@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{Graus2018, author = {Graus, Martin}, title = {Anwendung und Weiterentwicklung der Orbitaltomographie}, url = {http://nbn-resolving.de/urn:nbn:de:bvb:20-opus-163194}, school = {Universit{\"a}t W{\"u}rzburg}, year = {2018}, abstract = {Als Orbitaltomographie wird eine junge Methode innerhalb der Photoelektronenspektrokopie bezeichnet, welche es erm{\"o}glicht, Molek{\"u}lorbitale mit hoher Ortsaufl{\"o}sung abzubilden. Hierf{\"u}r werden die zu untersuchenden Molek{\"u}le durch elektromagnetische Strahlung angeregt und die mittels Photoeffekt emittierten Elektronen hinsichtlich ihres Impulses und ihrer kinetischen Energie charakterisiert. Moderne Photoemissionsexperimente erlauben die simultane Vermessung des gesamten Impulshalbraumes oberhalb der Probe. Die detektierte Intensit{\"a}tsverteilung stellt dann unter bestimmten Bedingungen das Betragsquadrat eines hemisph{\"a}rischen Schnittes durch den Fourierraum des spektroskopierten Orbitals dar, wobei der Radius der Hemisph{\"a}re von der Energie der anregenden Strahlung abh{\"a}ngt. Bei den in dieser Arbeit untersuchten Systemen handelt es sich um adsorbierte Molek{\"u}le, die hochgeordnete Schichten auf kristallinen Edelmetalloberfl{\"a}chen bilden. Im Fall eindom{\"a}nigen Wachstums liefern die parallel orientierten Molek{\"u}le identische Photoemissionssignale. Kommt es hingegen zur Ausbildung von Rotations- und Spiegeldom{\"a}nen, stellt die gemessene Impulsverteilung eine Superposition der unterschiedlichen Einzelbeitr{\"a}ge dar. Somit lassen sich R{\"u}ckschl{\"u}sse auf die Orientierungen der Molek{\"u}le auf den Substraten ziehen. Diese Charakterisierung molekularer Adsorptionsgeometrien wird anhand verschiedener Modellsysteme vorgestellt. Variiert man die Energie der anregenden Strahlung und somit den Radius der hemisph{\"a}rischen Schnitte durch den Impulsraum, ist es m{\"o}glich den Fourierraum des untersuchten Molek{\"u}lorbitals dreidimensional abzubilden. Kombiniert man die gemessenen Intensit{\"a}ten mit Informationen {\"u}ber die Phase der Wellenfunktion im Impulsraum, die durch zus{\"a}tzliche Experimente oder rechnerisch gewonnen werden k{\"o}nnen, l{\"a}sst sich durch eine Fouriertransformation ein dreidimensionales Bild des Orbitals generieren, wie Schritt f{\"u}r Schritt gezeigt wird. Im Zuge eines Photoemissionsprozesses kann das Molek{\"u}l in einen angeregten vibronischen Zustand {\"u}bergehen. Mittels Photoemissionsexperimenten mit hoher Energieaufl{\"o}sung lassen sich Unterschiede zwischen den Impulsverteilungen der schwingenden Molek{\"u}le und denen im vibronischen Grundzustand feststellen. Ein Vergleich der Messdaten mit Simulationen kann die Identifikation der angeregten Schwingungsmode erm{\"o}glichen, was eine neue Methode darstellt, Erkenntnisse {\"u}ber die Elektron-Phonon-Kopplung in molekularen Materialien zu gewinnen.}, subject = {ARPES}, language = {de} }