Scanning Tunneling Microscopy and Atomic Force Microscopy Measurements on Correlated Systems Dissertation zur Erlangung des akademischen Grades eines Doktors der Naturwissenschaften (Dr. rer. nat.) vorgelegt von Matthias Münks an der Mathematisch-Naturwissenschaftliche Sektion Fachbereich Physik Tag der mündlichen Prüfung: 17.10.2017 1. Referent: Prof. Dr. Klaus Kern 2. Referent: Prof. Dr. Fabian Pauly Konstanzer Online-Publikations-System (KOPS) URL: http://nbn-resolving.de/urn:nbn:de:bsz:352-2--1do79w04nugoh0 ii iii iv Zusammenfassung Die magnetischen Signaturen einzelner Atome auf Oberflächen oder eingebettet in molekularen Strukturen hängen größtenteils von der unmittelbaren Umgebung ab. Zu Beginn dieser Ar- beit präsentieren wir Messungen einzelner Co Atome auf einer h-BN/Rh(111) Oberfläche mit einem kombinierten Rastertunnel- und Rasterkraftmikroskop. Wir zeigen wie diese Co Atome CoHx Komplexe bilden, die je nach Ihrem Wasserstoffgehalt (Co, CoH, CoH2) verschiedene Spinsignale zeigen (S = 3=2;S = 1;S = 1=2). Die h-BN Entkopplungslage besitzt eine peri- odische Welligkeit von 100 pm senkrecht zur Rh(111) Oberfläche, wodurch nicht nur die mag- netische Anisotropie der CoHx Komplexe beeinflusst wird sondern auch deren Kopplungsstärke zu dem Rh Substrat. Diese Welligkeit führt zu einem sehr weichen Substrat welches aktiv mit der Spitze verformt werden kann während man parallel die involvierten kurzreichweitigen Kräfte und Tunnelströme misst. Eine einzigartige Eigenschaft der Rastersondenmikroskopie ist die präzise Kontrolle über beide Elektroden, der Probe und Spitze, welche die einzelnen Atome oder Moleküle im Tunnelkon- takt untersuchen. Im weiteren Verlauf zeigen wir, wie eine Pt Spitze mit einzelnen Co oder H Atomen funktionalisiert werden kann. Mit einer Co-funktionalisierten Spitze können wir kontrolliert an ein CoHx Komplex auf der h-BN/Rh(111) Oberfläche ankoppeln. Wir sehen Hinweise auf Spin-Spin Korrelationen zwischen dem stark hybridisierten Co Atom auf der Pt Spitze und dem Elektronenbad in der Pt Spitze wenn diese stark an ein schwach hybridisiertes CoHx auf der Probe angekoppelt wird. Die Tunnelspektroskopie zeigt eine Asymmetrie die nor- malerweise nur für spin-polarisierten Elektronentransport in Magnetfeldern auftritt. Wir zeigen, dass diese Asymmetrie hier ohne Magnetfelder auftritt, durch die Kopplungsstärke kontrolliert werden kann und der Ursprung der Spin-Spin Korrelationen in der funktionalisierten Spitze liegt. Wenn die Pt Spitze mit einem H Atom funktionalisiert und in die unmittelbare Nähe eines CoH Komplexes gebracht wird, können wir das H Atom reversibel an den CoH Kom- plex binden sowie entfernen und so den Spinzustand kontrollieren. Die Tunnelspektroskopie zeigt den Übergang des CoH S = 1 Komplexes mit magnetischer Anisotropie zu einem CoH2 S = 1=2 Komplex mit einer Kondo Resonanz während zur gleichen Zeit die Kraftmessungen einen Übergang zu einem energetisch günstigeren Potential zeigen. Stumpfe Spitzen scheinen aktiv die Adsorbierungslandschaft der CoH Komplexe auf der h-BN Schicht zu modifizieren. Unsere Experimente erlauben eine atomar präzise Kontrolle über molekulare magnetische Struk- turen. Wir zeigen, dass Schlüsselparameter wie die magnetische Anisotropie der CoHx Kom- plexe sowie deren Kondo-Interaktion oder Spin-Spin Korrelationen mit einer Metallelektrode gemessen und kontrolliert werden können. v vi Abstract The magnetic signatures of single atoms on surfaces or embedded in different molecular con- figurations vastly depend on their adjacent environment. Placing transition metal Co atoms on a h-BN/Rh(111) substrate allows us to probe their magnetic signatures with scanning tunneling microscopy and spectroscopy as well as atomic force measurements. We show how hydrogen adsorption creates cobalt hydride complexes, Co, CoH and CoH2, for which the hydrogen ac- tively controls the observed spin state, S = 3=2, S = 1, S = 1=2. A new approach is the h-BN decoupling layer that mediates the cobalt hydrides’ coupling to the Rh(111) metal due to its intrinsic spatial corrugation. This corrugation not only adjusts the magnetic anisotropy energies of the adsorbed complexes but also beds them on an ultrasoft substrate that can actively be ma- nipulated with tip interactions while monitoring the involved forces. A unique feature of scanning probe experiments is the precise control over both metal elec- trodes, tip and sample, that address the atoms or molecules in the tunnel junction. We show how Pt tips are functionalized with single Co or H atoms. In the case of a Co-functionalized Pt tip, we can controllably couple it to a CoHx system on the h-BN/Rh(111) surface. Signatures of correlations between the strongly hybridized spin on the tip and its electron bath are seen when it is coupled to the weakly hybridized cobalt hydride on the sample surface. Tunneling spec- troscopy uncovers an asymmetry reminiscent of spin-polarized transport in magnetic fields. We show that, even at zero field, this asymmetry is exclusively controlled by the coupling strength and related to spin-spin correlations in the functionalized tip. Furthermore, we can actively control the chemical composition of the CoHx systems. When the Pt tip is functionalized with a single hydrogen atom and brought in proximity to a CoH system, we can reversibly attach and remove the additional hydrogen atom in order to control the spin state of a CoH system and change a CoH S = 1 signature with magnetic anisotropy to a CoH2 S = 1=2 Kondo resonance. Blunt tip apexes seem to actively change the adsorption environments of the CoHx complexes when brought into close proximity to the h-BN substrate. All of these modifications are actively monitored in their tunneling current, frequency shift and spectroscopic signals. The results of our experiments allows an atomically precise control over magnetic molecular junctions. We show how key parameters such as the magnetic anisotropy energies of adsorbed CoHx systems, their Kondo exchange coupling to the metal electrode as well as hidden spin- spin correlations of these systems with a bare metal electrode can be monitored and actively controlled. Keywords: Combined STM/AFM, qPlus, Correlated and coupled atomic spins, Surface Mag- netism, Magnetic Anisotropy, Kondo. vii Publications Since the beginning of the Ph.D. thesis, three publications have been published and two more are in the process of being drafted: • P. Jacobson, T. Herden, M. Muenks, G. Laskin, O. Brovko, V. Stepanyuk, M. Ternes, K. Kern. Quantum engineering of spin and anisotropy in magnetic molecular junctions. Nature Communications 6, 8536 (2015). • M. Muenks, P. Jacobson, M. Ternes, K. Kern. Correlation-driven transport asymmetries through coupled spins in a tunnel junction. Nature Communications 8, 14119 (2017). • P. Jacobson, M. Muenks, G. Laskin, O. Brovko, V. Stepanyuk, Markus Ternes, Klaus Kern. Potential energy driven spin manipulation via a controllable hydrogen ligand. Science Advances, (2017)1. • M. Muenks, P. Jacobson, M. Ternes, K. Kern. Electron transport through correlated Kondo systems in magnetic field. In preparation. • M. Muenks, P. Jacobson, M. Ternes, K. Kern. Short-range force and magnetic anisotropy modulation via an ultrasoft substrate. In preparation. 1Manuscript is accepted and will be published in April 2017. M. Muenks and P. Jacobson with equal contribu- tion. viii I want to thank Prof. Dr. Klaus Kern for the possibility to execute research under the best possible circum- stances within the academic world. I especially appreciate the experienced freedom to follow my own scientific approach and the encouragement and trust you put into your entire group as well as your ability to extract the important content from a noisy background. Priv.-Doz. Dr. Markus Ternes for his day to day supervision. Not only your practical knowl- edge in the lab as a former ham radio enthusiast but also your deep understanding of physics helped and inspired me a lot. I am grateful for your help and the time you took to explain even the trivial details when I asked for it. I truly think you are an outstanding scientist and wish you all the best for your future endeavors! Dr. Peter Jacobson for being a constant source of inspiration and teaching me an eloquent scientific approach in his function as a mentor and co-author. Thank you for your help in the laboratory and the data analysis as well as the still ongoing discussions about science, politics and life in general. Prof. Dr. Fabian Pauly for his prompt commitment to being my second referee on this thesis and the discussions about our first results of correlated electron transport. Your expertise as a theoretician in quantum transport will provide a complementary view on this thesis. I wish you all the best for your upcoming research in Okinawa, Japan! Prof. Dr. Leitenstorfer for his prompt commitment to being on my thesis defense committee. Thank you for providing the knowledge and discussing the femtosecond laser project that is being implemented into the existing machine right now within a Konstanz/Stuttgart research collaboration. Abhishek Grewal, Yuqi Wang and Gennadii Laskin for their past and current commitment to the practical and theoretical tasks during their master and doctoral studies related to our ex- periments. We really enjoyed ourselves! Wolfgang Stiepany, Peter Andler and Marko Memmler for their vast technical expertise and guidance in all matters related to the practical aspects of scanning probe techniques. Mar- tin Siemers and Rafail Chaikevitch for their professional