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Dutreix, Clément. "Impurity and boundary modes in the honeycomb lattice." Thesis, Paris 11, 2014. http://www.theses.fr/2014PA112217/document.
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La présente thèse s’articule autour de deux sujets. Le premier concerne la localisation des électrons en présence d’impuretés ou d’interfaces dans le réseau hexagonal. Le deuxième, en revanche, traite de l’accumulation de spin dans un supraconducteur hors-Équilibre de type s.Le graphène est la principale motivation de la première partie. Ce matériau bidimensionnel consiste en un feuillet d’atomes de carbones et peut être décrit comme un réseau hexagonal, c’est-à-dire un réseau de Bravais triangulaire avec un motif diatomique. La structure de bande électronique révèle alors l’existence d’électrons de Dirac sans masse et chiraux à basse énergie.D’une part, il est possible d’annihiler ces fermions chiraux en étirant de façon uni-Axiale le matériau. Pour une valeur seuil de l’étirement, les électrons deviennent massiques et non-Relativistes, ce qui définit une transition de phase dite de Lifshitz. Afin de caractériser cette transition, nous étudions la diffusion des électrons sur des impuretés en fonction de l’étirement. Une impureté localisée induit des interférences quantiques dans la densité électronique, connues sous le nom d’oscillations de Friedel. Etant sensibles à la nature chirale des électrons, nous montrons que ces oscillations décroissent selon des lois de puissances qui permettent de caractériser chacune des phases de la transition. La même étude est réalisée dans le cas limite où le diffuseur est une lacune.D’autre part, le motif diatomique du réseau hexagonal propose aussi une incursion dans le monde des isolants et supraconducteurs topologiques. Pour ces systèmes, la caractérisation topologique de la structure de bande électronique permet de prédire l’existence d’états de bord aux interfaces. Nous développons notamment un modèle de supraconducteur topologique basé sur le réseau hexagonal du graphène, en présence de supraconductivité de type singulet (s ou d). Lorsque la symétrie par renversement du temps est brisée par un champ Zeeman, et en présence de couplage spin-Orbit Rashba, nous donnons une prescription qui permet de caractériser les différentes phases topologiques possibles et de prédire l’apparition d’états de bord (états de Majorana) dans des nano-Rubans de graphène.La seconde partie discute l’accumulation de spin dans un supraconducteur hors-Équilibre, joint à un ferromagnétique. Lorsqu’il est à l’équilibre, le supraconducteur est composé de quasiparticules et d’un condensat. L’injection de particules polarisées en charge et en spin, à savoir des électrons polarisés en spin, induit une accumulation de spin et de charge à l’intérieur du supraconducteur. Si l’injection cesse, les populations de spin et de charge vont relaxer vers l’équilibre, mais pas nécessairement sur des échelles de temps identiques. Récemment, la réalisation d’une expérience a mis en évidence que le la charge pouvait relaxer bien plus rapidement que le spin. Afin de confirmer cet effet, une nouvelle expérience a été réalisée grâce à des mesures établies dans le domaine fréquentiel. Ici, nous adressons un model relatif à cette dernière expérience, dans le but d’extraire le temps caractéristique de relaxation du spin qui s’avère être de l’ordre de quelques nanosecondesTwo fields of research define the framework in which the present thesis can be apprehended. The first one deals with impurity and boundary modes in the hexagonal lattice. The second one concerns a spin accumulation in an out-Of-Equilibrium superconductor.Two fields of research define the framework in which the present thesis can be apprehended. The first one deals with impurity and boundary modes in the hexagonal lattice. The second one concerns a spin accumulation in an out-Of-Equilibrium superconductor.Graphene is the main motivation of the first part. From a crystallographic perspective, the carbon atoms in graphene, a graphite layer, design a triangular Bravais lattice with a diatomic pattern. This gives rise to an extra degree of freedom in the electronic band structure that crucially reveals chiral massless Dirac electrons at low-Energy. First of all, it is possible to make these chiral fermions annihilate when a uniaxial strain stretches the graphene layer. For a critical value of the strain, all the fermions become massive and nonrelativistic, which defines a Lifshitz transition. We study the impurity scattering as a function of the strain magnitude. A localised impurity yields quantum interferences in the local density of states that are known as Friedel oscillations. Because they are affected by the chiral nature of the electrons, we show that the decaying laws of these oscillations are specific to the phase the system belongs to. Thus, the impurity scattering offers the possibility to fully characterise the transition.Second, the diatomic pattern of the graphene lattice can also be considered as an invitation to the world of topological insulators and superconductors. The existence of edge states in such systems relies on the topological characterization of the band structure. Here we especially introduce a model of topological superconductor based on the honeycomb lattice with induces spin-Singlet superconductivity. When a Zeeman field breaks the time-Reversal invariance, and in the presence of Rashba spin-Orbit interactions, we give a prescription to describe the topological phases of the system and predict the emergence of Majorana modes (edge states) in strained and doped nanoribbons.The second part discusses the study of a spin accumulation in an out-Of-Equilibrium s-Wave superconductor. At the equilibrium, the superconductor is made of particles coupled by a s-Wave pairing, as well as unpaired quasiparticles. Injecting spin-Polarised electrons into the superconductor induces charge and spin imbalances. When the injection stops, it may happen that charge and spin do not relax over the same time-Scale. The first experiment that points out such a spin-Charge decoupling has recently been realised. In order to confirm this chargeless spin-Relaxation time, a new experiment has been developed [96], based on measurements in the frequency domain. Here, we address a model that fits the experimental data and thus enables the extraction of this characteristic time that is of the order of a few nanoseconds
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McIntosh, Thomas Edward. "A Lanczos study of superconducting correlations on a honeycomb lattice." Thesis, Kingston, Ont. : [s.n.], 2008. http://hdl.handle.net/1974/1071.
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Jimenez, Segura Marco Polo. "Dimer solid-liquid transition in the honeycomb-lattice ruthenate Li2-xRuO3." 京都大学 (Kyoto University), 2016. http://hdl.handle.net/2433/216167.
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Gouw, Reza Raymond. "Nuclear design analysis of square-lattice honeycomb space nuclear rocket engine." [Florida] : State University System of Florida, 2000. http://etd.fcla.edu/etd/uf/2000/amt2440/master.pdf.
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Thesis (M.E.)--University of Florida, 2000.Title from first page of PDF file. Document formatted into pages; contains x, 69 p.; also contains graphics. Vita. Includes bibliographical references (p. 68).
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Plancher, Johann. "Thermal and fluid design analysis of a square lattice honeycomb nuclear rocket engine." [Gainesville, Fla.] : University of Florida, 2002. http://purl.fcla.edu/fcla/etd/UFE1000154.
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Thesis (M.S.)--University of Florida, 2002.Title from title page of source document. Document formatted into pages; contains xi, 80 p.; also contains graphics. Includes vita. Includes bibliographical references.
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Klafka, Tobias [Verfasser]. "Bose-Einstein condensation in higher Bloch bands of the optical honeycomb lattice / Tobias Klafka." Hamburg : Staats- und Universitätsbibliothek Hamburg Carl von Ossietzky, 2021. http://d-nb.info/1241249202/34.
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Halász, Gábor B. "Doping a topological quantum spin liquid : slow holes in the Kitaev honeycomb model." Thesis, University of Oxford, 2015. https://ora.ox.ac.uk/objects/uuid:928ba58d-c69c-4e85-8d49-677d7e9c0fdc.
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We present a controlled microscopic study of hole dynamics in both a gapped and a gapless quantum spin liquid. Our approach is complementary to previous phenomenological works on lightly doped quantum spin liquids as we introduce mobile holes into the ground state of the exactly solvable Kitaev honeycomb model. In the spatially anisotropic (Abelian) gapped phase of the model, we address the properties of a single hole [its internal degrees of freedom as well as its hopping properties], a pair of holes [their absolute and relative particle statistics as well as their interactions], and the collective state for a finite density of holes. Our main result is that the holes in the doped model possess internal degrees of freedom as they can bind the fractional excitations of the undoped model and that the resulting composite holes with different excitations bound are distinct fractional particles with fundamentally different single-particle properties and different experimental signatures in the multi-particle ground state at finite doping. For example, some hole types are free to hop in two dimensions, while others are confined to hop in one dimension only. Also, distinct hole types have different particle statistics and, in particular, some of them exhibit non-trivial (anyonic) relative statistics. At finite doping, the respective hopping dimensionalities manifest themselves in an electrical conductivity that is either approximately isotropic or extremely anisotropic. In the gapless phase of the model, we consider a single hole and address the possibility of a coherent quasiparticle description. Our main result is that a mobile hole has a finite quasiparticle weight which vanishes in the stationary limit. Although this result is obtained in terms of an approximate variational state, we argue that it is also applicable for the exact ground state of the doped model.
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Richter, Eva-Maria [Verfasser], and Daniela [Akademischer Betreuer] Pfannkuche. "Quantum phases and particle correlations in a honeycomb lattice / Eva-Maria Richter. Betreuer: Daniela Pfannkuche." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2016. http://d-nb.info/1095766392/34.
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Richter, Eva-Maria Verfasser], and Daniela [Akademischer Betreuer] [Pfannkuche. "Quantum phases and particle correlations in a honeycomb lattice / Eva-Maria Richter. Betreuer: Daniela Pfannkuche." Hamburg : Staats- und Universitätsbibliothek Hamburg, 2016. http://nbn-resolving.de/urn:nbn:de:gbv:18-78121.
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Shinjo, Kazuya. "Density-matrix renormalization group study of quantum spin systems with Kitaev-type anisotropic interaction." 京都大学 (Kyoto University), 2016. http://hdl.handle.net/2433/215292.
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Morvan, Alexis. "Honeycomb lattices of superconducting microwave resonators : Observation of topological Semenoff edge states." Thesis, Université Paris-Saclay (ComUE), 2019. http://www.theses.fr/2019SACLS037/document.
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Cette thèse décrit la réalisation et l’étude de réseaux bidimensionnels de résonateurs supraconducteurs en nid d’abeille. Ce travail constitue un premier pas vers la simulation de systèmes de la matière condensée avec des circuits supraconducteurs. Ces réseaux sont micro-fabriqués et sont constitués de plusieurs centaines de sites. Afin d’observer les modes propres qui y apparaissent dans une gamme de fréquence entre 4 et 8 GHz, nous avons mis au point une technique d’imagerie. Celle-ci utilise la dissipation locale créée par un laser avec lequel nous pouvons adresser chaque site du réseau. Nous avons ainsi pu mesurer la structure de bande et caractériser les états de bord de nos réseaux. En particulier, nous avons observé les états localisés qui apparaissent à l'interface entre deux isolants de Semenoff ayant des masses opposées. Ces états, dits de Semenoff, sont d'origine topologique. Nos observations sont en excellent accord avec des simulations électromagnétiques ab initioThis thesis describes the realization and study of honeycomb lattices of superconducting resonators. This work is a first step towards the simulation of condensed matter systems with superconducting circuits. Our lattices are micro-fabricated and typically contains a few hundred sites. In order to observe the eigen-modes that appear between 4 and 8 GHz, we have developed a mode imaging technique based on the local dissipation introduced by a laser spot that we can move across the lattice. We have been able to measure the band structure and to characterize the edge states of our lattices. In particular, we observe localized states that appear at the interface between two Semenoff insulators with opposite masses. These states, called Semenoff states, have a topological origin. Our observations are in good agreement with ab initio electromagnetic simulations
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Vinogradov, Nikolay. "Controlling Electronic and Geometrical Structure of Honeycomb-Lattice Materials Supported on Metal Substrates : Graphene and Hexagonal Boron Nitride." Doctoral thesis, Uppsala universitet, Institutionen för fysik och astronomi, 2013. http://urn.kb.se/resolve?urn=urn:nbn:se:uu:diva-194089.
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The present thesis is focused on various methods of controlling electronic and geometrical structure of two-dimensional overlayers adsorbed on metal surfaces exemplified by graphene and hexagonal boron nitride (h-BN) grown on transition metal (TM) substrates. Combining synchrotron-radiation-based spectroscopic and various microscopic techniques with in situ sample preparation, we are able to trace the evolution of overlayer electronic and geometrical properties in overlayer/substrate systems, as well as changes of interfacial interaction in the latter.It is shown that hydrogen uptake by graphene/TM substrate strongly depends on the interfacial interaction between substrate and graphene, and on the geometrical structure of graphene. An energy gap opening in the electronic structure of graphene on TM substrates upon patterned adsorption of atomic species is demonstrated for the case of atomic oxygen adsorption on graphene/TM’s (≥0.35 eV for graphene/Ir(111)). A non-uniform character of adsorption in this case – patterned adsorption of atomic oxygen on graphene/Ir(111) due to the graphene height modulation is verified. A moderate oxidation of graphene/Ir(111) is found largely reversible. Contrary, oxidation of h-BN/Ir(111) results in replacing nitrogen atoms in the h-BN lattice with oxygen and irreversible formation of the B2O3 oxide-like structure. Pronounced hole doping (p-doping) of graphene upon intercalation with active agents – halogens or halides – is demonstrated, the level of the doping is dependent on the agent electronegativity. Hole concentration in graphene on Ir(111) intercalated with Cl and Br/AlBr3 is as high as ~2×1013 cm-2 and ~9×1012 cm-2, respectively. Unusual periodic wavy structures are reported for h-BN and graphene grown on Fe(110) surface. The h-BN monolayer on Fe(110) is periodically corrugated in a wavy fashion with an astonishing degree of long-range order, periodicity of 2.6 nm, and the corrugation amplitude of ~0.8 Å. The wavy pattern results from a strong chemical bonding between h-BN and Fe in combination with a lattice mismatch in either [11 ̅1] or [111 ̅] direction of the Fe(110) surface. Two primary orientations of h-BN on Fe(110) can be observed corresponding to the possible directions of lattice match between h-BN and Fe(110). Chemical vapor deposition (CVD) formation of graphene on iron is a formidable task because of high carbon solubility in iron and pronounced reactivity of the latter, favoring iron carbide formation. However, growth of graphene on epitaxial iron films can be realized by CVD at relatively low temperatures, and the formation of carbides can be avoided in excess of the carbon-containing precursors. The resulting graphene monolayer creates a periodically corrugated pattern on Fe(110): it is modulated in one dimension forming long waves with a period of ~4 nm parallel to the [001] direction of the substrate, with an additional height modulation along the wave crests. The novel 1D templates based on h-BN and graphene adsorbed on iron can possibly find an application in 1D nanopatterning. The possibility for growing high-quality graphene on iron substrate can be useful for the low-cost industrial-scale graphene production.
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Duca, Lucia [Verfasser], and Immanuel [Akademischer Betreuer] Bloch. "Probing topological properties of Bloch bands with ultracold atoms in a honeycomb optical lattice / Lucia Duca. Betreuer: Immanuel Bloch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2015. http://d-nb.info/1079140212/34.
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Szulakowska, Ludmila. "Electron-electron Interactions and Optical Properties of Two-dimensional Nanocrystals." Thesis, Université d'Ottawa / University of Ottawa, 2020. http://hdl.handle.net/10393/40983.
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This thesis presents a theory of electron-electron interaction effects and optical properties
of nanostructures of two-dimensional (2D) honeycomb crystals - graphene and transition metal dichalcogenides (TMDC). Graphene, a semimetallic hexagonal lattice of carbon atoms can be described by a massless Dirac fermion model, with the conduction band (CB) and valence band (VB) touching in the corners of a hexagonal Brillouin zone, valleys K and -K. TMDC crystals sites host either a transition metal atom or a chalcogen dimer, which opens the energy gap and allows for describing their low-energy nature with massive Dirac fermion (mDf) model. The metal atom in TMDC crystals causes strong spin-orbit (SO) coupling, resulting in large SO splitting in bands at both valleys. For TMDCs it is possible to excite carriers in each valley with oppositely circularly polarised light, which offers promising prospects for devices based on electrons valley index, i.e. valleytronic devices. Additionally, the optical response of TMDCs is enhanced by the presence of secondary CB minima, at Q-points.
The dimensionality of 2D crystals can be further reduced to form quantum dots (QDs) - nanostructures con ned in all dimensions. This thesis first discusses hexagonal graphene QDs, which exhibit energy gap oscillation as a function of size, due to the edge type: zigzag or armchair. These QDs are divided into concentric rings, analysed with tight-binding (TB) model. An armchair edged QD is built from a zigzag edged QD by adding a 1D Lieb lattice of carbon atoms on its edge. The energy gap is formed differently for both edges: from the outer ring states for zigzag edge and from the 1D Lieb lattice zero-energy states for armchair edge, which causes the energy gap. The remaining portion of the thesis focuses on TMDC materials. First a TB model is presented for a member of TMDC group, MoS2, using three d orbitals of Mo atom and three p orbitals of the S2 dimers. The tunneling matrix elements between nearest-neighbor and next-nearest-neighbour sites are explicitly derived at K and -K to form a six band TB Hamiltonian. Its solutions are fitted to the bands obtained from the density functional theory ab initio calculations to obtain the correct behaviour of bands around K and additional minima at Q-points, which explains the role of d orbitals in TMDCs. Close to K the TB model is reduced to mDf model, which
is then studied in response to light, yielding the valley-dependent selection rules for
absorption. The interaction of mDf with light is further studied in the presence of strong external magnetic eld, which leads to the formation of Landau levels (LLs), asymmetric
between both valleys, and valley Zeeman splitting. These LLs are populated with electrons
to form a Hartree-Fock ground state (GS), which can exhibit valley polarisation due to the LL asymmetry. Quasi-electron-hole excitations out of the GS are then formed and their self-energy, vertex corrections and scattering energy is calculated. The effect of electron-electron interactions on valley Zeeman splitting is demonstrated and the Bethe-Salpeter equation is numerically solved to give magnetoexciton spectrum for both valleys. The results include a valley-dependent absorption spectrum for mDf magnetoexcitons that vary with the valley polarisation.
The final part of this thesis discusses the single particle and interacting effects in gated MoS2 QDs. First, I perform a single electron atomistic calculation for a million-atom computational box with periodic boundary conditions based on a TB model developed from ab initio methods for bulk MoS2. Electrons are then con ned with a parabolic electrostatic potential from top metallic gates. They exhibit twofold degenerate harmonic oscillator energy spectrum with shell spacing ω associated with valleys K as well as a sixfold degenerate energy spectrum derived from the Q-points. The degeneracy of electronic shells is broken due to valley contrasting Berry curvature,which acts as an effective magnetic eld splitting opposite angular momentum states in both valleys. I populate up to ve K-derived harmonic oscillator shells with up to six electrons and turn on the electron-electron interactions. The resulting GS phases form two regimes dependent on ω, which are dominated each by a broken-symmetry phase, i.e. valley and spin polarised GS for low ω and valley and spin unpolarised but spin intervalley antiferromagnetic GS for higher ω. This behaviour is explained as an effect of the strong SO splitting, weak intervalley exchange interaction and strong correlations. Means of detecting these effects in experiment based on the spin and valley blockade are proposed. These results advance the understanding of interaction-driven breaking of symmetry for valley systems, crucial for designing of valleytronic devices in the future.
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Pinto, Dias Daniela. "Topological properties of flat bands in generalized Kagome lattice materials." Thesis, KTH, Fysik, 2021. http://urn.kb.se/resolve?urn=urn:nbn:se:kth:diva-301294.
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Topological insulators are electronic materials that behave like an ordinary insulator in their bulk but have robust conducting states on their edge. Besides, in some materials the band structure presents completely flat bands, a special feature leading to strong interactions effects. In this thesis we present a study of the edge states of three particular two-dimensional models presenting flat bands: the honeycomb-Kagome, the $\alpha$--graphyne and a ligand decorated honeycomb-Kagome lattice models. We extend earlier work done on these lattice models by focusing on the topological nature of the edge states involving flat bands. We start by giving a review of the band structure theory and the tight-binding approximation. We then present several main topics in two-dimensional topological insulators such as the notion of topological invariants, the Kane-Mele model and the bulk-edge correspondence. Using these theoretical concepts we study the band structure of these lattices firstly without taking into account the spin and spin-orbit interations. We finally add these interactions to get their bulk band structures as well as the edge states. We observe how these spin-orbit interactions relieve degeneracies and allow for the emergence of edge states of topological nature. Since the lattices studied have an arrangement based on the honeycomb-Kagome lattice, two-dimensional materials having the structures of these lattices can be designed assembling metal ions and organic ligands. Therefore the results obtained could be used as a first hint to create new two-dimensional materials presenting topological properties.Topologiska isolatorer är elektroniska material som uppför sig som en vanlig isolator i sin bulk men har robusta ledande stater på kanten. Dessutom presenterar bandstrukturen i vissa material helt platta band, en speciell egenskap som leder till starka interaktionseffekter. I denna avhandling presenterar vi en studie av kanttillstånden för tre speciella tvådimensionella modeller som presenterar platta band: bikakan-Kagome, $\alpha$-grafynen och en liganddekorerad honungskaka-Kagome modeller. Vi utökar tidigare arbete med dessa gittermodeller genom att fokusera på den topologiska karaktären hos kanttillstånd som innefattar platta band. Vi börjar med att ge en genomgång av bandstruktursteorin och den tätt bindande approximationen. Vi presenterar sedan flera huvudämnen i tvådimensionella topologiska isolatorer såsom begreppet topologiska invarianter, Kane-Mele modellen och bulk-kant korrespondensen. Med hjälp av dessa teoretiska begrepp studerar vi bandstrukturen för dessa gitter först utan att ta hänsyn till spinnen och spinnsorbital interaktioner. Vi lägger sedan till dessa interaktioner för att få sina bulkbandstrukturer såväl som kanttillstånden. Vi observerar hur dessa spinnsorbital interaktioner lindrar degenerationer och möjliggör uppkomsten av kanttillstånd av topologisk naturen. Eftersom de undersökta gitterna har ett arrangemang baserat på honungskaka-Kagome gitteren, kan tvådimensionella material med strukturerna hos dessa gitter utformas genom att montera metalljoner och organiska ligander. Därför kan de erhållna resultaten användas som en första ledtråd för att skapa nya tvådimensionella material med topologiska egenskaper.
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Anisimov, Pavel S. [Verfasser], and Maria [Akademischer Betreuer] Daghofer. "Triplons in the excitonic Kitaev-Heisenberg model on the honeycomb lattice : condensation, interactions and topology / Pavel S. Anisimov ; Betreuer: Maria Daghofer." Stuttgart : Universitätsbibliothek der Universität Stuttgart, 2019. http://d-nb.info/1185487506/34.
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Nakamura, Yasuyuki. "Existence of a compensation temperature of a mixed spin-2 and spin-5/2 Ising ferrimagnetic system on a layered honeycomb lattice." The American Physical Society, 2000. http://hdl.handle.net/2237/7150.
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Manni, Soham [Verfasser], Philipp [Akademischer Betreuer] Gegenwart, Angela [Akademischer Betreuer] Rizzi, and Yogesh [Akademischer Betreuer] Singh. "Synthesis and investigation of frustrated Honeycomb lattice iridates and rhodates / Soham Manni. Gutachter: Philipp Gegenwart ; Angela Rizzi ; Yogesh Singh. Betreuer: Philipp Gegenwart." Göttingen : Niedersächsische Staats- und Universitätsbibliothek Göttingen, 2014. http://d-nb.info/1055814507/34.
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Jenderka, Marcus. "Pulsed Laser Deposition of Iridate and YBiO3 Thin Films." Doctoral thesis, Universitätsbibliothek Leipzig, 2017. http://nbn-resolving.de/urn:nbn:de:bsz:15-qucosa-219334.
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Die vorliegende Arbeit befasst sich mit dem Dünnfilmwachstum der ternären Oxide Na2IrO3, Li2IrO3, Y2Ir2O7 und YBiO3. All diesen oxidischen Materialien ist gemein, dass sie Verwirklichungen sogenannter Topologischer Isolatoren oder Spin-Flüssigkeiten sein könnten. Diese neuartigen Materiezustände versprechen eine zukünftige Anwendung in der Quantencomputation, in magnetischen Speichern und in elektrischen Geräten mit geringer Leistungsaufnahme. Die Herstellung der hier gezeigten Dünnfilme ist daher ein erster Schritt zur Umsetzung dieser Anwendungen in der Zukunft. Alle Dünnfilme werden mittels gepulster Laserplasmaabscheidung auf verschiedenen einkristallinen Substraten hergestellt. Die strukturellen, optischen und elektrischen Eigenschaften der Filme werden mittels etablierter experimenteller Verfahren wie Röntgenbeugung, spektroskopischer Ellipsometrie und elektrischenWiderstandsmessungen untersucht. Die strukturellen Eigenschaften von erstmalig in der Masterarbeit des Authors verwirklichten Na2IrO3-Dünnfilmen können durch Abscheidung einer ZnO-Zwischenschicht deutlich verbessert werden. Einkristalline Li2IrO3-Dünnfilme mit einer definierten Kristallausrichtung werden erstmalig hergestellt. Die Messung der dielektrischen Funktion gibt Einblick in elektronische Anregungen, die gut vergleichbar mit Li2IrO3-Einkristallen und verwandten Iridaten sind. Des Weiteren wird aus den Daten eine optische Energielücke von ungefähr 300 meV bestimmt. In Y2Ir2O7-Dünnfilmen wird eine mögliche (111)-Vorzugsorientierung in Wachstumsrichtung gefunden. Im Vergleich mit der chemischen Lösungsabscheidung zeigen die hier mittels gepulster Laserplasmaabscheidung hergestellten YBiO3-Dünnfilme eine definierte, biaxiale Kristallausrichtung in der Wachstumsebene bei einer deutlich höheren Schichtdicke. Über die gemessene dielektrische Funktion können eine direkte und indirekte Bandlücke bestimmt werden. Deren Größe gibt eine notwendige experimentelle Rückmeldung an theoretische Berechnungen der elektronischen Bandstruktur von YBiO3, welche zur Vorhersage der oben erwähnten, neuartigen Materiezuständen verwendet werden. Nach einer Einleitung und Motivation dieser Arbeit gibt das zweite Kapitel einen Überblick über den gegenwärtigen Forschungsstand der hier untersuchten Materialien. Die folgenden zwei Kapitel beschreiben die Probenherstellung und die verwendeten experimentellen Untersuchungsmethoden. Anschließend werden für jedes Material einzeln die experimentellen Ergebnisse dieser Arbeit diskutiert. Die Arbeit schließt mit einer Zusammenfassung und einem AusblickThe present thesis reports on the thin film growth of ternary oxides Na2IrO3, Li2IrO3, Y2Ir2O7 and YBiO3. All of these oxides are candidate materials for the so-called topological insulator and spin liquid, respectively. These states of matter promise future application in quantum computation, and in magnetic memory and low-power electronic devices. The realization of the thin films presented here, thus represents a first step towards these future device applications. All thin films are prepared by means of pulsed laser deposition on various single-crystalline substrates. Their structural, optical and electronic properties are investigated with established experimental methods such as X-ray diffraction, spectroscopic ellipsometry and resistivity measurements. The structural properties of Na2IrO3 thin films, that were previously realized in the author’s M. Sc. thesis for the first time, are improved significantly by deposition of an intermediate ZnO layer. Single-crystalline Li2IrO3 thin films are grown for the first time and exhibit a defined crystal orientation. Measurement of the dielectric function gives insight into electronic excitations that compare well with single crystal samples and related iridates. From the data, an optical energy gap of about 300 meV is obtained. For Y2Ir2O7 thin films, a possible (111) out-of-plane preferential crystal orientation is obtained. Compared to chemical solution deposition, the pulsed laser-deposited YBiO3 thin films presented here exhibit a biaxial in-plane crystal orientation up to a significantly larger film thickness. From the measured dielectric function, a direct and indirect band gap energy is determined. Their magnitude provides necessary experimental feedback for theoretical calculations of the electronic structure of YBiO3, which are used in the prediction of the novel states of matter mentioned above. After the introduction and motivation of this thesis, the second chapter reviews the current state of the science of the studied thin film materials. The following two chapters introduce the sample preparation and the employed experimental methods, respectively. Subsequently, the experimental results of this thesis are discussed for each material individually. The thesis concludes with a summary and an outlook
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Arguilla, Maxx Que. "Electronic and Magnetic Materials From Two-dimensional Honeycomb Tin Lattices." The Ohio State University, 2017. http://rave.ohiolink.edu/etdc/view?acc_num=osu1500481320103395.
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RIBEIRO, Fábio Gomes. "Strongly correlated electrons on the honeycombb lattice: magnetism and superconductivity." Universidade Federal de Pernambuco, 2015. https://repositorio.ufpe.br/handle/123456789/26571.
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In view of quite recent experimental activities on magnetic and superconducting properties of honeycomb and hexagonal lattice based materials, in this thesis we have used field-theoretic and many-body methods to investigate magnetic and superconducting properties of the large-U Hubbard model on the honeycomb lattice at half-filling and in the hole-doped regime. Within the framework of a functional-integral approach, we obtain the Lagrangian density associated with the charge (Grassmann fields) and spin [SU(2) gauge fields] degrees of freedom. The Hamiltonian related to the charge degrees of freedom is exactly diagonalized. In the strong-coupling regime, we derive a perturbative low-energy theory suitable to describe the (quantum) magnetic and superconducting phases at half-filling and in the hole-doped regime. At half-filling, we deal with the underlying spin degrees of freedom of the quantum antiferromagnetic (AF) Heisenberg model by employing a second-order spin-wave analysis, in which case we have calculated the ground-state energy and the staggered magnetization; the results are in very good agreement with previous studies. Further, in the continuum, we derive a nonlinear σ-model with a topological Hopf term that describes the AF-VBS (valence bond solid) competition. In the challenging hole-doped regime, our approach allows the derivation of a t-J Hamiltonian, and the analysis of the role played by charge and spin quantum fluctuations on the ground-state energy and, particularly, on the breakdown of the AF order at a critical hole doping; the results are benchmarked against recent Grassmann tensor product state simulations. In addition, we have performed an extensive study of the electronic structure of the doped system for each competing phase: AF, ferromagnetic (FM), and (spin-singlet pairing) s-, dx₂₋ʏ₂ – and idxʏ -wave superconducting (SC) state induced by purely electronic effects. In this context, an energetic analysis of the ground state of these phases reveal that the AF order prevails for low hole doping, while a dominantly chiral dx₂₋ʏ₂ + idxʏ superconducting state was found in the vicinity of the Van Hove singularity (high hole doping). We also stress that a thermodynamic analysis of the superconducting phase shows that the critical temperature is directly related to the exchange constant J = 4t²/U, in which t denotes the hopping amplitude and U the on-site Coulomb repulsion of the Hubbard model (purely electronic origin). Remarkably, the competition between the AF and dx₂₋ʏ₂ + idxʏ SC phases takes place by the occurrence of a first-order transition accompanied by a spatial phase separation of the referred phases.
Diante dos recentes resultados experimentais sobre propriedades magnéticas e supercontudoras de materias compostos com estruturas cristalinas “rede colmeia" (honeycomb) e hexagonal, nesta tese utilizamos métodos da teoria de campos e da teoria quântica de muitos corpos para investigar as propriedades magnéticas e supercondutoras do modelo de Hubbard no limite de acoplamento forte na rede honeycomb, incluindo os regimes de banda semicheia e dopada (buracos). No âmbito do formalismo de integração funcional, obtivemos uma densidade de lagrangiana associada aos graus de liberdade de carga (campos de Grassmann) e de spin [campos de calibre SU(2)]. O hamiltoniano relacionado aos graus de liberdade de carga é exatamente diagonalizado. No regime de acoplamento forte, derivamos uma teoria perturbativa de baixa energia adequada para descrever as fases (quânticas) magnéticas e supercondutoras nos regimes de banda semi-cheia e dopada por buracos. No regime de banda semi-cheia investigamos os efeitos das flutuações quânticas de spin na fase antiferromagnética (AF) no contexto do modelo de Heisenberg, utilizando uma teoria perturbativa de ondas de spin até O (1/S²), onde S é a magnitude do spin. Com efeito, calculamos a energia do estado fundamental e a magnetização por sítio, cujos resultados estão em boa concordância com estudos anteriores. Além disso, analisamos a competição AF-VBS (estado cristalino de ligação de valência) por meio do modelo σ não-linear com a presença do termo topológico de Hopf. No desafiante regime dopado por buracos, nossa abordagem possibilitou a derivação de um hamiltoniano t-J e a análise do papel desempenhado pelas flutuações quânticas de carga e de spin na energia do estado fundamental da fase AF e, principalmente, no colapso da fase AF para uma dopagem crítica; os resultados são aferidos com recentes simulações de Grassmann tensor product state. Em adição, realizamos um estudo extensivo das estruturas eletrônicas do sistema dopado para cada fase competidora, na ausência de flutuações quânticas de spin: AF, ferromagnética (FM) e supercondutora (SC) induzida por efeitos puramente eletrônicos com simetria (pareamento tipo singleto) s, dx₂₋ʏ₂ ou dxʏ. Neste contexto, uma análise energética do estado fundamental dessas fases revela que a fase AF prevalece no regime de baixa dopagem, enquanto que o estado supercondutor com simetria quiral dx₂₋ʏ₂ + idxʏ predomina nas proximidades da singularidade de Van Hove (regime de alta dopagem). Destacamos ainda que uma análise termodinâmica da fase supercondutora demonstra que a temperatura crítica está diretamente relacionada à constante de troca J = 4t²/U, onde t é a amplitude de hopping e U é a repulsão coulombiana intra-sítio do modelo de Hubbard (origem puramente eletrônica). Finalmente, ressaltamos que a competição entre as fases AF - dx₂₋ʏ₂ + idxʏ SC se manifesta pela ocorrência de uma transição de primeira ordem acompanhada da separação espacial das referidas fases.
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Real, Elgueda Bastián Maximiliano. "Transport and driven-dissipative localization in exciton-polariton lattices." Electronic Thesis or Diss., Université de Lille (2022-....), 2022. http://www.theses.fr/2022ULILR025.
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La simulation des Hamiltoniens de réseaux dans les plateformes photoniques a permis de mieux comprendre les nouvelles propriétés de transport et de localisation dans le contexte de la physique de l'état solide. En particulier, les exciton-polaritons constituent un système polyvalent permettant d'étudier ces propriétés dans des réseaux avec des structures de bande intrigantes en présence de pertes et de gains, et d'interactions entre particules. Les polaritons sont des quasi-particules hybrides lumière-matière résultant du couplage fort entre les photons et les excitons dans les microcavités semi-conductrices, dont les propriétés peuvent être directement accessibles dans les expériences de photoluminescence. Dans cette thèse, nous étudions premièrement les caractéristiques des réseaux en nid d'abeille déformés, composés de résonateurs de polaritons couplés, à haut contenu photonique. Dans un réseau déformé de façon critique, nous mettons en évidence à la fois un transport semi-Dirac et une localisation anisotrope des photons. Deuxièmement, nous montrons qu'un forçage judicieux dans des réseaux de résonateurs à pertes permet l'apparition de nouveaux modes localisés. En utilisant des réseaux de polaritons sous un forçage résonant par plusieurs faisceaux optiques, nous démontrons la possibilité de localiser la lumière sur différentes géométries, voir jusqu'à un seul site. Enfin, nous profitons de l'interaction de polaritons dépendant de la polarisation pour démontrer un effet optique de type Zeeman dans un seul micropilier. En combinant le couplage spin-orbite optique, inhérent aux microstructures semi-conductrices, avec l'effet Zeeman, induit par l'interaction, nous montrons l'émission de faisceaux de vortex avec une chiralité bien définie. Cette thèse met en lumière la puissance des plateformes de polaritons pour étudier les Hamiltoniens de réseaux avec des propriétés sans précédent. Elle apporte également un premier pas vers la génération, entièrement optique, de phases topologiques dans les réseauxThe simulation of lattice Hamiltonians in photonic platforms has been enlightening in the understanding of novel transport and localization properties in the context of solid-state physics. In particular, exciton-polaritons provide a versatile system to investigate these properties in lattices with intriguing band structures in the presence of gain and loss, and particle interactions. Polaritons are hybrid light-matter quasiparticles arising from the strong coupling between photons and excitons in semiconductor microcavities, whose properties can be directly accessed in photoluminescence experiments. In this thesis, we firstly study the features of strained honeycomb lattices made of coupled polariton resonators having high photonic content. In a critically strained lattice, we evidence both a semi-Dirac transport and an anisotropic localization of photons. Secondly, we show that a judicious driving in lattices of lossy resonators allows the appearance of novel localized modes. Using polariton lattices driven resonantly with several optical beams, we demonstrate the localization of light in at-will geometries down to a single site. Finally, we take advantage of the polarization-dependent polariton interaction to demonstrate an optical Zeeman-like effect in a single micropillar. In combination with optical spin-orbit coupling inherent to semiconductor microstructures, the interaction-induced Zeeman effect results in emission of vortex beams with a well-defined chirality. This thesis brings to light the power of polariton platforms to study lattice Hamiltonians with unprecedented properties and it also provides a first step towards the fully-optical generation of topological phases in lattices
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Milicevic, Marijana. "Manipulation of Dirac Cones and Edge states in Polariton Honeycomb Lattices." Electronic Thesis or Diss., Sorbonne université, 2018. http://www.theses.fr/2018SORUS515.
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Les polaritons de cavité sont des particules mixtes lumière-matière résultant du couplage fort entre excitons de puits quantiques et photons de cavité dans des microcavités semi-conductrices. Ils s'avèrent être une plate-forme extraordinaire pour émuler des Hamiltoniens 1D et 2D. Cela est dû au contrôle de site unique lors de la fabrication de réseaux de polaritons, ainsi qu'à la possibilité de visualiser directement la dispersion, les fonctions propres résolus en espace et la propagation des polaritons via des expériences de photoluminescence. En gravant une microcavité à base de GaAs, un réseau en nid d'abeille pour polaritons a été fabriqué. Les deux bandes d’énergie les plus basses de cette structure émulent pour les photons les bandes π et π * du graphène. Remarquablement, le système permet également d'explorer des degrés de liberté orbitaux, inaccessibles dans le graphène réel. Dans la première partie de cette thèse, cet émulateur à polaritons est utilisé pour aborder la physique des états de bord dans un réseau en nid d'abeille. De nouveaux états de bord, à caractère plat et dispersif, ont été découverts et visualisés dans le graphène orbital. Dans la deuxième partie de la thèse, nous démontrons expérimentalement une méthode pour adapter la dispersion de Dirac pour les photons. En mettant en œuvre une déformation uniaxiale dans le réseau en nid d'abeille, des photons de Dirac qui combinent des masses effectives nulle, finie et infinie sont créés. Les résultats présentés ouvrent de nouvelles perspectives pour l'ingénierie d’interfaces entre différents types de dispersions de Dirac. De plus, la partie excitonique des polaritons assure une sensibilité au champ magnétique, créant la possibilité de briser la symétrie d'inversion temporelle du système et d'étudier les états de bord topologiques photoniques dans des cônes de Dirac exotiquesExciton polaritons are mixed light-matter particles arising from strong coupling of quantum well excitons and cavity photons in semiconductor microcavities. They prove to be an extraordinary platform to emulate 1D and 2D Hamiltonians. This is due to the single site control when fabricating polariton lattices as well as to the possibility to directly visualize dispersion, spatial eigenfunctions and propagation of polaritons in photoluminescence experiments. By etching GaAs-based microcavity a honeycomb lattice for polaritons has been fabricated. The lowest two bands of this structure emulate for photons the π and π* bands of graphene. Remarkably, the system also permits exploring orbital degrees of freedom, inaccessible in actual graphene. In the first part of this thesis polariton emulator is used to address the physics of edge states in honeycomb lattice. New edge states, with flat and dispersive character have been discovered and visualised in orbital graphene. In the second part of the thesis we demonstrate experimentally a method to tailor the Dirac dispersion for photons. By implementing uni-axial strain in the honeycomb lattice Dirac photons that combine zero, finite and infinite effective masses are created. Presented results open new perspectives for the engineering of interfaces between various types of Dirac dispersions. Furthermore, the excitonic part of polaritons assures sensitivity to the magnetic field, creating the possibility to break the time reversal symmetry of the system and study photonic topological edge states in exotic Dirac cones. Finally, nonlinear Dirac physics can be probed in this system owing to polariton-polariton interactions
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Wintersperger, Karen [Verfasser], and Immanuel [Akademischer Betreuer] Bloch. "Realization of Floquet topological systems with ultracold atoms in optical honeycomb lattices / Karen Wintersperger ; Betreuer: Immanuel Bloch." München : Universitätsbibliothek der Ludwig-Maximilians-Universität, 2020. http://d-nb.info/1221524461/34.
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Kerdi, Banan Khaled. "Transport quantique des trous dans une monocouche de WSe2 sous champ magnétique intense." Thesis, Toulouse 3, 2021. http://www.theses.fr/2021TOU30009.
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Les dichalcogénures des métaux de transition sont constitués d'un empilement de monocouches atomiques liées entre elles par des liaisons faibles de type Van der Waals. Lorsqu'une monocouche de ce matériau est isolée, la symétrie d'inversion du cristal est brisée et la présence d'un couplage spin-orbite fort introduit une levée de dégénérescence des états électroniques ayant des spins différents. Le facteur de Landé effectif (g*) qui intervient dans l'énergie Zeeman est un paramètre qui caractérise, entre autres, la structure de bande du matériau. Il est exceptionnellement grand dans le système WSe_2 en raison de la présence de tungstène et des interactions électroniques. Sa détermination au travers des mesures de résistance électrique sous champ magnétique intense est l'objet de cette thèse. Dans un premier temps, des monocouches de WSe_2 sont produites par l'exfoliation mécanique du matériau massif et leur adressage électrique à l'échelle micrométrique est réalisé par des procédés technologiques de salle blanche impliquant la lithographie électronique. La magnétorésistance des échantillons produits est ensuite étudiée dans des conditions extrêmes de basse température et de champ magnétique intense. La densité de porteur de charges, des trous dans le cas cette thèse, peut être ajustée in-situ par effet de champ. Dans les monocouches de WSe_2, la quantification de l'énergie des niveaux de Landau modifiée par l'effet Zeeman est révélée par la présence d'oscillations complexes de la magnéto-résistance (oscillations de Shubnikov-de Haas). Le développement d'un modèle théorique dédié, où le désordre est pris en compte par un élargissement Gaussien des niveaux de Landau, est nécessaire afin d'interpréter quantitativement les résultats expérimentaux. Il simule l'évolution des composantes du tenseur de résistivité où les paramètres d'ajustement sont la mobilité électronique, l'énergie des bords de mobilité des niveaux de Landau ainsi que le facteur de Landé effectif. L'ajustement théorique aux résultats expérimentaux permet d'extraire l'évolution de g* des trous en fonction de leur densité dans une gamme variant de 5.10^12 à 7,5.10^12 cm^-2, qui s'inscrit dans la continuité des résultats issus de la littérature. Au-delà des approches novatrices sur le plan des conditions expérimentales et de modélisation, cette étude confirme l'importance des interactions électroniques dans la compréhension des propriétés électroniques de ce matériauTransition metal dichalcogenides are made up of a stack of atomic monolayers bound together by weak Van der Waals interactions. When a single layer of this material is isolated, the crystal inversion symmetry is broken, leading to the degeneracy lifting of the electronic states having different spins in the presence of strong spin-orbit coupling. The effective Landé factor (g*) which arises in the Zeeman energy is a parameter which characterizes, among others, the band-structure of the material. It is exceptionally large in WSe_2 monolayers thanks to the presence of heavy tungsten atoms as well as electronic interactions. Its experimental determination through electrical resistance measurements under intense magnetic field constitutes the objective of this thesis. First, WSe_2 monolayers are produced by mechanical exfoliation of the mother material and their electrical addressing at the micrometric scale is achieved by clean room processes involving electron-beam lithography. Their magneto-resistance is studied under extreme conditions of low temperature and high magnetic field. The charge carrier density, holes in the thesis, can be varied in situ thanks to field effect. In WSe_2 monolayers, the quantization of the Landau level energy modified by the Zeeman effect is revealed by the presence of complex magneto-resistance oscillations (Shubnikov-de Haas oscillations). A dedicated theoretical model, where disorder is introduced through a Gaussian broadening of the Landau levels, is necessary for a quantitative understanding of the experimental results. The components of the resistivity tensor are simulated by this model where the main fitting parameters are the electronic mobility, the mobility edge of the Landau levels and the effective Landé factor. The fitting of the experimental results allows the extraction of g* for a hole density ranging from 5.10^12 to 7.5.10^12 cm^-2, which follows the trend reported in the literature. Beyond the innovative approaches in terms of experimental conditions and modelling, this study confirms the importance of electronic interactions in understanding the electronic properties of this material
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Curcella, Alberto. "Looking for silicene: studies of silicon deposition on metallic and semiconductor substrates." Master's thesis, Alma Mater Studiorum - Università di Bologna, 2015. http://amslaurea.unibo.it/9314/.
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Nel presente lavoro espongo i risultati degli esperimenti svolti durante la mia internship all’Institut des NanoSciences de Paris (INSP), presso l’Università Pierre et Marie Curie (Paris VI), nel team "Phisico-Chimie et Dynamique des Surfaces", sotto la supervisione del Dott. Geoffroy Prévot. L’elaborato è stato redatto e in- tegrato sotto la guida del Dott. Pasquini, del dipartimento di Fisica e Astronomia dell’Università di Bologna. La tesi s’inserisce nel campo di ricerca del silicene, i.e. l’allotropo bidimensionale del silicio. Il cosidetto free-standing silicene è stato predetto teoricamente nel 2009 utilizzando calcoli di Density Functional Theory, e da allora ha stimolato un’intensa ricerca per la sua realizzazione sperimentale. La sua struttura elettronica lo rende particolarmente adatto per eventuali appli- cazioni tecnologiche e sperimentali, mentre lo studio delle sue proprietà è di grande interesse per la scienza di base.
Nel capitolo 1 presento innanzitutto la struttura del silicene e le proprietà previste dagli studi pubblicati nella letteratura scientifica. In seguito espongo alcuni dei risultati sperimentali ottenuti negli ultimi anni, in quanto utili per un paragone con i risultati ottenuti durante l’internship.
Nel capitolo 2 presento le tecniche sperimentali che ho utilizzato per effettuare le misure. Molto tempo è stato investito per ottenere una certa dimistichezza con gli apparati in modo da svolgere gli esperimenti in maniera autonoma.
Il capitolo 3 è dedicato alla discussione e analisi dei risultati delle misure, che sono presentati in relazione ad alcune considerazioni esposte nel primo capitolo.
Infine le conclusioni riassumono brevemente quanto ottenuto dall’analisi dati. A partire da queste considerazioni propongo alcuni esperimenti che potrebbero ulteriormente contribuire alla ricerca del silicene. I risultati ottenuti su Ag(111) sono contenuti in un articolo accettato da Physical Review B.
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Ciou, We Lun, and 邱瑋倫. "Quantum Anomalous Hall Effect in Honeycomb Lattice." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/ja6786.
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碩士國立清華大學
物理系
105
In this thesis, we explore two methods that may generate Quantum Anomalous Hall effect in room temperature on a honey comb lattice. In the first case, we consider the honeycomb lattice in external magnetic fields. It is shown that the distribution of Chern numbers over bands in Hofstadter butterfly obeys the Diophantine equation. Furthermore, we find that the distribution of Chern numbers can be altered by including the spin orbital interaction when its strength exceeds some threshold. It is argued that by appropriate using the spin orbit interaction, the Quantum Anomalous Hall in a subband of a honeycomb lattice in a magnetic field can be engineered. In the second work, we propose to induce magnetization in the honeycomb lattice by placing the honeycomb lattice in proximity to a ferromagnetic material. We find that under strain, pseudo magnetic fields generate flat bands similar to Landau levels in the honeycomb lattice. These flat bands possess non-vanishing Chern number and can exhibit Quantum Anomalous Hall effects in room temperatures.
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Wong, Jian-Hao, and 翁健豪. "Mode Structures for Photonic Crystals of Connected- Honeycomb Lattice." Thesis, 2008. http://ndltd.ncl.edu.tw/handle/80363106071586195438.
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碩士逢甲大學
光電研究所
96
In This thesis, we employed the plane-wave expansion method to study the mode structures for 2D photonic crystals of connected honeycomb lattice. We calculated the photonic band structures, the DOS and the electric field of the eigen modes. By combining the density of state and the field distribution of the eigen modes, we further worked out the position-dependent LDOS which is crucial in determining the decay rate of the spontaneous emission of an active medium embedded in photonic crystals. We have calculated the LDOS at various position in the photonic crystal for a two-level atom with dipole directed perpendicular to the plane of periodicity. The calculated results show that the spontaneous emission of the dipole can be enhanced or suppressed by the photonic crystal, depending on the position of the dipole. Since light propagating in 2D PCs is not necessarily confined to the plane of periodicity, we have also calculated the photonic band structures at various off-plane components of wave vectors. The effective index model is employed to evaluate the band gaps at various off-plane angles. We found that the forbidden angles at certain frequencies can take up more than 50% of the solid angle. This indicated that being 2D as it is, a connected honeycomb PC is possible to prohibit 3D light propagation considerably.
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Manni, Soham. "Synthesis and investigation of frustrated Honeycomb lattice iridates and rhodates." Doctoral thesis, 2014. http://hdl.handle.net/11858/00-1735-0000-0022-5F49-5.
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Yu, Meng-Ru, and 游孟儒. "Andreev reflection in transport of noncentrosymmetric superconductor on honeycomb lattice." Thesis, 2017. http://ndltd.ncl.edu.tw/handle/6urn7u.
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碩士國立交通大學
電子物理系所
105
In this thesis, we investigate transport at an interface between monolayer honeycomb lattice with spin-orbit coupling (SOC) and superconductor with mixed spin-singlet and spin-triplet superconducting pairing and calculate the differential conductance (G/G0). Our analysis method is based on the BTK (G.E.Blonder, M.Tinkham, T.M.Klapwijk) and its extension to graphene by C.W.J.Beenakker. The quantum phenomenon called Andreev reflection (AR) at an interface between normal and superconducting side of tunneling junction is our focus. It converts an electron into hole excitation by the superconducting pairing potential. A hole is reflected in the normal side, and generates a Cooper pair which induces supercurrent in the superconducting side. It is a process that transfers disspative current into non-disspative supercurrent and increases the differential conductance. The SOC induces Rashba type and Kane-Mele type, and both leads to non-trivial topological properties. Antiferromagnetic spin-exchange coupling correlates neighbor site and leads to the spin-singlet d+id’-wave superconducting pairing. The Rashba SO coupling favors spin-triplet p-wave superconducting pairing. Both superconducting conditional states show topological non-trivial properties induce chiral/helical Majorana mode edge state. We compare AR calculation in three different limits of our general model : (i) Rashba SOC + spin-triplet p-wave pairing (ii) K.M. SOC + Rashba SOC + spin-singlet d+id’-wave pairing (iii) generic case with both (i) and (ii) on equal footing. We find different signatures of Andreev reflection conductance in these three cases. Our results are relevant to possible topological superconductor on Silicene, MoS2 and doped graphene.
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Lee, Shi-Ting, and 李詩婷. "Effects Of Strong Impurities On Topological States in 2D Honeycomb Lattice." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/86260462503867087449.
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Lee, Der-Hau, and 李德浩. "Topological phases of matter on honeycomb lattice with spin-orbit coupling." Thesis, 2018. http://ndltd.ncl.edu.tw/handle/k3m2zc.
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博士國立交通大學
電子物理系所
106
Due to the exotic metallic electronic states at their boundary, topological matters attract the intensive attentions of broad research communities, such as physics, chemistry, material science, and microelectronics. In this thesis, we investigate two-dimensional topological materials on honeycomb lattice. Based on second quantization approaches, we have computed the electronic properties of zigzag nanoribbons within various interactions, such as nearest-neighbor (tight-binding) and next-nearest-neighbor hoppings, Kane-Mele and Rashba spin-orbital couplings, singlet d+id and triplet p+ip-wave pairing channels. We also employ non-equilibrium Green's function (NEGF) techniques to analyze transport properties of finite system. The main aim of this thesis is to examine non-trivial topology of electronic structures on honeycomb lattice. The specific objectives of this thesis focus on topological phase transitions in the corresponding quantum spin Hall insulator and non-centrosymmetric superconductor. In order to address theses research objectives, weak-coupling renormalization group approach and topological index (Chern number) computation are performed. In the normal state, the findings indicate a finite-size effect on a zigzag nanoribbon leads to the overlapping of edge state wave functions when Kane-Mele interaction is present. The effective inter-edge hopping terms are considered in the non-interacting calculation. In the superconducting state, topological phase diagrams for non-centrosymmetric superconductors on honeycomb lattice are demonstrated. In addition to the energetically favourable spin singlet d+id channel on honeycomb lattice, Rashba-interaction-based triplet p+ip pairing is also considered to comprise the parity-mixing state. The co-existence of d+id and p+ip-wave pairings appears over a certain parameter space. The results also highlight the co-existence of helical and chiral Majorana fermions at edges of zigzag nanoribbons. Our findings extend the recent investigations and can be referred to further study on topologically graphene-based materials, such as transition metal dichalcogenides (TMDs), group-IV elements (like silicene and germanene) and their binary compounds, and the group III-V compounds, etc.
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Inglis, Stephen. "Quantum groundstates of the spin-1/2 XXZ model on a fully-frustrated honeycomb lattice." Thesis, 2010. http://hdl.handle.net/10012/5581.
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In this thesis we present results from quantum Monte Carlo for the fully-frustrated honeycomb lattice.
The XXZ model is of interest in the classical limit, as there is a mapping between the classical fully-frustrated honeycomb Ising model groundstates and the classical hard-core dimer model groundstate.
The aim of this work is to explore the effect of quantum fluctuations on the fully-frustrated honeycomb model to see what sort of interesting physics arises.
One might expect unusual physics due to the quantum hard-core dimer model, where interesting physics are known to exist.
This is because there is a duality mapping between the classical dimer model and the classical fully-frustrated honeycomb Ising model.
Indeed, by studying the fully-frustrated honeycomb XXZ model we find that in some cases the system orders into crystal-like structures, a case of order-by-disorder.
The most interesting case, when the frustrating bonds are chosen randomly, reveals to us a novel state without any discernible order while at the same time avoiding the freezing one would expect of a glass.
This state is a featureless system lacking low temperature magnetic susceptibility---a candidate ``quantum spin liquid''.
Future work that might more easily measure quantum spin liquid criteria is suggested.
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Kim, Jeehoon. "Scanning tunneling microscopy in La₂₋₂xSr₁₊₂xMn₂O₇ and honeycomb lattice in HOPG with a CNT-STM tip." Thesis, 2007. http://hdl.handle.net/2152/3116.
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Kim, Jeehoon 1970. "Scanning tunneling microscopy in La₂₋₂xSr₁₊₂xMn₂O₇ and honeycomb lattice in HOPG with a CNT-STM tip." 2007. http://hdl.handle.net/2152/13303.
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Prasad, Yogeshwar. "Studies of "clean" and "disordered" Bilayer Optical Lattice Systems Circumventing the 'fermionic Cooling-problem'." Thesis, 2018. http://etd.iisc.ac.in/handle/2005/4030.
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The advancement in the eld of cold-atoms has generated a lot of interest in the condensed matter community. Cold-atom experiments can simulate clean, disor-der/impurity free systems very easily. In these systems, we have a control over various parameters like tuning the interaction between particles by the Feshbach resonance, tuning the hopping between lattice sites by laser intensity and so on. As a result, these systems can be used to mimic various theoretical models, which was hindered because of various experimental limitations. Thus, we have an ex-perimental tool in which we can start with a simple theoretical model and later tune the model experimentally and theoretically to simulate the real materials. This will be helpful in studying the physics of the real materials as we can control interactions as well as the impurities can also be taken care of. But the advance-ment in the eld of cold atoms has seen a roadblock for the fermions in optical lattices. The super uid and anti-ferromagnetic phases has not been achieved for fermions in optical lattices due to the \cooling problem" (entropy issues).
In this thesis, we have addressed the issue of the \cooling problem" for fermions in optical lattice systems and studied the system with determinant quantum Monte Carlo technique. We start by giving a general idea of cold-atoms and optical lat-tice potentials, and a brief review of the experimental work going on in the cold-atomic systems. Experimental limitations like \fermionic cooling problem" have been discussed in some detail. Then we proposed a bilayer band-insulator model to circumvent the \entropy problem" and simultaneously increasing the transi-tion temperature for fermions in optical lattices. We have studied the attractive Hubbard model, which is the minimal model for fermions in optical lattices. The techniques that we have used to study the model are mean- eld theory, Gaussian uctuation theory and determinant quantum Monte Carlo numerical technique. . Chapter-1 : provides a general introduction to the ultra-cold atoms, optical lattice and Feshbach resonance. In this chapter we have discussed about cold-atom experiments in optical lattice systems. Here, we have brie y discussed the control over various parameters in the experiments. The goal of these experiments is to realize or mimic many many-body Hamiltonians in experiments, which until now was just a theoretical tool to describe various many-body physics. In the end we give a brief idea for introducing disorder in the cold-atom experiments discuss the limitations of these experiments in realizing the \interesting" super uid and anti-ferromagnetic phases of fermionic Hubbard model in optical lattices.
Chapter-2 : gives a brief idea of \Determinant Quantum Monte-Carlo" (DQM C) technique that has been used to study these systems. In this chapter we will discuss the DQM C algorithm and the observables that can be calculated. We will discuss certain limitation of the DQM C algorithm like numerical instability and sign problem. We will brie y discuss how sign problem doesn't occur in the model we studied.
Chapter-3 : discusses the way by which we can bypass the \cooling problem" (high entropy state) to get a fermionic super uid state in the cold atom experi-ments. In this chapter we propose a model whose idea hinges on a low-entropy band-insulator state, which can be tuned to super uid state by tuning the on-site attractive interaction by Feshbach resonance. We show through Gaussian uctua-tion theory that the critical temperature achieved is much higher in our model as compared to the single-band Hubbard model. Through detailed variational Monte Carlo calculations, we have shown that the super uid state is indeed the most stable ground state and there is no other competing order. In the end we give a proposal for its realization in the ultra-cold atom optical lattice systems.
Chapter-4 : discusses the DQM C study of the model proposed in chapter-
3. Here we have studied the various single-particle properties like momentum distribution, double occupancies which can be easily measured in cold-atom ex-periments. We also studied the pair-pair and the density-density correlations in detail through DQM C algorithm and mapped out the full T U phase diagram. We show that the proposed model doesn't favor the charge density wave for the interaction strengths we are interested in.
Chapter-5 : gives a brief idea of the e ect of adding an on-site random disorder in our proposed bilayer-Hubbard model. We study the e ect of random disorder on various single-particle properties which can be easily veri ed in cold-atom ex-periments. We studied the suppression of the pair-pair correlations as we increase the disorder strength in our proposed model. We nd that the critical value of the interaction doesn't change in the weak-disorder limit. We estimated the critical disorder strength needed to destroy the super uid state and argued that the tran-sition from the super uid to Bose-glass phase in presence of disorder lies in the universality class of (d + 1) XY model. In the end, we give a schematic U V phase diagram for our system.
Chapter-6 : We studied the bilayer attractive Hubbard model in different lattice geometry, the bilayer honeycomb lattice, both in presence and in absence of the on-site random disorder. We discussed how the pair-pair and density-density cor-relations behave in the presence and absence of disorder. Through the finite-size scaling analysis we see the co-existence of the super fluid and the charge density wave order at half- lling. An in nitesimal disorder destroys the CDW order com-pletely while the super uid phase found to be robust against weak-disorder. We estimated the critical interaction strength, the critical temperature and the critical disorder strength through nite-size scaling, and provide a putative phase diagram for the system considered.
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Prasad, Yogeshwar. "Studies of "clean" and "disordered" Bilayer Optical Lattice Systems Circumventing the 'fermionic Cooling-problem'." Thesis, 2018. http://etd.iisc.ernet.in/2005/4003.
Full textAbstract:
The advancement in the eld of cold-atoms has generated a lot of interest in the condensed matter community. Cold-atom experiments can simulate clean, disor-der/impurity free systems very easily. In these systems, we have a control over various parameters like tuning the interaction between particles by the Feshbach resonance, tuning the hopping between lattice sites by laser intensity and so on. As a result, these systems can be used to mimic various theoretical models, which was hindered because of various experimental limitations. Thus, we have an ex-perimental tool in which we can start with a simple theoretical model and later tune the model experimentally and theoretically to simulate the real materials. This will be helpful in studying the physics of the real materials as we can control interactions as well as the impurities can also be taken care of. But the advance-ment in the eld of cold atoms has seen a roadblock for the fermions in optical lattices. The super uid and anti-ferromagnetic phases has not been achieved for fermions in optical lattices due to the \cooling problem" (entropy issues).
In this thesis, we have addressed the issue of the \cooling problem" for fermions in optical lattice systems and studied the system with determinant quantum Monte Carlo technique. We start by giving a general idea of cold-atoms and optical lat-tice potentials, and a brief review of the experimental work going on in the cold-atomic systems. Experimental limitations like \fermionic cooling problem" have been discussed in some detail. Then we proposed a bilayer band-insulator model to circumvent the \entropy problem" and simultaneously increasing the transi-tion temperature for fermions in optical lattices. We have studied the attractive Hubbard model, which is the minimal model for fermions in optical lattices. The techniques that we have used to study the model are mean- eld theory, Gaussian uctuation theory and determinant quantum Monte Carlo numerical technique. . Chapter-1 : provides a general introduction to the ultra-cold atoms, optical lattice and Feshbach resonance. In this chapter we have discussed about cold-atom experiments in optical lattice systems. Here, we have brie y discussed the control over various parameters in the experiments. The goal of these experiments is to realize or mimic many many-body Hamiltonians in experiments, which until now was just a theoretical tool to describe various many-body physics. In the end we give a brief idea for introducing disorder in the cold-atom experiments discuss the limitations of these experiments in realizing the \interesting" super uid and anti-ferromagnetic phases of fermionic Hubbard model in optical lattices.
Chapter-2 : gives a brief idea of \Determinant Quantum Monte-Carlo" (DQM C) technique that has been used to study these systems. In this chapter we will discuss the DQM C algorithm and the observables that can be calculated. We will discuss certain limitation of the DQM C algorithm like numerical instability and sign problem. We will brie y discuss how sign problem doesn't occur in the model we studied.
Chapter-3 : discusses the way by which we can bypass the \cooling problem" (high entropy state) to get a fermionic super uid state in the cold atom experi-ments. In this chapter we propose a model whose idea hinges on a low-entropy band-insulator state, which can be tuned to super uid state by tuning the on-site attractive interaction by Feshbach resonance. We show through Gaussian uctua-tion theory that the critical temperature achieved is much higher in our model as compared to the single-band Hubbard model. Through detailed variational Monte Carlo calculations, we have shown that the super uid state is indeed the most stable ground state and there is no other competing order. In the end we give a proposal for its realization in the ultra-cold atom optical lattice systems.
Chapter-4 : discusses the DQM C study of the model proposed in chapter-
3. Here we have studied the various single-particle properties like momentum distribution, double occupancies which can be easily measured in cold-atom ex-periments. We also studied the pair-pair and the density-density correlations in detail through DQM C algorithm and mapped out the full T U phase diagram. We show that the proposed model doesn't favor the charge density wave for the interaction strengths we are interested in.
Chapter-5 : gives a brief idea of the e ect of adding an on-site random disorder in our proposed bilayer-Hubbard model. We study the e ect of random disorder on various single-particle properties which can be easily veri ed in cold-atom ex-periments. We studied the suppression of the pair-pair correlations as we increase the disorder strength in our proposed model. We nd that the critical value of the interaction doesn't change in the weak-disorder limit. We estimated the critical disorder strength needed to destroy the super uid state and argued that the tran-sition from the super uid to Bose-glass phase in presence of disorder lies in the universality class of (d + 1) XY model. In the end, we give a schematic U V phase diagram for our system.
Chapter-6 : We studied the bilayer attractive Hubbard model in different lattice geometry, the bilayer honeycomb lattice, both in presence and in absence of the on-site random disorder. We discussed how the pair-pair and density-density cor-relations behave in the presence and absence of disorder. Through the finite-size scaling analysis we see the co-existence of the super fluid and the charge density wave order at half- lling. An in nitesimal disorder destroys the CDW order com-pletely while the super uid phase found to be robust against weak-disorder. We estimated the critical interaction strength, the critical temperature and the critical disorder strength through nite-size scaling, and provide a putative phase diagram for the system considered.
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Jenderka, Marcus. "Pulsed Laser Deposition of Iridate and YBiO3 Thin Films." Doctoral thesis, 2016. https://ul.qucosa.de/id/qucosa%3A15345.
Full textAbstract:
Die vorliegende Arbeit befasst sich mit dem Dünnfilmwachstum der ternären Oxide Na2IrO3, Li2IrO3, Y2Ir2O7 und YBiO3. All diesen oxidischen Materialien ist gemein, dass sie Verwirklichungen sogenannter Topologischer Isolatoren oder Spin-Flüssigkeiten sein könnten. Diese neuartigen Materiezustände versprechen eine zukünftige Anwendung in der Quantencomputation, in magnetischen Speichern und in elektrischen Geräten mit geringer Leistungsaufnahme. Die Herstellung der hier gezeigten Dünnfilme ist daher ein erster Schritt zur Umsetzung dieser Anwendungen in der Zukunft. Alle Dünnfilme werden mittels gepulster Laserplasmaabscheidung auf verschiedenen einkristallinen Substraten hergestellt. Die strukturellen, optischen und elektrischen Eigenschaften der Filme werden mittels etablierter experimenteller Verfahren wie Röntgenbeugung, spektroskopischer Ellipsometrie und elektrischenWiderstandsmessungen untersucht.
Die strukturellen Eigenschaften von erstmalig in der Masterarbeit des Authors verwirklichten Na2IrO3-Dünnfilmen können durch Abscheidung einer ZnO-Zwischenschicht deutlich verbessert werden. Einkristalline Li2IrO3-Dünnfilme mit einer definierten Kristallausrichtung werden erstmalig hergestellt. Die Messung der dielektrischen Funktion gibt Einblick in elektronische Anregungen, die gut vergleichbar mit Li2IrO3-Einkristallen und verwandten Iridaten sind. Des Weiteren wird aus den Daten eine optische Energielücke von ungefähr 300 meV bestimmt. In Y2Ir2O7-Dünnfilmen wird eine mögliche (111)-Vorzugsorientierung in Wachstumsrichtung gefunden. Im Vergleich mit der chemischen Lösungsabscheidung zeigen die hier mittels gepulster Laserplasmaabscheidung hergestellten YBiO3-Dünnfilme eine definierte, biaxiale Kristallausrichtung in der Wachstumsebene bei einer deutlich höheren Schichtdicke. Über die gemessene dielektrische Funktion können eine direkte und indirekte Bandlücke bestimmt werden. Deren Größe gibt eine notwendige experimentelle Rückmeldung an theoretische Berechnungen der elektronischen Bandstruktur von YBiO3, welche zur Vorhersage der oben erwähnten, neuartigen Materiezuständen verwendet werden.
Nach einer Einleitung und Motivation dieser Arbeit gibt das zweite Kapitel einen Überblick über den gegenwärtigen Forschungsstand der hier untersuchten Materialien. Die folgenden zwei Kapitel beschreiben die Probenherstellung und die verwendeten experimentellen Untersuchungsmethoden. Anschließend werden für jedes Material einzeln die experimentellen Ergebnisse dieser Arbeit diskutiert. Die Arbeit schließt mit einer Zusammenfassung und einem Ausblick.The present thesis reports on the thin film growth of ternary oxides Na2IrO3, Li2IrO3, Y2Ir2O7 and YBiO3. All of these oxides are candidate materials for the so-called topological insulator and spin liquid, respectively. These states of matter promise future application in quantum computation, and in magnetic memory and low-power electronic devices. The realization of the thin films presented here, thus represents a first step towards these future device applications. All thin films are prepared by means of pulsed laser deposition on various single-crystalline substrates. Their structural, optical and electronic properties are investigated with established experimental methods such as X-ray diffraction, spectroscopic ellipsometry and resistivity measurements. The structural properties of Na2IrO3 thin films, that were previously realized in the author’s M. Sc. thesis for the first time, are improved significantly by deposition of an intermediate ZnO layer. Single-crystalline Li2IrO3 thin films are grown for the first time and exhibit a defined crystal orientation. Measurement of the dielectric function gives insight into electronic excitations that compare well with single crystal samples and related iridates. From the data, an optical energy gap of about 300 meV is obtained. For Y2Ir2O7 thin films, a possible (111) out-of-plane preferential crystal orientation is obtained. Compared to chemical solution deposition, the pulsed laser-deposited YBiO3 thin films presented here exhibit a biaxial in-plane crystal orientation up to a significantly larger film thickness. From the measured dielectric function, a direct and indirect band gap energy is determined. Their magnitude provides necessary experimental feedback for theoretical calculations of the electronic structure of YBiO3, which are used in the prediction of the novel states of matter mentioned above. After the introduction and motivation of this thesis, the second chapter reviews the current state of the science of the studied thin film materials. The following two chapters introduce the sample preparation and the employed experimental methods, respectively. Subsequently, the experimental results of this thesis are discussed for each material individually. The thesis concludes with a summary and an outlook.
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Kalz, Ansgar. "Phase diagrams of two-dimensional frustrated spin systems." Doctoral thesis, 2012. http://hdl.handle.net/11858/00-1735-0000-000D-F080-B.
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Luo, Shiang-Wen, and 羅祥文. "Study of photonic crystal energy gap between honeycomb and it´s corresponding conjugated triangle lattices." Thesis, 2007. http://ndltd.ncl.edu.tw/handle/k8786d.
Full textAbstract:
碩士國立東華大學
電機工程學系
95
Photonic crystals are engineered periodic structures made of two or more materials with different dielectric constants. A photonic band gap is a frequency range for which no photons having frequencies within that range can propagate through the photonic crystal. The goal of this thesis is to simulate photonic band gap by changing structure parameters to observe photonic band gap variation. The main purpose of this thesis is to study of photonic crystal energy gap between honeycomb and its conjugated triangle lattices. First, we employ Maxwell’s equation to acquire wave equation and add in plane wave expansion method to calculate photonic crystal dispersion curve then find photonic energy’s range. The influence of structure parameter on photonic energy gap is important. We observe photonic energy gap variation by changing different dielectric constant and filling ratio. The structure parameter is set in the simulation, in triangle lattice part, filling ratio is between 0.41 and 0.495, and background dielectric constant is between 6 and 20; in honeycomb lattice part, filling ratio is between 0.2 and 0.4, and rod dielectric constant is between 9 and 14. From our simulated results, we sum up the relationship between dielectric constant, filling ratio and photonic energy gap. When dielectric constant or filling ratio is increasing, photonic energy gap is bigger. There is the biggest photonic energy gap in the triangle lattice when filling ratio is 0.48. Filling ratio near 0.5 has the biggest photonic energy gap in the honeycomb lattice. In the case of photonic energy gap is easy to over 20％ in the triangle lattice, which is about 15％ in the honeycomb lattice. Furthermore, we also study frequency range which happen photonic energy gap for photonic crystal. Regardless of triangle or honeycomb lattice, photonic energy gap center frequency moves to high frequency via filling ratio increases. If dielectric constant increases, center frequency will move to low frequency. Triangle and honeycomb lattices are different in the structure, but structure parameter and photonic energy gap have similar shift results from our simulated results and the analyzed illustration.
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Li, Zong-Hang, and 李宗翰. "Design of Asymmetric Y-Junction Polarization Splitter in Two-Dimensional Compound-Type Photonic Crystal of Honeycomb and Triangular Lattices." Thesis, 2013. http://ndltd.ncl.edu.tw/handle/74297548112177672070.
Full textAbstract:
碩士龍華科技大學
電機工程系碩士班
101
This thesis is focused on the design and analysis of polarization splitter in two-dimensional compound-type photonic crystal. The plane wave expansion method and the finite-difference time-domain method are employed to calculate photonic bandgap and to simulate wave propagation, respectively. Firstly, Three line-defect waveguides which can guide only the transverse electric (TE) wave, only the transverse magnetic (TM) wave, and both the TE and TM waves are designed in a compound-type lattice with a complete photonic bandgap. Secondly, an asymmetric Y-junction polarization splitter is designed based on the three line-defect waveguides. Transmission efficiencies of the polarization splitter can be improved by modifying the radius of the air holes. According to simulation results, transmission efficiencies larger than 85% and 95% can be obtained for TM and TE waves, respectively.
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Wang, Sheng. "Artificial Graphene in Nano-patterned GaAs Quantum Wells and Graphene Growth by Molecular Beam Epitaxy." Thesis, 2016. https://doi.org/10.7916/D8DF6RGW.
Full textAbstract:
In this dissertation I present advances in the studies of artificial lattices with honeycomb topology, called artificial graphene (AG), in nano-patterned GaAs quantum wells (QWs). AG lattices with very small lattice constants as low as 40 nm are achieved for the first time in GaAs. The high quality AG lattices are created by optimized electron-beam (E-beam) lithography followed by inductively coupled plasma reactive-ion etching (ICP-RIE) process. E-beam lithography is used to define a honeycomb lattice etch mask on the surface of the GaAs QW sample and the optimized anisotropic ICP-RIE process is developed to transfer the pattern into the sample and create the AG lattices. Such high-resolution AG lattices with small lattice constants are essential to form AG miniband structures and create well-developed Dirac cones.
Characterization of electron states in the nanofabricated artificial lattices is by optical experiments. Optical emission (photoluminescence) yields a determination of the Fermi energy of the electrons. A significant reduction of the Fermi energy is due to the nano-patterning process. Resonant inelastic light scattering (RILS) spectra reveal novel transitions related to the electron band structures of the AG lattices. These transitions exhibit a remarkable agreement with the predicted joint density of states (JDOS) based on the band structure calculation for the honeycomb topology.
I calculate the electron band structures of AG lattices in nano-patterned GaAs QWs using a periodic muffin-tin potential model. The evaluations predict linear energy-momentum dispersion and Dirac cones, where the massless Dirac fermions (MDFs) appear, occur in the band structures. Requirements of the parameters of the AG potential to achieve isolated and well-developed Dirac cones are discussed. Density of states (DOS) and JDOS from AG band structures are calculated, which provide a basis to interpret quantitatively observed transitions of electrons involving AG bands.
RILS of intersubband transitions reveal intriguing satellite peaks that are not present in the as-grown QWs. These additional peaks are interpreted as combined intersubband transitions with simultaneous change of QW subband and AG band index. The calculated JDOS for the electron transitions within the AG lattice model provide a remarkably accurate description of the combined intersubband excitations.
Novel low-lying excitation peaks in RILS spectra, interpreted as direct transitions between AG bands without change in QW subband, provide a more direct insight on the AG band structures. We discovered that RILS transitions around the Dirac cones are resonantly enhanced by varying the incident photon energies. The spectral lineshape of these transitions provides insights into the formation of Dirac cones that are characteristic of the honeycomb symmetry of the AG lattices. The results confirm the formation of AG miniband structures and well-developed Dirac cones. The realization of AG lattices in a nanofabricated high mobility semiconductor offers the advantage of tunability through methods suitable for device scalability and integration.
The last part of this thesis describes the growth of nanocrystalline single layer and bilayer graphene on sapphire substrates by molecular beam epitaxy (MBE) with a solid carbon source. Raman spectroscopy reveals that fabrication of single layer, bilayer or multilayer graphene crucially depends on MBE growth conditions. Etch pits revealed by atomic force microscopy indicate a removal mechanism of carbon by reduction of sapphire. Tuning the interplay between carbon deposition and its removal, by varying the incident carbon flux and substrate temperature, should enable the growth of high quality graphene layers on large area sapphire substrates.
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Gotfryd, Dorota. "The Low-Energy Models of Mott Insulators with a Finite Spin-Orbit Coupling." Doctoral thesis, 2021. https://depotuw.ceon.pl/handle/item/3899.
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