The emerging field of spintronics relies on the manipulation of electron spin in order to use it in spin-based electronics. Such a paradigm change has to tackle several challenges including finding materials with sufficiently long spin lifetimes and materials which are efficient in generating pure spin currents. This thesis predicts that two types of material families could be a solution to the aforementioned challenges: complex oxides and bismuth based materials. We derived a general approach for constructing an effective spin-orbit Hamiltonian which is applicable to all nonmagnetic materials. This formalism is useful for calculating spin-dependent properties near an arbitrary point in momentum space. We also verified this formalism through comparisons with other approaches for III-V semiconductors, and its general applicability is illustrated by deriving the spin-orbit interaction and predicting spin lifetimes for strained SrTiO3 and a two-dimensional electron gas in SrTiO3 (such as at the LaAIO3/SrTiO3 interface). Our results suggest robust spin coherence and spin transport properties in SrTiO3 related materials even at room temperature. In the second part of the study we calculated intrinsic spin Hall conductivities for bismuth-antimony Bi1-xSbx semimetals with strong spin-orbit couplings, from the Kubo formula and using Berry curvatures evaluated throughout the Brillouin zone from a tight-binding Hamiltonian. Nearly crossing bands with strong spin-orbit interaction generate giant spin Hall conductivities in these materials, ranging from 474 ((ћ/e)Ω-1cm-1) for bismuth to 96((ћ/e)Ω-1cm-1) for antimony; the value for bismuth is more than twice that of platinum. The large spin Hall conductivities persist for alloy compositions corresponding to a three-dimensional topological insulator state, such as Bi0.83Sb0.17. The spin Hall conductivity could be changed by a factor of 5 for doped Bi, or for Bi0.83Sb0.17, by changing the chemical potential by 0.5 eV, suggesting the potential for doping or voltage tuned spin Hall current. We have also calculated intrinsic spin Hall conductivities of Bi2Se3 and Bi2Te3 topological insulators from an effective tight-binding Hamiltonian including two nearest-neighbor interactions. We showed that both materials exhibit giant spin Hall conductivities calculated from the Kubo formula in linear response theory and the clean static limit. We conclude that bismuth-antimony alloys and bismuth chalcogenides are primary candidates for efficiently generating spin currents through the spin Hall effect.

This thesis describes two topological phases of matter, the Weyl semimetal and the line node semimetal, that are related to but distinct from topological insulator phases. These new topological phases are semimetallic, having electronic energy bands that touch at discrete points or along a continuous curve in momentum space. These states are achieved by breaking time-reversal symmetry near a transition between an ordinary insulator and a topological insulator, using a model based on alternating layers of topological and ordinary insulators, which can be tuned close to the transition by choosing the thicknesses of the layers. The semimetallic phases are topologically protected, with corresponding topological surface states, but the protection is due to separation of the band-touching points in momentum space and discrete symmetries, rather than being protected by an energy gap as in topological insulators. The chiral surface states of the Weyl semimetal give it a non-zero Hall conductivity, while the surface states of the line node semimetal have a flat energy dispersion in the region bounded by the line node. Some transport properties are derived, with a particular emphasis on the behaviour of the conductivity as a function of the impurity concentrations and the temperature.

Atomically two-dimensional (2-D) graphene, as well as the hexagonal boron nitride dielectric have been and are continuing to be widely investigated for the next generation nanoelectronic devices. More recently, other 2-D materials and electronic systems including the surface states of topological insulators (TIs) and monolayers of transition metal dichalcogenides (TMDs) have also attracted considerable interest. In this work I have focused on these latter two material systems on possible device applications. TIs are characterized by an insulating bulk band gap and metallic Dirac surface states which are spin-polarized. Here, the electronic structures of bulk and thin film TIs are studied using ab-initio density functional theory (DFT). Band inversion, an essential characteristic of TIs, is shown in the bulk band structures. Properties of TI surface bands in thin film such as the critical film thickness to induce a gap, the thickness dependent gap size, and the localization length of surface states are reported. Effects of crystalline dielectric materials on TI surface states are also addressed by ab-initio calculations. I discuss the sensitivity of Dirac point degeneracy and linear band dispersion of TI with respect to different dielectric surface terminations as well as different relative atom positions of the dielectric and TI. Additionally, this work presents research on exciton condensation in TI using a tight-binding model combined with self-consistent non-local Hartree-Fock mean-field theory. Possibility of exciton condensation in the TI Bi₂Se₃ thin film is assessed. Non-equilibrium Green's function (NEGF) simulations with the atomistic tight-binding (TB) Hamiltonian are carried out to explore the performance of metal-oxide-semiconductor field-effect-transistor (MOSFET) and tunnel field-effect-transistor (TFET) based on the Bi₂Se₃ TI thin film. How the high dielectric constant of Bi₂Se₃ affects the performance of MOSFET and TFET is presented. Bulk TMDs such as MoS₂, WS₂ and others are the van der Waals-bonded layered material, much like graphite, except monolayer (and Bulk) TMDs have a large band gap in-contrast to graphene (and graphite). Here, the performance of nanoscale monolayer MoS₂ n-channel MOSFETs are examined through NEGF simulations using an atomistic TB Hamiltonian. N- and p-channel MOSFETs of various monolayer TMDs are also compared by the same approach. I correlate the performance differences with the band structure differences. Finally, ab-initio calculations of adatom doping effects on the monolayer MoS₂ is shown. I discuss the most stable atomic configurations, the bonding type and the amount of charge transfer from adatom to the monolayer MoS₂. / text

This thesis describes two topological phases of matter, the Weyl semimetal and the line node semimetal, that are related to but distinct from topological insulator phases. These new topological phases are semimetallic, having electronic energy bands that touch at discrete points or along a continuous curve in momentum space. These states are achieved by breaking time-reversal symmetry near a transition between an ordinary insulator and a topological insulator, using a model based on alternating layers of topological and ordinary insulators, which can be tuned close to the transition by choosing the thicknesses of the layers. The semimetallic phases are topologically protected, with corresponding topological surface states, but the protection is due to separation of the band-touching points in momentum space and discrete symmetries, rather than being protected by an energy gap as in topological insulators. The chiral surface states of the Weyl semimetal give it a non-zero Hall conductivity, while the surface states of the line node semimetal have a flat energy dispersion in the region bounded by the line node. Some transport properties are derived, with a particular emphasis on the behaviour of the conductivity as a function of the impurity concentrations and the temperature.

Les isolants topologiques 3D sont un nouvel état de la matière décrit par un volume iso-lant électriquement et recouvert par des états de surface métalliques. Une jonction Joseph-son topologique (TJJ) formée autour de ces états de surface peut théoriquement contenirun mode lié d’Andreev ayant une périodicité doublée par rapport aux modes liés d’An-dreev conventionnels 2p périodiques. Le mode d’Andreev 4p périodique serait la briqueélémentaire de l’ordinateur quantique topologique. Ainsi, nous étudions la dynamique dece mode particulier lors de mesures de Shapiro sur des jonctions Josephson fabriquées surdes isolants topologiques à base de bismuth.A?n d’identi?er les e?ets d’un mode 4p-périodique dans une mesure de Shapiro, nousutilisons un model phénoménologique permettant de simuler la caractéristique courant-tension d’une TJJ lors de telles mesures. Nous prédisons deux signatures du mode 4p-périodique et estimons leur robustesse face aux e?ets de chau?age par e?et Joule et face àun modèle d’empoisonnement thermiquement activé du mode 4p-périodique.Par des mesures de Shapiro, nous étudions la dynamique des TJJ basées sur le matériausimple qu’est le Bi2Se3. L’observation des deux mêmes signatures précédemment anticipéespar nos simulations, à savoir un ordre d’apparition non conventionnel des pas de Shapiroainsi que la persistance d’un supercourant à la fermeture du plateau de Shapiro n = 0prouve la présence d’un mode 4p-périodique.Notre étude s’est également portée sur un autre isolant topologique le BiSbTeSe2. Nousavons e?ectué sa croissance par cristallisation liquide-solide et avons mis en évidence,par des mesures d’interférométrie supraconductrice une supraconductivité de surface sanstransport électronique par le volume. / Three dimensional topological insulators (3D TI) are a new state of matter composedof an electrically insulating bulk covered by metallic surface states. Theoretically, a topo-logical Josephson junction composed of these surface states can host an Andreev Boundstate (ABS) that has twice the periodicity of the conventional 2p periodic ABSs. The4p periodic ABS is expected to be the building block of topological quantum computing.Therefore, we study the dynamic of this particular ABS by performing Shapiro measure-ment on Josephson junctions built with bismuth based 3D TI.To identify the e?ects of a 4p periodic ABS in a Shapiro measurement, we use a phe-nomenological model that simulates the voltage-current characteristics of a TJJ. We predicttwo signatures of the 4p periodic ABS and estimate their robustness against Joule heatingand thermally activated quasiparticle poisoning of the 4p periodic mode.We study the Josephson junctions dynamics by performing Shapiro measurements onjunctions built on Bi2Se3. We observe the two previously anticipated signatures, whichare the non-conventional appearance order of the Shapiro steps and the remaining of asupercurrent at the closing of the Shapiro step n = 0. They prove the presence of a 4pperiodic ABS.We also study the topological insulator BiSbTeSe2 that we have grown by using themelting growth method. By superconducting interferometric measurements, we show asuperconducting surface transport without bulk electronic conduction.

Topological insulators and topological crystalline insulators are materials that have a bulk band structure that is gapped, but that also have toplogically protected non-gapped surface states. This implies that the bulk is insulating, but that the material can conduct electricity on some of its surfaces. The robustness of these surface states is a consequence of time-reversal symmetry, possibly in combination with invariance under other symmetries, like that of the crystal itself. In this thesis we review some of the basic theory for such materials. In particular we discuss how topological invariants can be derived for some specific systems. We then move on to do band structure calculations using the tight-binding method, with the aim to see the topologically protected surface states in a topological crystalline insulator. These calculations require the diagonalization of block tridiagonal matrices. We finish the thesis by studying the properties of such matrices in more detail and derive some results regarding the distribution and convergence of their eigenvalues.

Die drei-dimensionalen Topologische Isolatoren Bi2Se3 and Bi2Te3 sind Modell-Systeme einer neuen Klasse von Isolatoren mit metallischen Oberflächenzuständen. Ihre kleinen Bandlücken und die schweren Elemente sind essentiell für die topologisch nicht-triviale Bandstruktur, sind aber ebenso verantwortlich für andere bemerkenswerte Eigenschaften, wie etwa für ihre Leistungsfähigkeit als Thermoelektrika. Diese Arbeit untersucht die elektronischen Eigenschaften der Topologischen Isolatoren Bi2Se3 und Bi2Te3 mittels zahlreicher experimenteller Methoden. Es wird gezeigt, dass Ferromagnetismus in Mn gedoptem Bi2Te3 durch sintern unterdrückt werden kann. Zudem werden ein überraschend großer Magnetoresistiver Effekt und ein ladungsträgerunabhängiger Vorzeichenwechsel des Seebeck-Koeffizienten mit zunehmenden Mn Gehalt diskutiert. Kernmagnetische Resonanz (NMR) von 125Te Kernen in Bi2Te3 Einkristallen lässt auf eine ungewöhnliche elektronische SpinSuszeptibilität and komplexe NMR Verschiebungen schließen. Es wird gezeigt dass die Quadrupolwechselwirkung von 209Bi Kernen in Bi2Se3 Einkristallen eine Signatur der Bandinversion ist, in quantitativer Ubereinstimmung mit DFT Rechnungen. Weiterhin wird argumentiert dass die starke Spin-Bahn Kopplung der Leitungselektronen zu einer nicht-trivialen Orientierungsabh¨angigkeit der 209Bi Quadrupolaufspaltung führt.:Contents List of Figures List of Tables List of abbreviations Introduction 1 Topological insulators in three dimensions 2 Theoretical background 3 Methods I: structural, electronic and magnetic properties 4 Methods II: nuclear magnetic resonance 5 Sample preparation and basic characterisation6 Magnetic and electronic properties of Mn doped Bi2Te3 7 NMR of spin 1/2 nuclei: 125Te in Bi2Te3 8 NMR of quadrupole nuclei: 209Bi in Bi2Se3 Conclusions and outlook Appendix Bibliography / The three-dimensional topological insulators Bi2Se3 and Bi2Te3 are model systems of a new class of materials with an insulating bulk and gapless surface states. Their small band gaps and the heavy elements are essential for the topologically non-trivial band structure, but these features are similarly responsible for other remarkable properties, such as their high thermoelectric performance. This thesis investigates the electronic properties of the topological insulators Bi2Se3 and Bi2Te3 with a broad range of experimental methods. Ferromagnetism in Mn doped Bi2Te3 is shown to disappear under sample sintering. A surprisingly large magnetoresistance and a charge carrier independent change in the sign of the thermopower with increasing Mn content are discussed.125Te nuclear magnetic resonance (NMR) of Bi2Te3 single crystals suggest an unusual electronic spin susceptibility and complex NMR shifts. The quadrupole interaction of 209Bi nuclei in Bi2Se3 single crystals is shown to be a signature of the band inversion in quantitative agreement with first-principle calculations. Furthermore, it is proposed that the strong spin-orbit coupling of conduction electrons causes a non-trivial orientation dependent quadrupole splitting of the 209Bi resonance.:Contents List of Figures List of Tables List of abbreviations Introduction 1 Topological insulators in three dimensions 2 Theoretical background 3 Methods I: structural, electronic and magnetic properties 4 Methods II: nuclear magnetic resonance 5 Sample preparation and basic characterisation6 Magnetic and electronic properties of Mn doped Bi2Te3 7 NMR of spin 1/2 nuclei: 125Te in Bi2Te3 8 NMR of quadrupole nuclei: 209Bi in Bi2Se3 Conclusions and outlook Appendix Bibliography

京都大学 / 0048 / 新制・課程博士 / 博士(理学) / 甲第20162号 / 理博第4247号 / 新制||理||1611(附属図書館) / 京都大学大学院理学研究科物理学・宇宙物理学専攻 / (主査)教授 川上 則雄, 教授 松田 祐司, 教授 前野 悦輝 / 学位規則第4条第1項該当 / Doctor of Science / Kyoto University / DFAM

<pre>The degree of freedom brought about by breaking the directional symmetry of space through the use of anisotropic media finds applications in numerous photonic systems. Almost all these systems are based on physical principles that are generalized extensions of their isotropic counterparts, much in the same way an ellipse is related to a circle. However, as we show, there are examples where, in the presence of loss, disorder or even coupling to the measurement apparatus, emerges a completely new behavior which is qualitatively different from the isotropic case. In this work we study these emergent phenomena found in open anisotropic photonic systems.</pre> <pre><br></pre> <pre>We demonstrate that open systems based on biaxial anisotropic medium can support exceptional points which are singularities in the parameter space of the system where the mode frequencies as well as the modes themselves coalesce. We also show that topological insulators, which are novel materials that behave as dielectric in the bulk but metallic in the surface and exhibit bianisotropy through the coupling of their electric and magnetic response, can emit thermal radiation that carries nonzero spin angular momentum. Next, after describing how the strong anisotropy of hyperbolic metamaterial can support electromagnetic fields propagating with high wavenumbers unbounded by the frequency, we show that a super-resolution imaging scheme based on such material is quite robust against substantial loss and disorder. Finally, we consider an example of an incoherent perfect absorber and show that loss and anisotropy in this case can work together to recover the ideal lossless limit for the absorbing performance. In addition to making new conceptual connections between photonics and other branches of science such as condensed matter physics, biotechnology and quantum mechanics, these new emergent phenomena are shown to have thermal, imaging and sensing applications.</pre>

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.