronic band-structure. In G/SiC, close to the quantum Hall breakdown regime, we measure an ultra-broad quantum Hall plateau at R=h/2e² covering a magnetic field range of more than 70 T (from 7 T to 80 T). Accordingly, the longitudinal resistance is close to zero, but displays unexpected weak 1/B-periodic oscillations. Based on microscopic observations, this 2DEG is modeled as a low charge carrier density graphene matrix decorated by micrometers-size puddles with larger doping. Numerical simulations of the transport properties reproduce well both the broad Quantum Hall plateau and the presence of the oscillations. Besides the SiC substrate which acts as a charge reservoir and stabilizes the quantum Hall state at filling factor ν=2, a magnetic field dependent transfer of charges involving the puddles is responsible for the presence of the oscillating features. This original study provides new insights for resistance metrology purposes. The 2DEG arising at the interface between the complex oxides LAO and STO is nowadays envisioned for future multi-functional devices. Their electronic properties are still a matter of debate and require further investigations. The high field magneto-resistance of this 2DEG displays quasi-periodic Shubnikov-de Haas Oscillations (SdHO) and a linear Hall effect up to 55 T at low temperature. We observe a large discrepancy between the carrier density extracted from the period of the SdHO and the slope of the Hall resistance, which constitutes a strong evidence for the presence of many sub-bands crossing the Fermi energy. The quasi-periodic oscillations of the magneto-resistance are well reproduced by numerical simulations taking into account the strong Rashba effect at the interface. In addition, from the back-gate voltage evolution of the SdHO at sub-kelvin temperature, we identify the electronic sub-bands contributing to transport, the orbital symmetry from which they derive, as well as their spatial localization along the interface.

Im Rahmen dieser Doktorarbeit werden de Haas-van Alphen-Untersuchungen an den nichtmagnetischen Borkarbidsupraleitern LuNi2B2C und YNi2B2C präsentiert. Aus den Quantenoszillationen in der normalleitenden Phase in Kombination mit Bandstrukturrechnungen konnten Informationen über die verzweigte Fermiflächenarchitektur und über die Elektron-Phonon-Kopplung der Borkarbide gewonnen werden. Die Kopplung ist stark anisotrop und fermiflächenabhängig. Dies spricht für einen Mehrbandmechanismus der Supraleitung in der Materialklasse. Zusätzlich konnten de Haas-van-Alphen-Oszillationen mehrerer Fermiflächen unterhalb von Bc2 tief in der Shubnikov-Phase beobachtet werden. Das Verhalten dieser Oszillationen lässt sich nicht mit bisher bekannten Theorien beschreiben. Allerdings weist das Bestehen der Oszillationen weit unterhalb von Bc2 auf ein Bestehen von elektronischen Zuständen in der Shubnikov-Phase hin.

Mittels Photolumineszenz(PL)-Spektroskopie werden die Lumineszenzeigenschaften von wurtzitischen MgZnO-Dünnfilmen mit Mg-Konzentrationen im Bereich von 0 ≤ x ≤ 0,35 experimentell untersucht und die gefundenen Zusammenhänge anhand mehrerer im Rahmen dieser Arbeit entwickelter Modelle theoretisch beschrieben. Dabei werden Erklärungen für verschiedene Auswirkungen der Mischkristallunordnung auf die Lumineszenz dieses ternären Mischhalbleiters vorgestellt, welche in der Literatur bisher nicht bzw. nur unvollständig untersucht wurden. Aufgrund der Mischkristallverbreiterung überlagern sich in MgZnO für x > 0,02 die Lumineszenzbeiträge von störstellengebundenen, freien und in Potentialmulden lokalisierten Exzitonen. Sie können daher mittels zeitintegrierter PL nicht spektral getrennt werden. In dieser Arbeit wird gezeigt, dass die verschiedenen Übergänge dennoch durch zeitaufgelöste PL-Messungen unterschieden und identifiziert werden können. Die gemessenen PL-Transienten werden angepasst und die Linienform der Lumineszenzabklingkurven in Abhängigkeit von der PL-Energie analysiert. Die Bewegung der Exzitonen im Mischkristall wird unter Verwendung eines Effektiv-Masse-Modells quantenmechanisch beschrieben und der Einfluss der Mischkristallunordnung auf die optischen Übergänge qualitativ untersucht. Dabei wird insbesondere auf die Mischkristallverbreiterung sowie auf das nichtexponentielle Abklingen der Lumineszenz eingegangen. Daneben wird ein Tunnelmodell vorgestellt, mit welchem die zeitverzögerten PL-Spektren von MgZnO quantitativ reproduziert werden können. Dabei wird die asymmetrische Linienform sowie die zeitabhängige Rotverschiebung des Emissionsmaximums modelliert und die Parameter auf mikroskopische Eigenschaften der Exzitonen zurückgeführt. Außerdem wird die für Mischkristalle typische S-förmige Verschiebung des temperaturabhängigen PL-Maximums durch ein modifiziertes Arrheniusmodell erklärt.

Materials play an important role in society. Historically, and even at present, materials have been discovered by trial and error, and many of the most useful materials have been discovered by chance. The high-throughput approach aims to remove (as much as possible) chance and guesswork at the experimental level by filtering out materials candidates that are not predicted to exist. Many successes have been recorded. In the high-throughput approach to materials discovery, machined-learned models of materials are created from databases of theoretical materials. These databases are the result of millions of density-functional-theory (DFT) simulations. The size and accuracy of the data in the databases (and, consequently, the predictions of machined-learned models) are most affected by the band energy calculation; most of the computation of a DFT simulation is computing the band energy in the self-consistency cycle, and most of the error in the simulation comes from band energy error. The band energy is obtained from a two-part multidimensional numerical integral over the Brillouin or irreducible Brillouin zone. A quadratic approximation and integration algorithm for computing the band energy in 2D and 3D is described. Analytic and semi-analytic integration of quadratic polynomials over simplices improves the accuracy and efficiency of the calculation. A method is proposed for estimating the error bounds of the quadratic approximation that does not require additional eigenvalues. Error propagation of approximation errors leads to an adaptive refinement approach that is driven by band energy error. Because adaptive meshes have little symmetry, integration is performed within the irreducible Brillouin zone, and a general algorithm for computing the irreducible Brillouin zone is described. The efficiency of quadratic integration is tested on realistic empirical pseudopotentials. When compared to current integration methods, uniform quadratic integration over the irreducible Brillouin zone sometimes results in fewer k-points for a given accuracy. Adaptive refinement fails to improve integration performance because band energy error bounds are inaccurate, especially at accidental crossings at the Fermi level.

In the last few years a renewed interest has reappeared in materials that were highly investigated in the 50s-70s, like manganese perovskites, spinel chalcogenides and vanadium oxides. The first two classes of materials are nowadays intensively studied due to the colossal magnetoresistance effect, which is the magnetoresistance associated with a ferromagnetic-paramagnetic transition. Vanadium oxides are known to form many compounds and most of them undergo metal-to-insulator phase transitions, with a high increase in the electrical conductivity (MIT). Many technological applications derive from the variation of the physical properties around the phase transition temperature. Although many efforts have been done in order to understand their electronic structures and to elucidate the MIT mechanisms, the vanadium oxides are still matter of debate in science.The present study has been performed in order to understand the electronic structure of these very intriguing materials. The role of different dopants that induce strong changes in the electronic and magnetic properties has been investigated making use of two spectroscopic techniques, namely X-ray photoelectron and X-ray emission spectroscopy.

The details of the interplay between the electronic structure and the magnetic properties of matter represent a state of the art challenge. In the present work spectroscopic investigations on the electronic structure of some interesting materials are presented. The achieved information has been used in order to answer specific questions related to the magnetic behavior of the investigated materials. For the transition metal dicyanamide compounds it is shown that the electronic states arising from carbon and nitrogen remain roughly unchanged for all compositions. A model for the magnetic superexchange interaction was proposed. In this model the geometry of the crystallographic structure accounts for a particular interaction pattern while the occupancy of the 3d transition metal band is the factor which triggers the changeover from antiferromagnetic to ferromagnetic interaction. In the case of six-membered ferric-wheel molecules the comparison between experimental and theoretical data issued estimations for the magnitude of magnetic exchange interactions. The information on the electronic structure of the LaNi5-xMex (Me=Cu, Al) allowed a better understanding of their magnetic behavior. The changes induced in the electronic structure of the parent compound by partial substitutions of nickel by copper or aluminum are discussed.

This work investigates the defect energies, band alignments, and charge carrier recombination in polycrystalline Cu(In1-xGax)(Se1-ySy)2 chalcopyrite thin films and the interrelationship with the alloy composition. Photoluminescence spectroscopy of investigated Cu-poor Cu(In,Ga)(Se,S)2 layers generally shows broad emission lines with the corresponding maxima shifting towards higher energies under decreasing temperature or under increasing excitation power. Admittance spectroscopy of Cu-poor ZnO/CdS/Cu(In,Ga)(Se,S)2 chalcopyrite devices shows that the activation energies of the dominant defect distributions involving donors at the CdS/absorber interface and deep acceptors in the chalcopyrite bulk, increase upon alloying CuInSe2 with S. The band alignments within the Cu(In1-xGax)(Se1-ySy)2 system are determined using the energy position of the bulk acceptor state as a reference. The band gap enlargement under Ga alloying is accommodated almost exclusively in the rise of the conduction band edge, whereas the increase of band gap upon alloying with S is shared between comparable valence and conduction band offsets. The extrapolated band discontinuities [delta]EV(CuInSe2/CuInS2) = -0.23 eV, [delta]EC(CuInSe2/CuInS2) = 0.21 eV, [delta]EV(CuInSe2/CuGaSe2) = 0.036 eV, and [delta]EC(CuInSe2/CuGaSe2) = 0.7 eV are in good agreement with theoretical predictions. Current-voltage analysis of Cu-poor ZnO/CdS/Cu(In,Ga)(Se,S)2 devices reveals recombination barriers which follow the band gap energy of the absorber irrespective of alloy composition, as expected for dominant recombination in the chalcopyrite bulk. In turn, the recombination at the active junction interface prevails in Cu-rich devices which display substantially smaller barriers when compared to the band gap energy of the absorber. The result indicates that the Cu-stoichiometry is the driving compositional parameter for the charge carrier recombination in the chalcopyrite heterojunctions under investigations.

This master’s thesis focuses on investigating the electronic properties of the superconducting spinel compound LiTi2O4 by means of computational and experimental effort. The title compound has been extensively studied in the past years, being the only known superconducting spinel oxide with relatively high Tc = 11.5 K. Even so, the origin of its superconducting mechanism is under debate, and its anomalous superconductivity is still inquired. Thanks to the recently developed ability to produce high-quality epitaxial LiTi2O4 thin films, a renewed research interest in this compound has matured. With this work, we partake in this challenge and present combined experimental and computational results on the electronic band structure of the material. Density functional theory (DFT) has been employed for the first principle electronic structure calculations performed with the Quantum ESPRESSO software. Furthermore, thin-film samples were in-situ realized with the pulsed laser deposition (PLD) method and investigated through the angle-resolved photoemission spectroscopy (ARPES) technique conducted at the ULTRA end-station of the SLS synchrotron facility at PSI in Switzerland. Here, we report the computed electronic band structure of LiTi2O4, with a detailed investigation of its density of states and Fermi surface. Further, we compare these calculations with the obtained experimental ARPES data. Emerging from this study are results supporting the non-conventional superconducting nature of LiTi2O4, which presents coexisting correlation effects, such as electron-phonon coupling and enhanced electron-electron interactions. / Denna masteruppsats fokuserar på att undersöka de elektroniska egenskaperna hos det supraledande spinellmaterialet LiTi2O4 med hjälp av datorsimuleringar samt experimentella mätningar. LiTi2O4 har studerats omfattande under de senaste åren, eftersom den är den enda kända supraledande spinelloxiden med relativt hög Tc = 11.5 K. Trots det är ursprunget till dess supraledande mekanism debatterad, och meaknismen för dess okonventionella supraledning är fortfarande inte helt förstådd. Tack vare den nyligen utvecklade förmågan att producera tunna högkvalitativa epitaxiella LiTi2O4 filmer, har ett förnyat forskningsintresse för denna förening mognat. Med detta arbete deltar vi i denna utmaning och presenterar kombinerade experimentella och beräkningsresultat om materialets elektroniska bandstruktur. Densitetsfunktionsteori (DFT) har använts för principiella elektroniska strukturberäkningar utförda med Quantum ESPRESSO-mjukvaran. Vidare realiserades tunnfilmsprover in-situ med pulsed laser deposition (PLD) medoden och undersöktes experimentellt via vinkelupplöst fotoemissionsspektroskopi (ARPES) som utfördes vid ULTRA-ändstationen på SLS synkrotronanläggningen vid PSI i Schweiz. Här rapporterar vi den beräknade elektroniska bandstrukturen för LiTi2O4, med en detaljerad undersökning av dess tillståndstäthet och Fermi-yta. Vidare jämför vi dessa teoretiska beräkningar med de erhållna experimentella ARPES data. Resultat från denna studie stöder den icke-konventionella supraledande naturen hos LiTi2O4, som också uppvisare samexisterande korrelationseffekter, såsom elektron-fononkoppling samt starka elektron-elektron-interaktioner.

В работе проведено исследование структурных, оптических и электронных свойств графитоподобного нитрида углерода (g-C3N4), синтезированных методом пиролиза мочевины при температурах 450–600°С. Метод функционала плотности использован для подтверждения выводов, основанных на экспериментальных данных. Был выполнен анализ литературных данных по g-C3N4, были рассмотрены структурные особенности получаемого материала и их оптическое поглощение. Также приведено описание используемых экспериментальных методов и математическое описание используемого метода моделирования – теории функционала плотности. Структура синтезированных образцов g-C3N4 определена по спектрам рентгеновской дифракции и ИК поглощения. Оптическое поглощение образцов исследованы по спектрам диффузного отражения, преобразованным в соответствии с теорией Кубелки-Мунка, оценка ширины энергетической щели для прямых и непрямых оптических переходов проводилась по методу Тауца. Была установлена линейная зависимость параметров кристаллической решетки и ширины щели от температуры синтеза. Для подтверждения экспериментальных результатов было проведено моделирование возможных структур g-C3N4 методом функционала плотности в пакете Quantum ESPRESSO. Была проведена оптимизация положений атомов для различных модельных структур, рассчитаны ИК спектры и зонные структуры электронных состояний. / In this work we investigated structural, optical and electrical properties of graphitic carbon nitride (g-C3N4) synthesized by thermal treatment of urea at 450– 600°C. Density functional theory calculations were performed to support suggestions based on experimental data. Available research data for g-C3N4 was studied, with interest in structural and absorption properties of the material. Additionally, the experimental methods and theory behind DFT-calculations were described. We determined the structure of g-C3N4 samples based on XRD and IR spectra. Optical absorption was investigated using diffuse reflectance spectra transformed with Kubelka-Munk function. Energy band gap was evaluated with Tauc plot for direct and indirect optical transitions. We show that crystal lattice parameters and band gap linearly depend on the synthesis temperature. To further prove our assumptions based on experimental data we also performed DFT-calculations for g-C3N4 in Quantum ESPRESSO software package. We calculated IR spectra and electronic band structures for models of each sample.

Diese Arbeit befasst sich mit den optischen Eigenschaften von dünnen GaN-Schichten gewachsen in verschiedenen Orientierungen. Hierbei werden die optischen Eigenschaften von verspannten M- und A-plane sowie unverspannten C-plane GaN-Schichten untersucht und die Ergebnisse im Rahmen von Bandstrukturberechnungen diskutiert. Im Rahmen dieser Arbeit werden die Bandstrukturverschiebungen theoretisch mittels eines k.p-Näherungsansatzes untersucht. Diese Bandverschiebungen beeinflussen sowohl die Übergangsenergien als auch die Oszillatorstärken. Man findet, dass die C-plane Schicht im Falle einer isotropen Verspannung in der Filmebene keine Anisotropie der optischen Polarisation zeigt. In beiden Fällen zeigen die drei Übergänge von den drei oberen Valenzbändern in das untere Leitungsband andere Polarisationseigenschaften als die entsprechenden Übergänge in C-plane GaN-Schichten. Es wird beobachtet, dass für einen bestimmten Wertebereich der Verspannung in der Filmebene diese Übergänge nahezu vollständig x-,z- bzw. y-artig polarisiert sind. Die verwendeten Schichten wurden auch mittels Transmissionspektroskopie untersucht. Im Falle der M-plane GaN-Schichten können zwei fundamentale Übergänge identifiziert werden, wobei der elektrische Feldvektor E des einfallenden Lichtes einmal parallel (z-Polarisation) und einmal senkrecht (x-Polarisation) auf der c-Achse steht. Die M-plane GaN-Schicht besitzt unterschiedliche Dielektrizitätskonstanten für z-Polarisation und x-Polarisation, welche zu zusätzlichem Dichroismus und Doppelbrechung führen. Als Resultat findet eine Filterung der Polarisation für einfallendes, linear polarisiertes Licht statt. Die elektrische Feldkomponente mit x-Polarisation wird stärker absorbiert als die Komponente mit z-Polarisation. Diese Polarisationsfilterung äußert sich für schmalbandiges Licht in Form einer Drehung der Polarisationsebene in Richtung der c-Achse, wobei ein maximaler Rotationswinkel von 40 Grad gefunden wurde. / In this work, we focus on the optical response of GaN thin films grown along various orientations. The optical properties of strained M- and A- and unstrained C-plane GaN thin films are investigated, and the results are explained with help of band-structure calculations. We calculate the strain-induced band-structure modification using the k.p perturbation approach. The valence-band (VB) states are modified affecting both the transition energies as well as the oscillator strengths. We observe that C-plane GaN does not show any in-plane polarization anisotropy, when an isotropic in-plane strain is applied. For the case of M- and A-plane GaN, one expects to see an in-plane polarization anisotropy even for the unstrained case. Additionally, the in-plane strain significantly changes the band structure and the symmetry of the VB states. The three transitions, involving electrons in the conduction band (CB) and holes in the top three VBs, will exhibit a very different polarization characteristic than the ones for C-plane GaN. These transitions are predominantly x, z, and y polarized, respectively, for a certain range of in-plane strain values, present in our samples. For M-plane GaN thin films, two fundamental transitions can be identified, which occur when the electric field vector E is perpendicular (x-polarization) and parallel c (z-polarization). These transitions give rise to a transmittance spectrum separated by 50 meV at room temperature with respect to each other. This result in a polarization filtering of an incident linearly polarized light beam after transmission, because the electric field component with x-polarization is more strongly absorbed than with z-polarization. This filtering manifests as a rotation of the polarization vector toward the c axis and can be as large as 40 degrees for an initial angle of 60 degrees, for our samples.