@phdthesis{Halbig2019,
  author    = {Halbig, Benedikt},
  title     = {Surface Raman Spectroscopy on Ordered Metal Adsorbates on Semiconductor Substrates and Thin Intermetallic Films},
  doi       = {10.25972/OPUS-18138},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-181385},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2019},
  abstract  = {Surface systems attract great scientific attention due to novel and exotic properties. The atomically structured surfaces lead to a reduced dimensionality which alters electronic correlations, vibrational properties, and their impact on each other. The emerging physical phenomena are not observed for related bulk materials. In this thesis, ordered (sub)monolayers of metal atoms (Au and Sn) on semiconductor substrates (Si(111) and Ge(111)) and ultrathin intermetallic films (CePt5 and LaPt5) on metal substrate (Pt(111)) are investigated by polarized in situ surface Raman spectroscopy. The surface Raman spectra exhibit features of specific elementary excitations like surface phonons and electronic excitations, which are suitable to gain fundamental insights into the surface systems. The Au-induced surface reconstructions (5x2) and (r3xr3) constitute quasi-one- and two-dimensional Au structures on the Si(111) substrate, respectively. The new reconstruction-related Raman peaks are analyzed with respect to their polarization and temperature behavior. The Raman results are combined with firstprinciples calculations to decide between different proposed structural models. The Au-(5x2)/Si(111) reconstruction is best described by the model of Kwon and Kang, while for Au-(r3xr3)/Si(111) the conjugate honeycomb-chained-trimer model is favored. The Sn-induced reconstructions with 1/3 monolayer on Ge(111) and Si(111) are investigated to reveal their extraordinary temperature behavior. Specific surface phonon modes are identified that are predicted within the dynamical fluctuation model. Contrary to Sn/Si(111), the corresponding vibrational mode of Sn/Ge(111) exhibits a nearly harmonic character. The reversible structural phase transition of Sn/Ge(111) from (r3xr3) to (3x3) is observed, while no phase transition is apparent for Sn/Si(111). Moreover, Raman spectra of the closely related systems Sn-(2r3x2r3)/Si(111) and thin films of a-Sn as well as the clean semiconductor surfaces Si(111)-(7x7) and Ge(111)-c(2x8) are evaluated and compared. The CePt5/Pt(111) system hosts 4f electrons whose energy levels are modified by the crystal field and are relevant for a description of the observed Kondo physics. In contrast, isostructural LaPt5/Pt(111) has no 4f electrons. For CePt5/Pt(111), distinct Raman features due to electronic Raman scattering can be unambiguously related to transitions between the crystal-field states which are depth-dependent. This assignment is supported by comparison to LaPt5/Pt(111) and group theoretical considerations. Furthermore, the vibrational properties of CePt5 and LaPt5 reveal interesting similarities but also striking differences like an unusual temperature shift of a vibration mode of CePt5, which is related to the influence of 4f electrons.},
  subject      = {Raman-Spektroskopie},
  language  = {en}
}
@phdthesis{Seibel2016,
  author    = {Seibel, Christoph},
  title     = {Elektronische Struktur von Halbleiteroberfl{\"a}chen mit starker Spin-Bahn-Wechselwirkung: Topologie, Spinpolarisation und Robustheit},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-140418},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2016},
  abstract  = {Neue Erkenntnisse {\"u}ber elektronische Eigenschaften von Festk{\"o}rpern legen den Grundstein f{\"u}r innovative Anwendungen der Zukunft. Von zentraler Bedeutung sind insbesondere die Eigenschaften der Elektronenspins. Um diese besser zu verstehen, befasst sich die vorliegende Arbeit mit der experimentellen Analyse der elektronischen Struktur von topologischen Isolatoren (Sb\$_2\$Te\$_3\$ , Bi\$_2\$Se\$_x\$Te\$_{3-x}\$, Bi\$_{1.5}\$Sb\$_{0.5}\$Te\$_{1.8}\$Se\$_{1.2} und Bi\$_{1.4}\$Sb\$_{1.1}\$Te\$_{2.2}\$S\$_{0.3}\$) und Kristallen mit starker Spin-Bahn-Wechselwirkung (BiTeI) mittels Photoelektronenspektroskopie. Zu Beginn werden die zum Verst{\"a}ndnis dieser Arbeit ben{\"o}tigten Grundlagen erkl{\"a}rt sowie die unterschiedlichen zum Einsatz kommenden Techniken eingef{\"u}hrt. Der Hauptteil der Arbeit teilt sich in drei Forschungsschwerpunkte. Der erste Teil befasst sich mit den elektronischen Eigenschaften der Valenzbandstruktur von Sb2Te3 und den auftretenden Oberfl{\"a}chenzust{\"a}nden. Durch gezielte Variation der Energie der anregenden Strahlung wird der Charakter der Wellenfunktion des topologischen Oberfl{\"a}chenzustands und dessen Wechselwirkung mit Valenzzust{\"a}nden erforscht. Dabei spielt die Topologie der Volumenbandstruktur eine grundlegende Rolle. Der zus{\"a}tzliche Vergleich zu Photoemissionsrechnungen erm{\"o}glicht detaillierte Einblicke in die Wechselwirkung zwischen Oberfl{\"a}chen- und Volumenzust{\"a}nden und gibt Aufschluss dar{\"u}ber, wie diese vermittelt werden. Im zweiten Abschnitt wird durch die Analyse des gemessenen Photoelektronenspins das Zusammenspiel der Spintextur des Grundzustands und Endzust{\"a}nden in Bi2Te3 untersucht. Dabei treten, im Gegensatz zu Grundzustandsrechnungen, Radialkomponenten des Polarisationsvektors in nichtsymmetrischer Messgeometrie auf. Sowohl deren Energieabh{\"a}ngigkeit als auch deren Auftreten in Photoemissionsrechnungen (1-Schritt-Modell) deutet darauf hin, dass diese ihren Ursprung in {\"U}bergangsmatrixelementen des Photoemissionsprozesses haben. Dieses Ergebnis wird mit Spinpolarisationsmessungen am Oberfl{\"a}chenzustand des nicht-topologischen Schichtsystems BiTeI verglichen. Im dritten Teil werden Auswirkungen unterschiedlicher Manipulationen der untersuchten Materialien auf deren elektronische Eigenschaften beschrieben. Die Adsorption von Bruchteilen einer monoatomaren Lage des Alkalimetalls Caesium auf die Oberfl{\"a}che des topologischen Isolators Sb2Te3 wird systematisch untersucht. Dadurch kann dessen intrinsische p-Dotierung teilweise abgebaut werden, wobei die Valenzbandstruktur trotz der Reaktivit{\"a}t des Adsorbats intakt bleibt. Des Weiteren werden Auswirkungen von {\"A}nderungen der Kristallst{\"o}chiometrie durch Volumendotierung vergleichend diskutiert. Ausblickend befasst sich das Kapitel mit dem Verhalten geringer Mengen ferromagnetischer Materialen (Fe, Ni) auf den Oberfl{\"a}chen der topologischen Isolatoren. F{\"u}r die verschiedenen Adsorbate werden Trends aufgezeigt, die von Temperatur und Zusammensetzung des Substratkristalls abh{\"a}ngen.},
  subject      = {Elektronenstruktur},
  language  = {de}
}
@phdthesis{Aulbach2018,
  author    = {Aulbach, Julian},
  title     = {Gold-Induced Atomic Wires on Terraced Silicon Surfaces: Formation and Interactions of Silicon Spin Chains},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-169347},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2018},
  abstract  = {Atomic nanowires formed by self-assembled growth on semiconducting surfaces represent a feasible physical realization of quasi-1D electron systems and can be used to study fascinating 1D quantum phenomena. The system in the focus of this thesis, Si(553)-Au, is generated by Au adsorption onto a stepped silicon surface. It features two different chain types, interspersed with each other: A Au chain on the terrace, and a honeycomb chain of graphitic silicon located at the step edge. The silicon atoms at the exposed edges of the latter are predicted to be spin-polarized and charge-ordered [1], leading to an ordered array of local magnetic moments referred to as ``spin chains''. The present thesis puts this spin chain proposal to an experimental test. A detailed scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS) scrutiny reveals a distinct unoccupied density of states (DOS) feature localized at every third Si step-edge atom, which aligns perfectly with the density functional theory (DFT) prediction. This finding provides strong evidence for the formation of spin chains at the Si(553)-Au step edges, and simultaneously rules out the interpretation of previous studies which attributed the x3 step-edge superstructure to a Peierls instability. To study the formation of spin chains in further detail, an additional member of the so-called Si(hhk)-Au family -- Si(775)-Au -- is analyzed. Based on DFT modeling (performed by S.C. Erwin, Naval Research Laboratory, USA) and detailed STM and STS experiments, a new structure model for this surface is developed, and the absence of spin chains at the Si(775)-Au step edges is demonstrated. The different step-edge charge distributions of all known Si(hhk)-Au surfaces are traced back to an electron transfer between the terrace and the step edge. Accordingly, an unintentional structure defect should create a localized spin at the Si(775)-Au step edge. This prediction is verified experimentally, and suggest that surface chemistry can be used to create and destroy Si spin chains. Having clarified why spin chains form on some Si(hhk)-Au surfaces but not on others, various interaction effects of the Si(553)-Au spin chains are inspected. A collaborative analysis by SPA-LEED (M. Horn-von Hoegen group, University of Duisburg-Essen, Germany), DFT (S.C. Erwin), and STM reveals strong lateral coupling between adjacent spin chains, bearing interesting implications for their magnetic ordering. The centered geometry uncovered leads to magnetic frustration, and may stabilize a 2D quantum spin liquid. Moreover, a complex interplay between neighboring Au and Si chains is detected. Specifically, the interaction is found effectively ``one-way'', i.e., the Si step edges respond to the Au chains but not vice versa. This unidirectional effect breaks the parity of the Si chains, and creates two different configurations of step edges with opposite directionality. In addition to the static properties of the Si(553)-Au surface mentioned above, the occurrence of solitons in both wire types is witnessed in real space by means of high-resolution STM imaging. The solitons are found to interact with one another such that both move in a coupled fashion along the chains. Likewise, STM experiments as a function of the tunneling current suggest an excitation of solitons along the step edge by the STM tunneling tip. Solitons are also found to play an essential role in the temperature-dependent behavior of the Si(553)-Au step edges. It is an accepted fact that the distinct x3 superstructure of the Si(553)-Au step edges vanishes upon heating to room temperature. As a first step in exploring this transition in detail over a large temperature range, a previously undetected, occupied electronic state associated with the localized step-edge spins is identified by means of angle-resolved photoemission spectroscopy (ARPES). A tracking of this state as a function of temperature reveals an order-disorder-type transition. Complementary STM experiments attribute the origin of this transition to local, thermally activated spin site hops, which correspond to soliton-anitsoliton pairs. Finally, a manipulation of the Si(553)-Au atomic wire array is achieved by the stepwise adsorption of potassium atoms. This does not only increase the filling of the Au-induced surface bands culminating in a metal-insulator transition (MIT), but also modifies the Si step-edge charge distribution, as indicated by STM and ARPES experiments. [1] S. C. Erwin and F. Himpsel, Intrinsic magnetism at silicon surfaces, Nat. Commun. 1, 58 (2010).},
  subject      = {Rastertunnelmikroskopie},
  language  = {en}
}
@phdthesis{Reis2022,
  author    = {Reis, Felix},
  title     = {Realization and Spectroscopy of the Quantum Spin Hall Insulator Bismuthene on Silicon Carbide},
  doi       = {10.25972/OPUS-25825},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-258250},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2022},
  abstract  = {Topological matter is one of the most vibrant research fields of contemporary solid state physics since the theoretical prediction of the quantum spin Hall effect in graphene in 2005. Quantum spin Hall insulators possess a vanishing bulk conductivity but symmetry-protected, helical edge states that give rise to dissipationless charge transport. The experimental verification of this exotic state of matter in 2007 lead to a boost of research activity in this field, inspired by possible ground-breaking future applications. However, the use of the quantum spin Hall materials available to date is limited to cryogenic temperatures owing to their comparably small bulk band gaps. In this thesis, we follow a novel approach to realize a quantum spin Hall material with a large energy gap and epitaxially grow bismuthene, i.e., Bi atoms adopting a honeycomb lattice, in a \((\sqrt{3}\times\sqrt{3})\) reconstruction on the semiconductor SiC(0001). In this way, we profit both from the honeycomb symmetry as well as the large spin-orbit coupling of Bi, which, in combination, give rise to a topologically non-trivial band gap on the order of one electronvolt. An in-depth theoretical analysis demonstrates that the covalent bond between the Si and Bi atoms is not only stabilizing the Bi film but is pivotal to attain the quantum spin Hall phase. The preparation of high-quality, unreconstructed SiC(0001) substrates sets the basis for the formation of bismuthene and requires an extensive procedure in ultra-pure dry H\(_2\) gas. Scanning tunneling microscopy measurements unveil the (\(1\times1\)) surface periodicity and smooth terrace planes, which are suitable for the growth of single Bi layers by means of molecular beam epitaxy. The chemical configuration of the resulting Bi film and its oxidation upon exposure to ambient atmosphere are inspected with X-ray photoelectron spectroscopy. Angle-resolved photoelectron spectroscopy reveals the excellent agreement of probed and calculated band structure. In particular, it evidences a characteristic Rashba-splitting of the valence bands at the K point. Scanning tunneling spectroscopy probes signatures of this splitting, as well, and allows to determine the full band gap with a magnitude of \(E_\text{gap}\approx0.8\,\text{eV}\). Constant-current images and local-density-of-state maps confirm the presence of a planar honeycomb lattice, which forms several domains due to different, yet equivalent, nucleation sites of the (\(\sqrt{3}\times\sqrt{3}\))-Bi reconstruction. Differential conductivity measurements demonstrate that bismuthene edge states evolve at atomic steps of the SiC substrate. The probed, metallic local density of states is in agreement with the density of states expected from the edge state's energy dispersion found in density functional theory calculations - besides a pronounced dip at the Fermi level. By means of temperature- and energy-dependent tunneling spectroscopy it is shown that the spectral properties of this suppressed density of states are successfully captured in the framework of the Tomonaga-Luttinger liquid theory and most likely originate from enhanced electronic correlations in the edge channel.},
  subject      = {Zweidimensionales Material},
  language  = {en}
}
@phdthesis{Bauernfeind2023,
  author    = {Bauernfeind, Maximilian Josef Xaver},
  title     = {Epitaxy and Spectroscopy of Two-Dimensional Adatom Systems: the Elemental Topological Insulator Indenene on SiC},
  doi       = {10.25972/OPUS-31166},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-311662},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2023},
  abstract  = {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.},
  subject      = {Dreiecksgitter},
  language  = {en}
}
@phdthesis{Wagner2024,
  author    = {Wagner, Tim Matthias},
  title     = {Characterization of 2D antimony lattices},
  doi       = {10.25972/OPUS-36329},
  url       = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-363292},
  school      = {Universit{\"a}t W{\"u}rzburg},
  year      = {2024},
  abstract  = {Two-dimensional lattices are in the focus of research in modern solid state physics due to their novel and exotic electronic properties with tremendous potential for seminal future applications. Of particular interest within this research field are quantum spin Hall insulators which are characterized by an insulating bulk with symmetry-protected metallic edge states. For electrons within these one-dimensional conducting channels, spin-momentum locking enables dissipationless transport - a property which promises nothing short of a revolution for electronic devices. So far, however, quantum spin Hall materials require enormous efforts to be realized such as cryogenic temperatures or ultra-high vacuum. A potential candidate to overcome these shortcomings are two-dimensional lattices of the topological semi-metal antimony due to their potential to host the quantum spin Hall effect while offering improved resilience against oxidation. In this work, two-dimensional lattices of antimony on different substrates, namely Ag(111), InSb(111) and SiC(0001), are investigated regarding their atomic structure and electronic properties with complimentary surface sensitive techniques. In addition, a systematic oxidation study compares the stability of Sb-SiC(0001) with that of the two-dimensional topological insulators bismuthene-SiC(0001) and indenene-SiC(0001). A comprehensive experimental analysis of the \((\sqrt{3}\times\sqrt{3})R30^\circ\) Sb-Ag(111) surface, including X-ray standing wave measurements, disproves the proclaimed formation of a buckled antimonene lattice in literature. The surface lattice can instead be identified as a metallic Ag\(_2\)Sb surface alloy. Antimony on InSb(111) shows an unstrained Volmer-Weber island growth due to its large lattice mismatch to the substrate. The concomitant moir\'{e} situation at the interface imprints mainly in a periodic height corrugation of the antimony islands which as observed with scanning tunneling microscopy. On islands with various thicknesses, quasiparticle interference patterns allow to trace the topological surface state of antimony down to the few-layer limit. On SiC(0001), two different two-dimensional antimony surface reconstructions are identified. Firstly, a metallic triangular \$1\times1\$ lattice which constitutes the antimony analogue to the topological insulator indenene. Secondly, an insulating asymmetric kagome lattice which represents the very first realized atomic surface kagome lattice. A comparative, systematic oxidation study of elemental (sub-)monolayer materials on SiC(0001) reveals a high sensitivity of indenene and bismuthene to small dosages of oxygen. An improved resilience is found for Sb-SiC(0001) which, however, oxidizes nevertheless if exposed to oxygen. These surface lattices are therefore not suitable for future applications without additional protective measures.},
  subject      = {Antimon},
  language  = {en}
}