In spite of the failures to find an ultimate theory of unconventional superconductivity, after many years of research the scientific community possesses a considerable store of theoretical knowledge about the problem. Over time, the focus is gradually shifted from finding a theoretical description of an experimentally observed phenomenon to distinguishing between multiple models that offer comparably reasonable descriptions. From the point of view of an experimentalist, this means that any qualitative under-standing of an experimental observation would no longer suffice. Instead, the empha-sis in the experimental research should be shifted to accurate quantification of obser-vations, which becomes possible only if the results available from all the available ex-perimental methods are connected together by the theoretical glue. Among the meth-ods that are to be unified, ARPES plays a central role. The reason for this is that it gives access to the single-particle excitation spectrum of the material as a function of both momentum and energy with very high resolution. Other experimental techniques, such as inelastic neutron scattering (INS), Raman spectroscopy, or the newly estab-lished Fourier-transform scanning tunneling spectroscopy (FT-STS) probe more com-plicated two-particle spectra of the electrons and up to now can not achieve the mo-mentum resolution comparable with that of ARPES. Such reasoning serves as the mo-tivation for the present work, in which some steps are done towards understanding the anomalous effects observed in the single-particle excitation spectra of cuprates and relating the ARPES technique to other experimental methods. First, the electronic properties of BSCCO are considered — the superconducting cuprate most studied by surface-sensitive methods. The recent progress in un-derstanding the electronic structure of this material is reported, focusing mainly on the many-body effects (renormalization) and their manifestation in the ARPES spectra. The main result of this part of the work is a model of the Green’s function that is later used for calculating the two-particle excitation spectrum. Then, the matrix element effects in the photoemission spectra of cuprates are discussed. After a general introduction to the problem, the thesis focuses on the recently discovered anomalous behavior of the ARPES spectra that partially originates from the momentum-dependent photoemission matrix element. The momentum- and excitation energy dependence of the anomalous high-energy dispersion, termed “waterfalls”, is covered in full detail. Understanding the role of the matrix element effects in this phenomenon proves crucial, as they obstruct the view of the underlying excitation spectrum that is of indisputable interest. Finally, the work describes the relation of ARPES with other experimental methods, with the special focus on the INS spectroscopy. For the optimally doped bilayer Bi-based cuprate, the renormalized two-particle correlation function in the superconducting state is calculated from ARPES data within an itinerant model based on the random phase approximation (RPA). The results are compared with the experimental INS data on BSCCO and YBCO. The calculation is based on numerical models for the normal and anomalous Green’s functions fitted to the experimental single-particle spectra. The renormalization is taken into account both in the single-particle Green’s function by means of the self-energy, and in the two-particle correlation function by RPA. Additionally, two other applications of the same approach are briefly sketched: the relation of ARPES to FT-STS, and the nesting properties of Fermi surfaces in two-dimensional charge density wave systems.

This thesis is devoted to studies of high temperature superconductors and related materials using the angle-resolved photoemission spectroscopy (ARPES). Though there is no accepted theory of superconductivity, encompassing high-$T_{\\rm c}$ materials, there is enough evidence to believe that superconductivity can always be interpreted as stemming from pairing of electrons by interaction with bosons, and $T_{\\rm c}$ is determined by effectiveness of such a pairing. ARPES, owing to the possibility of recording energy- and momentum-resolved electronic spectrum, is a powerful probe of the normal-state electronic structure, which is an important prerequisite for the superconductivity, and implications of the electron pairing, such as emergence of the superconducting gap and finer features below $T_{\\rm c}$. Based on ARPES data one can quantify the electronic interactions by analysis of kinks in the dispersion curves, spectral line widths etc. In current work new methods of ARPES data analysis were developed and applied to the spectra taken from cuprate and iron-based high-$T_{\\rm c}$. The possibility to analyze the macroscopic response of solids in the normal state as well as in the superconducting and charge-density-wave phases basing on the experimentally measured renormalized band dispersion and anisotropic superconducting and charge-density-wave gap was shown. The thesis consists of five parts. Part 1 introduces the employed notions of electrons in solids and methods of their investigation. Part 2 describes the Voigt fitting procedure, allowed for purification of the spectra from resolution effects, and, consequently, for determination of the quasiparticle scattering rate with enhanced precision. In Part 3 the calculation of the temperature-dependent Hall coefficient in the charge-density-wave-bearing 2H-TaSe$_2$ from the band dispersion, measured in ARPES, is presented, and comparison to the independent magnetotransport measurements is shown. The extraction of the band dispersion of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ and LiFeAs from ARPES data can be found in Part 4. Agreement with Hall effect measurements on the same samples is demonstrated. Part 5 introduces the extraction of the momentum-dependent superconducting gap in iron arsenides from fitting of ARPES spectra to Dynes function. The superfluid density was calculated from the band dispersion and the superconducting gap, measured in ARPES, and compared to the ones measured by different techniques.

L'électronique basée sur le graphène fait face à un verrou technologique, qui est l'absence d'une bande interdite (gap) permettant une commutation entre les états logiques allumé et éteint. Les nano-rubans de graphène rendent possible l'obtention de ce gap mais il est difficile de produire de tels rubans avec une largeur précise à l'échelle atomique et des bords bien ordonnés. Le confinement électronique est une façon élégante d'ouvrir un gap et peut en principe être réglé en ajustant la largeur des nano-rubans. Cette thèse est consacrée à la compréhension de l'ouverture du gap par nano-structuration. Nous avons suivi deux approches: l'introduction d'un potentiel super-périodique sur le graphène par des substrats vicinaux de métaux nobles et le confinement électronique dans des nano-rubans sur des facettes artificielles du SiC. Des potentiels super-périodiques ont été introduits avec deux substrats nano-structurés: l'Ir(332) et un cristal courbé de Pt(111) multi-vicinale. Le graphène modifie les marches initiales des substrats et les transforme en une succession de terrasses (111) et de régions d'accumulation de marches, observés par STM. La nano-structuration du substrat induit alors un potentiel super-périodique dans le graphène entraînant l'ouverture de gaps sur la bande π du graphène observée par ARPES, ce qui est cohérent avec la périodicité structurale observé par STM et LEED. Les gaps peuvent être convenablement expliqués par un modèle de type hamiltonien de Dirac; ce dernier nous permet de retrouver la force du potentiel à la jonction entre les terrasses (111) et la région d'accumulation des marches. La force du potentiel dépend du substrat, de la périodicité associée à la surface et du type de bord des marches (soit type A ou B). Nous avons aussi changé le potentiel de surface en intercalant du Cu sur l'Ir(332), qui reste préférentiellement au niveau de l'accumulation des marches. La surface présente des régions dopées n alors que les régions non-intercalées restent dopées p, conduisant à une succession de rubans dopés n et p pour une même couche de graphène continue. La seconde approche pour contrôler le gap est par confinement électronique dans des nanorubans de graphène synthétisés sur du SiC. Ces rubans sont obtenus sur des facettes du SiC ordonnées périodiquement. Comme l'ouverture d'un gap d'origine inconnue avait été observée par ARPES, nous avons réalisé les premières études atomiquement résolues par STM. Nous démontrons la régularité et la chiralité des bords, nous localisons précisément les nanorubans de graphène sur les facettes et nous identifions des mini-facettes sur du SiC. Afin de comprendre le couplage entre le graphène et le substrat, nous avons étudié une coupe transversale par STEM/EELS, en complément des études par ARPES et STM/STS. Nous observons que la facette (1-107) où le graphène se trouve présente un sub-facettage sur les extrémités haute et basse. Le sub-facettage comprend des mini-terrasses (0001) et des mini-facettes (1-105). Le graphène s'étend tout au long du la région sub-facettée, et est couplé au substrat dans les mini-terrasses (0001), ce qui le rend semi-conducteur. En revanche, le graphène au-dessus des mini-facettes (1-105) est découplé du substrat mais présente un gap observé par EELS, et compatible avec les observations faites par ARPES. L'origine du gap est expliquée par le confinement électronique sur des nano-rubans de graphène de 1 - 2 nm de largeur localisés sur ces mini-facettes (1-105). / The major challenge for graphene-based electronic applications is the absence of the band-gap necessary to switch between on and off logic states. Graphene nanoribbons provide a route to open a band-gap, though it is challenging to produce atomically precise nanoribbon widths and well-ordered edges. A particularly elegant method to open a band-gap is by electronic confinement, which can in principle be tuned by adjusting the nanoribbon width. This thesis is dedicated to understanding the ways of opening band-gaps by nanostructuration. We have used two approaches: the introduction of a superperiodic potential in graphene on vicinal noble metal substrates and the electronic confinement in artificially patterned nanoribbons on SiC. Superperiodic potentials on graphene have been introduced by two nanostructured substrates, Ir(332) and a multivicinal curved Pt(111) substrate. The growth of graphene modifies the original steps of the pristine substrates and transforms them into an array of (111) terraces and step bunching areas, as observed by STM. This nanostructuration of the underlying substrate induces the superperiodic potential on graphene that opens mini-gaps on the π band as observed by ARPES and consistent with the structural periodicity observed in STM and LEED. The mini-gaps are satisfactorily explained by a Dirac-hamiltonian model, that allows to retrieve the potential strength at the junctions between the (111) terraces and the step bunching. The potential strength depends on the substrate, the surface periodicity and the type of step-edge (A or B type). The surface potential has also been modified by intercalating Cu on Ir(332), that remains preferentially on the step bunching areas, producing there n-doped ribbons, while the non-intercalated areas remain p-doped, giving rise to an array of n- and p- doped nanoribbons on a single continuous layer. In the second approach to control the gap, we have studied the gap opening by electronic confinement in graphene nanoribbons grown on SiC. These ribbons are grown on an array of stabilized sidewalls on SiC. As a band-gap opening with unclear atomic origin had been observed by ARPES, we carried-out a correlated study of the atomic and electronic structure to identify the band gap origin. We performed the first atomically resolved study by STM, demonstrating the smoothness and chirality of the edges, finding the precise location of the metallic graphene nanoribbon on the sidewalls and identifying an unexpected mini-faceting on the substrate. To understand the coupling of graphene to the substrate, we performed a cross-sectional study by STEM/EELS, complementary of our ARPES and STM/STS studies. We observe that the (1-107) SiC sidewall facet is sub-faceted both at its top and bottom edges. The subfacetting consists of a series of (0001) miniterraces and (1-105) minifacets. Graphene is continuous on the whole subfacetting region, but it is coupled to the substrate on top of the (0001) miniterraces, rendering it there semiconducting. On the contrary, graphene is decoupled on top of the (1-105) minifacets but exhibits a bandgap, observed by EELS and compatible with ARPES observations. Such bandgap is originated by electronic confinement in the 1 - 2 nm width graphene nanoribbons that are formed over the (1-105) minifacets.

L'étude de l'état isolant de Mott est un des domaines très actif de la recherche en matière condensée car les fortes corrélations qui en sont à l'origine donnent naissance à des états de la matière très variés et avec des applications potentielles. Sr₂IrO₄ est un isolant de Mott exotique car induit par un fort couplage spin-orbite. Il permet d'étudier l'impact des corrélations électroniques sur les propriétés de basses énergies sous un angle nouveau. L'objet de cette thèse est l'étude expérimentale des propriétés électroniques de ces composés iridates par des mesures d'ARPES permettant des observations directes de la structure électronique dans l'espace réciproque et de RMN et μSR, qui donnent une vision locale dans l'espace réel. Nous nous sommes en particulier intéressés à la transition isolant métal pouvant survenir en dopant ce composé. Une façon originale de doper Sr₂IrO₄ que nous avons étudiée en détails est de substituer l'iridium par du rhodium. Les deux sont isovalents, mais le rhodium capture un électron conduisant à un dopage effectif en trous. Grâce à l’ARPES, nous avons mis en évidence les différentes bandes de la structure électronique. Nous avons étudié attentivement le caractère orbital de ces bandes et mis en évidence des anisotropies résiduelles en certains points de l’espace réciproque, survivant malgré la présence du fort couplage spin-orbite. Ceci, ainsi que des effets de repliement de la structure électronique, donnent lieu à des variations brutales d'intensité, qui doivent être prises en compte pour analyser correctement les spectres. Lors du dopage avec le Rh, la phase métallique obtenue reste très incohérente, avec une absence de pic de quasiparticule et un pseudogap uniforme sur l'ensemble de la surface de Fermi. Le gap de Mott ne semble pas se fermer. Le pseudogap peut révéler une brisure de symétrie mais aussi l’effet du désordre introduit par le Rh et nous discuterons son origine, en lien avec la physique d’autres systèmes corrélés. Nous montrons que pour de faibles taux de substitution Ir/Rh, l’ajout de porteurs trous contrôle le comportement du système alors qu’à des taux de substitutions plus élevés, le nombre de porteurs est stable mais le désordre augmente et contrôle à son tour la physique. Nous nous sommes aussi intéressés aux propriétés électroniques et magnétiques sondées par la RMN de l'oxygène 17 sur poudre et poudre orientée et par μSR. La RMN permet de différencier les deux sites d'oxygène de Sr₂IrO₄ nous permettant de déterminer certains paramètres nucléaires préalables à l'étude fine des propriétés électroniques. Dans le composé pur, nous avons étudié la transition magnétique et observé ce qui semble être le développement d'un moment sur l'oxygène apical. Dans les composés dopés, nous ne voyons pas de désordre structural important malgré des taux de dopage allant jusqu'à 15% de rhodium. Les propriétés magnétiques présentent néanmoins des signes d’inhomogénéité, plus marqués dans le cas du dopage lanthane. Les fluctuations dans le composé métallique montrant une prédominance des corrélations antiferromagnétiques. De son côté, la μSR a permis de construire le diagramme de phases de la transition antiferromagnétique et de mettre en évidence l'inhomogénéité de la transition magnétique dans les échantillons faiblement dopés. À basse température, nous confirmons que la phase magnétique évolue, peut-être avec l’apparition d’un moment sur l’oxygène, et cet effet est même renforcé dans les composés faiblement dopés. Au-dessus de la température de transition antiferromagnétique, nous n'avons pas trouvé de signature d'une transition vers une phase de boucles de courant observée par d'autres techniques. Cette étude permet d’attribuer à Sr₂IrO₄ dopé rhodium le caractère assez rare de matériau 2D fortement corrélé à désordre contrôlé. De manière plus générale, cet exemple devrait permettre de mieux comprendre les effets éventuels de désordre associés à d’autre façons de doper les iridates. / The study of the insulating Mott state is a very active field of research in condensed matter because of the strong correlations usually at play which can lead to a large variety of states of matter, with potential applications. Sr₂IrO₄ is an exotic Mott insulator because it is induced by a strong spin-orbit coupling. It allows studying the impact of electronic correlations on the low energy properties from a new viewpoint. The subject of this thesis is the experimental study of the electronic properties of these iridate compounds by ARPES measurements allowing direct observations of the electronic structure in reciprocal space and NMR and μSR, which give a local view in real space. We have in particular studied the metal to insulator transition which can occur in this compound upon doping. An original way to dope Sr₂IrO₄ that we have investigated in details is to substitute iridium by rhodium. Both are isovalent but the rhodium captures an electron leading to an effective hole doping. Thanks to ARPES we have identified the different bands in the electronic structure. We have studied in details the orbital character of these bands and pointed out residual anisotropies at some points in the reciprocal space, which survive despite the strong spin-orbit coupling. This, as well as the folding effects of the electronic structure, give rise to sudden variations in intensity, which must be taken into account in order to correctly analyze the spectra. Upon doping with Rh, the obtained metallic phase remains very incoherent, with no quasiparticle peak and a uniform pseudogap over the full Fermi surface. The Mott gap does not seem to be closing. The pseudogap can reveal symmetry breaking but also the effect of the disorder introduced by the Rh and we will discuss its origin, in relation to the physics of other correlated systems. We show that for low Ir/Rh substitution rates, the addition of hole carriers controls the behavior of the system while at higher substitution rates, the number of carriers is stable but the disorder increases and in turn controls physics.We were also interested in the electronic and magnetic properties probed by 17 oxygen NMR on powder and oriented powder samples and by μSR. NMR makes it possible to differentiate the two oxygen sites in Sr₂IrO₄ allowing determining some nuclear parameters necessary to the fine study of the electronic properties. In the pure compound, we have studied the magnetic transition and observed what appears to be the development of a moment on the apical oxygen. In the doped compounds, we do not see any significant structural disorder despite doping levels up to 15% rhodium. However, the magnetic properties nevertheless show signs of inhomogeneity, which are more pronounced in the case of lanthanum doping. The fluctuations in the correlated metal compound show a predominance of antiferromagnetic correlations. From our μSR investigation, we could construct the magnetic phase diagram which highlights the inhomogeneity of the magnetic transition in the low-doped samples. At low temperature, we confirm that the magnetic phase evolves, perhaps with the appearance of a moment on the oxygen, and this effect is even enhanced in the lightly doped compounds. Above the antiferromagnetic transition temperature, we did not find signatures of the current loop phase observed by other techniques. This study makes it possible to attribute to Sr₂IrO₄ doped with rhodium the rather rare character of strongly correlated 2D material with controlled disorder. More generally, this example should provide a better understanding of the potential effects of disorder associated with other ways to dope iridates.

This thesis is devoted to studies of high temperature superconductors and related materials using the angle-resolved photoemission spectroscopy (ARPES). Though there is no accepted theory of superconductivity, encompassing high-$T_{\\rm c}$ materials, there is enough evidence to believe that superconductivity can always be interpreted as stemming from pairing of electrons by interaction with bosons, and $T_{\\rm c}$ is determined by effectiveness of such a pairing. ARPES, owing to the possibility of recording energy- and momentum-resolved electronic spectrum, is a powerful probe of the normal-state electronic structure, which is an important prerequisite for the superconductivity, and implications of the electron pairing, such as emergence of the superconducting gap and finer features below $T_{\\rm c}$. Based on ARPES data one can quantify the electronic interactions by analysis of kinks in the dispersion curves, spectral line widths etc. In current work new methods of ARPES data analysis were developed and applied to the spectra taken from cuprate and iron-based high-$T_{\\rm c}$. The possibility to analyze the macroscopic response of solids in the normal state as well as in the superconducting and charge-density-wave phases basing on the experimentally measured renormalized band dispersion and anisotropic superconducting and charge-density-wave gap was shown. The thesis consists of five parts. Part 1 introduces the employed notions of electrons in solids and methods of their investigation. Part 2 describes the Voigt fitting procedure, allowed for purification of the spectra from resolution effects, and, consequently, for determination of the quasiparticle scattering rate with enhanced precision. In Part 3 the calculation of the temperature-dependent Hall coefficient in the charge-density-wave-bearing 2H-TaSe$_2$ from the band dispersion, measured in ARPES, is presented, and comparison to the independent magnetotransport measurements is shown. The extraction of the band dispersion of Ba$_{1-x}$K$_{x}$Fe$_2$As$_2$ and LiFeAs from ARPES data can be found in Part 4. Agreement with Hall effect measurements on the same samples is demonstrated. Part 5 introduces the extraction of the momentum-dependent superconducting gap in iron arsenides from fitting of ARPES spectra to Dynes function. The superfluid density was calculated from the band dispersion and the superconducting gap, measured in ARPES, and compared to the ones measured by different techniques.

In spite of the failures to find an ultimate theory of unconventional superconductivity, after many years of research the scientific community possesses a considerable store of theoretical knowledge about the problem. Over time, the focus is gradually shifted from finding a theoretical description of an experimentally observed phenomenon to distinguishing between multiple models that offer comparably reasonable descriptions. From the point of view of an experimentalist, this means that any qualitative under-standing of an experimental observation would no longer suffice. Instead, the empha-sis in the experimental research should be shifted to accurate quantification of obser-vations, which becomes possible only if the results available from all the available ex-perimental methods are connected together by the theoretical glue. Among the meth-ods that are to be unified, ARPES plays a central role. The reason for this is that it gives access to the single-particle excitation spectrum of the material as a function of both momentum and energy with very high resolution. Other experimental techniques, such as inelastic neutron scattering (INS), Raman spectroscopy, or the newly estab-lished Fourier-transform scanning tunneling spectroscopy (FT-STS) probe more com-plicated two-particle spectra of the electrons and up to now can not achieve the mo-mentum resolution comparable with that of ARPES. Such reasoning serves as the mo-tivation for the present work, in which some steps are done towards understanding the anomalous effects observed in the single-particle excitation spectra of cuprates and relating the ARPES technique to other experimental methods. First, the electronic properties of BSCCO are considered — the superconducting cuprate most studied by surface-sensitive methods. The recent progress in un-derstanding the electronic structure of this material is reported, focusing mainly on the many-body effects (renormalization) and their manifestation in the ARPES spectra. The main result of this part of the work is a model of the Green’s function that is later used for calculating the two-particle excitation spectrum. Then, the matrix element effects in the photoemission spectra of cuprates are discussed. After a general introduction to the problem, the thesis focuses on the recently discovered anomalous behavior of the ARPES spectra that partially originates from the momentum-dependent photoemission matrix element. The momentum- and excitation energy dependence of the anomalous high-energy dispersion, termed “waterfalls”, is covered in full detail. Understanding the role of the matrix element effects in this phenomenon proves crucial, as they obstruct the view of the underlying excitation spectrum that is of indisputable interest. Finally, the work describes the relation of ARPES with other experimental methods, with the special focus on the INS spectroscopy. For the optimally doped bilayer Bi-based cuprate, the renormalized two-particle correlation function in the superconducting state is calculated from ARPES data within an itinerant model based on the random phase approximation (RPA). The results are compared with the experimental INS data on BSCCO and YBCO. The calculation is based on numerical models for the normal and anomalous Green’s functions fitted to the experimental single-particle spectra. The renormalization is taken into account both in the single-particle Green’s function by means of the self-energy, and in the two-particle correlation function by RPA. Additionally, two other applications of the same approach are briefly sketched: the relation of ARPES to FT-STS, and the nesting properties of Fermi surfaces in two-dimensional charge density wave systems.

Topology, an important topic in physics since several years, is handled as possible solution to many current-state problems in electronics and energy. It could allow to dramatically shrink computational devices or increase their speed without the current problem of heat dissipation, or topological principles can be used to introduce room temperature high-conduction paths within materials. Unfortunately, while many promising materials have been presented yet, the one breakthrough material is still missing. Current style materials are either consisting of toxical elements, obstructing possible use cases, or their electronic structure is too complex to investigate the interplay of all the facets of the electronic structure present in the mateirals. In this thesis, two very promising materials will be thoroughly introduced, namely TaIrTe4 and GaGeTe. Both materials have the potential, to lift one of the shortcomings mentioned. First, TaIrTe4 will be presented. TaIrTe4 is a simplistic Weyl semimetal in terms of its electronic and topological structure - the simplest yet known material. It hosts four Weyl points, the minimum amount of Weyl nodes possible in a non-centrosymmetric material. Predictions state, that these nodes are well separated throughout the Brillouin zone, and are connected by nearly parallel Fermi arcs. The existance of the topological states is proved in this thesis through angle-resolved photoemission spectroscopy (ARPES) and confirmed by spin polarization measurements on these states. GaGeTe is predicted to be a Bi2Se3-style topological insulator, but ARPES data presented shows, that no direct band gap could be observed. Yet, a topological state is still believed to be present. This makes this material interesting in many ways: its elemental composition is less toxic than bismuth and selenium, as well as it is the first realization of such a specific electronic structure. A full discussion of the electronic states close to the Fermi level including the possible existance of topological states is shown in this thesis.

This thesis presents experimental studies of the electronic and magnetic properties of several iron-based unconventional superconductors, primarily using the techniques of magnetotransport and torque magnetometry in high magnetic fields and synchrotron-based angle-resolved photo-emission spectroscopy (ARPES). Superconductivity in the iron-based superconductors is always found in proximity to a magnetic phase, and the details of the electronic structure and Fermi surface are also important in determining the strength of interactions, and ultimately superconductivity. This motivates the experimental studies of electronic, magnetic and superconducting properties of Fe-based superconductors presented in this thesis. First, quantum oscillation measurements using high-field torque magnetometry are used to provide a partial determination of the Fermi surface of superconducting LiFeAs. The data are compared with density functional theory calculations, finding strong mass enhancements on the observed electron bands, however the hole bands are not observed. A large portion of this thesis concerns experiments on FeSe, which uniquely has a structural transition but is not magnetically ordered at any temperature. High field magnetotransport measurements show quantum oscillations, revealing small quasi-two dimensional Fermi surfaces, and it is argued that both hole and electron pockets are observed. The low-temperature Fermi surface consisting of one hole pocket and two electron pockets is also deduced from low-field magnetotransport. ARPES studies show that both hole and electron pockets undergo a significant elongation when cooling through the structural transition at ~90 K, interpreted as the result of orbital order. Measurements of the resistivity anisotropy above the structural transition are used to show that the structural distortion is electronically-driven. By combining these data sets, a complete picture of the symmetry-broken electronic structure of FeSe is constructed. The final chapter concerns another iron-based superconductor with a more complex crystal structure, the so-called ``10-3-8" phase, and in particular finds an unusual field-induced magnetic transition.

Two-dimensional (2D) topological insulators are a new class of materials with properties that are promising for potential future applications in quantum computers. For example, stanene represents a possible candidate for a topological insulator made of Sn atoms arranged in a hexagonal lattice. However, it has a relatively fragile low-energy spectrum and sensitive topology. Therefore, to experimentally realize stanene in the topologically non-trivial phase, a suitable substrate that accommodates stanene without compromising these topological properties must be found. A heterostructure consisting of a SiC substrate with a buffer layer of adsorbed group-III elements constitutes a possible solution for this problem. In this work, 2D adatom systems of Al and In were grown epitaxially on SiC(0001) and then investigated structurally and spectroscopically by scanning tunneling microscopy (STM) and photoelectron spectroscopy. Al films in the high coverage regime \( (\Theta_{ML}\approx2\) ML\( ) \) exhibit unusually large, triangular- and rectangular-shaped surface unit cells. Here, the low-energy electron diffraction (LEED) pattern is brought into accordance with the surface topography derived from STM. Another Al reconstruction, the quasi-one-dimensional (1D) Al phase, exhibits a striped surface corrugation, which could be the result of the strain imprinted by the overlayer-substrate lattice mismatch. It is suggested that Al atoms in different surface areas can occupy hexagonal close-packed and face-centered cubic lattice sites, respectively, which in turn lead to close-packed transition regions forming the stripe-like corrugations. On the basis of the well-known herringbone reconstruction from Au(111), a first structural model is proposed, which fits well to the structural data from STM. Ultimately, however, thermal treatments of the sample could not generate lower coverage phases, i.e. in particular, a buffer layer structure. Strong metallic signatures are found for In high coverage films \( (\Theta_{ML}\approx3\) to \(2\) ML\() \) by scanning tunneling spectroscopy (STS) and angle-resolved photoelectron spectroscopy (ARPES), which form a \( (7\times7) \), \( (6\times4\sqrt{3}) \), and \( (4\sqrt{3}\times4\sqrt{3}) \) surface reconstruction. In all these In phases electrons follow the nearly-free electron model. Similar to the Al films, thermal treatments could not obtain the buffer layer system. Surprisingly, in the course of this investigation a triangular In lattice featuring a \( (1\times1) \) periodicity is observed to host massive Dirac-like bands at \( K/K^{\prime} \) in ARPES. Based on this strong electronic similarity with graphene at the Brillouin zone boundary, this new structure is referred to as \textit{indenene}. An extensive theoretical analysis uncovers the emergence of an electronic honeycomb network based on triangularly arranged In \textit{p} orbitals. Due to strong atomic spin-orbit coupling and a comparably small substrate-induced in-plane inversion symmetry breaking this material system is rendered topologically non-trivial. In indenene, the topology is intimately linked to a bulk observable, i.e., the energy-dependent charge accumulation sequence within the surface unit cell, which is experimentally exploited in STS to confirm the non-trivial topological character. The band gap at \( K/K^{\prime} \), a signature of massive Dirac fermions, is estimated by ARPES to approximately 125 meV. Further investigations by X-ray standing wave, STM, and LEED confirm the structural properties of indenene. Thus, this thesis presents the growth and characterization of the novel quantum spin Hall insulator material indenene. / Zweidimensionale (2D) topologische Isolatoren sind eine neue Materialklasse mit vielversprechenden Eigenschaften für potenzielle zukünftige Anwendungen in Quantencomputern. Stanene stellt hier beispielsweise einen möglichen Kandidaten für einen topologischen Isolator dar. Diese 2D-Schicht besteht aus Sn-Atomen, angeordnet in einem hexagonalen Gitter. Allerdings weist dieses Gitter ein relativ fragiles Niederenergiespektrum und eine empfindliche Topologie auf. Um Stanene daher in der topologisch nicht-trivialen Phase experimentell realisieren zu können, muss ein geeignetes Substrat gefunden werden, das Stanene aufnehmen kann, ohne die topologischen Eigenschaften zu beeinträchtigen. Eine Heterostruktur aus einem SiC-Substrat mit einer Pufferschicht aus adsorbierten Gruppe-III Elementen stellt hier eine mögliche Lösung für dieses Problem dar. Im Hinblick darauf wurden für diese Arbeit 2D-Adatomsysteme aus Al und In epitaktisch auf SiC(0001) gewachsen und mittels Rastertunnelmikroskopie (engl.: scanning tunneling microscopy, STM) und Photoelektronenspektroskopie strukturell und spektroskopisch untersucht. Al-Schichten mit hoher Bedeckung \( (\Theta_{ML}\approx2\) ML\( ) \) weisen ungewöhnlich große, dreieckig und rechteckig geformte Oberflächeneinheitszellen auf. Hierbei wird das Beugungsmuster der niederenergetischen Elektronenbeugung (engl.: low-energy electron diffraction, LEED) mit der aus STM abgeleiteten Oberflächentopographie in Einklang gebracht. Eine andere Al-Rekonstruktion, die quasi-eindimensionale (1D) Al-Phase, zeigt eine gestreifte Oberflächenkorrugation, die ein Ergebnis der Verspannung durch die Fehlanpassung des Al-Gitters auf dem Substratgitter sein könnte. Es wird vorgeschlagen, dass Al-Atome in verschiedenen Oberflächenbereichen sowohl jeweils hexagonal-dichtgepackte als auch kubisch flächenzentrierte Gitterplätze einnehmen können. In Übergangsregionen zwischen beiden Bereichen erzeugt dies dicht gepackte Al-Atome, die wiederum die streifenartigen Korrugationen hervorrufen. Auf der Basis der bekannten Fischgrätenrekonstruktion von Au(111) wird ein erstes Strukturmodell vorgeschlagen, das gut mit strukturellen STM-Daten übereinstimmt. Letztendlich konnten jedoch durch thermische Behandlungen der Probe keine Phasen mit geringerer Bedeckung, das heißt insbesondere die Pufferschichtstruktur, erzeugt werden. In-Hochbedeckungsphasen \( (\Theta_{ML}\approx3\) to \(2\) ML\() \) weisen ein ausgeprägtes metallisches Verhalten auf in der Rastertunnelspektroskopie (engl.: scanning tunneling spectroscopy, STS) und winkelaufgelösten Photoelektronenspektroskopie (engl.: angle-resolved photoelectron spectroscopy, ARPES). Zudem bilden diese Phasen eine \( (7\times7) \), \( (6\times4\sqrt{3}) \), and \( (4\sqrt{3}\times4\sqrt{3}) \)-Oberflächenrekonstruktion aus. In all diesen Phasen folgen die Elektronen dem Modell der quasifreien Elektronen. Ähnlich zu den Al-Filmen konnte auch hier nach thermischen Behandlungen der Probe keine Pufferschichtstruktur erzeugt werden. Überraschenderweise tritt im Laufe dieser Untersuchung ein Dreiecksgitter aus In-Atomen mit einer \( (1\times1) \)-Periodizität auf, das bei \( K/K^{\prime} \) massive Dirac-artige Bänder in ARPES zeigt. Aufgrund der starken Ähnlichkeit mit der Graphene-Bandstruktur am Brillouinzonenrand, wird dieses neuartige Materialsystem \textit{Indenene} benannt. Eine umfangreiche theoretische Untersuchung legt die Entstehung eines elektronischen Honigwabennetzwerks offen, dass sich aufgrund von dreieckig angeordneten In \textit{p}-Orbitalen bildet. Durch starke atomare Spin-Bahn-Wechselwirkung und einen vergleichsweisen schwachen substratinduzierten Inversionssymmetriebruch in der Ebene, ist dieses Materialsystem topologisch nicht-trivial. In Indenene ist die Topologie eng mit einer Volumenobservablen, genauer die energieabhängige Ladungsakkumulationsequenz innerhalb der Oberflächeneinheitszelle, verknüpft. Diese Sequenz wird mittels STS experimentell ausgenutzt, um den topologisch nicht-trivialen Charakter zu bestätigen. Die Bandlücke bei \( K/K^{\prime} \), charakteristisch für massive Dirac-Fermionen, wird mittels ARPES auf ungefähr 125 meV abgeschätzt. Weitere Untersuchungen basierend auf stehenden Röntgenwellen, STM, und LEED bestätigen die strukturellen Eigenschaften von Indenene. Dementsprechend wird in dieser Arbeit dasWachstum und auch die Charakterisierung des neuartigen Quanten Spin Hall Isolators Indenene vorgestellt.

The projects presented in this thesis cover the examination of the electronic and structural properties of organic thin films at noble metal-organic interfaces. Angle-resolved photoemission spectroscopy is used as the primary investigative tool due to the connection of the emitted photoelectrons to the electronic structure of the sample. The surveyed materials are of relevance for fundamental research and practical applications on their own, but also serve as archetypes for the photoemission techniques presented throughout the four main chapters of this thesis. The techniques are therefore outlined with their adaptation to other systems in mind and a special focus on the proper description of the final state. The most basic description of the final state that is still adequate for the evaluation of photoemission data is a plane wave. Its simplicity enables a relatively intuitive interpretation of photoemission data, since the initial and final state are related to one another by a Fourier transform and a geometric factor in this approximation. Moreover, the initial states of some systems can be reconstructed in three dimensions by combining photoemission measurements at various excitation energies. This reconstruction can even be carried out solely based on experimental data by using suitable iterative algorithms. Since the approximation of the final state in the photoemission process by a plane wave is not valid in all instances, knowledge on the limitations of its applicability is indispensable. This can be gained by a comparison to experimental data as well as calculations with a more detailed description of the photoemission final state. One possible appraoch is based on independently emitting atoms where the coherent superposition of partial, atomic final states produces the total final state. This approach can also be used for more intricate studies on organic thin films. To this end, experimental data can be related to theoretical calculations to gain extensive insights into the structural and electronic properties of molecules in organic thin films. / Die in dieser Arbeit vorgestellten Projekte behandeln die Untersuchung der elektronischen und strukturellen Eigenschaften organischer Dünnschichtfilme an Grenzflächen zwischen Edelmetallen und organischen Materialien. Als maßgebliche Messmethode wird die winkelaufgelöste Photoelektronenspektroskopie aufgrund der Verbindung der emittierten Photoelektronen mit der elektronischen Struktur der untersuchten Probe angewandt. Die verwendeten Materialien sind sowohl in der Grundlagenforschung als auch für praktische Anwendungen relevant, und dienen gleichzeitig auch als Beispiele für die Photoemissionstechniken, die in den vier Hauptkapiteln der Arbeit präsentiert werden. Diese Techniken werden daher auch bezüglich ihrer Ubertragbarkeit auf andere Systeme dargestellt, wobei besonders auf die korrekte Beschreibung des Endzustands in der Photoemission eingegangen wird. Die simpelste Beschreibung des Endzustands, die für die Auswertung von Photoemissionsdaten noch sinnvoll verwendet werden kann, stellt eine ebene Welle dar. Ihre Einfachheit ermöglicht eine relativ intuitive Interpretation von Photoemissionsdaten, da Anfangs- und Endzustand in dieser Näherung lediglich durch eine Fourier-Transformation und einen geometrischen Faktor verknüpft sind. Kombiniert man die Photoemissionsmessungen bei unterschiedlichen Anregungsenergien, lassen sich zusätzlich die Anfangszustände bestimmter Systeme in guter Näherung dreidimensional rekonstruieren. Mit Hilfe geeigneter iterativer Algorithmen ist diese Rekonstruktion darüber hinaus mit ausschließlich experimentellen Daten realisierbar. Da die Näherung des Endzustands mit einer ebenen Welle nur unter bestimmten Bedingungen ausreichend präzise das reale System widerspiegelt, ist die Kenntnis über die Grenzen ihrer Anwendbarkeit von Bedeutung. Dies kann über den Vergleich mit experimentellen Daten sowie Rechnungen mit detailierteren Beschreibungen des Endzustands in der Photoemission geschehen. Ein möglicher Ansatz basiert auf unabhängig voneinander emittierenden Atomen, deren kohärent überlagerte, partielle Endzustände den gesamten Endzustand formen. Dieser Ansatz kann des Weiteren für komplexere Untersuchungen an organischen Dünnschichten verwendet werden. So können über den Vergleich von experimentellen Messung mit theoretischen Rechnungen umfangreiche Einblicke auf die strukturellen und elektronischen Eigenschaften der Moleküle in organischen Dünnschichten gewonnen werden.