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Josephson Tunneling at the Atomic Scale: the Josephson EEct As a Local Probe

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Josephson Tunneling at the Atomic Scale: the Josephson EEct As a Local Probe Josephson Tunneling at the Atomic Scale: The Josephson Eect as a Local Probe THIS IS A TEMPORARY TITLE PAGE It will be replaced for the nal print by a version provided by the service academique. Thèse n. 6750 2015 présentée le 22 juillet 2015 à la Faculté des Sciences des Bases Laboratoire á l'Echelle Nanométrique Programme Doctoral en Physique École Polytechnique Fédérale de Lausanne pour l'obtention du grade de Docteur des Sciences par Berthold Jäck acceptée sur proposition du jury: Prof. Andreas Pautz, président du jury Prof. Klaus Kern, directeur de thèse Prof. Davor Pavuna, rapporteur Prof. Hermann Suderow, rapporteur Dr. Fabien Portier, rapporteur Lausanne, EPFL, 2015 Non est ad astra mollis e terris via — Lucius Annaeus Seneca Acknowledgements It took me about three and a half years, with many ups and downs, to complete this thesis. At this point, I want to say thank you to all the people that supported me on my way by any means. First of all, I am grateful to Prof. Klaus Kern for giving me the opportunity to perform my doctoral studies under his supervision at the Max-Planck-Institute for Solid State Research in Stuttgart, Germany. For the entire time of my studies, I enjoyed a significant scientific freedom, such that I had the great possibility to pursue and realize my own ideas. In particular, I want to acknowledge his personal support, which allowed me to attend conferences, workshops and schools, have interesting scientific discussions as well as a wonderful time in the whole of Europe and across the USA. I highly appreciate his support and the trust he put in me. I am very grateful to Dr. Christian Ast and Dr. Markus Etzkorn for supervising my stud- ies, their continuous support and many fruitful discussions about physics. I highly benefited and learned from their exceptional experimental, theoretical and computational knowledge as well as experience. I am thankful to Prof. Andreas Pautz, Prof. Davor Pavuna, Prof. Hermann Suderow and Dr. Fabien Portier for being part of my thesis committee. Special thanks go to my fellow PhD colleague Matthias Eltschka. We really worked hard to tame the beast, a.k.a. the milli-Kelvin STM, and spent many day and night shifts together in the MPI. Our shared working ethics certainly helped to accomplish this thesis. All the results that I presented in this thesis would not exist without the permanent help and support from many people at the MPI. In particular, I am grateful to our technicians Wolfgang Stiepany, Peter Andler and Marco Memmler for their permanent support on taming the beast and their immediate help on repairing parts that I had damaged. Moreover, they were also great football team mates in Kernspaltung. I am also very thankful to Sabine Birtel who always had a helping hand for me. At this point, I also want to express my thanks to the low temperature service group, the IT service group, the crystal growth service group and the technology service group of our institute. I am very thankful to Dr. Christian Urbina and Dr. Fabien Portier from CEA Saclay. I benefited a lot from their comprehensive knowledge on Josephson physics and I highly appreciated i Acknowledgements the opportunity to visit the Quantronics group at the CEA Saclay. I also want to express my gratitude to Prof. Joachim Ankerhold, Prof. Alex Balatsky, Prof. Gerd-Ludwig Ingold, Prof. Dimitri Roditchev and Prof. Alexey Ustinov for fruitful discussions about Josephson physics. I am very grateful to Dr. Thomas White for thoroughly proofreading my thesis within the shortest possible time. Working on a PhD thesis in experimental physics focuses on lab work, data analysis and scientific discussion. However, this job certainly runs better if you are in good company, which I certainly was. I’m grateful to have met Dr. Laura Rizzi and Daniel Weber in the guesthouse on my first day here in Stuttgart. Their company gave me an easy start and I’m very happy to be still in friendly contact with Dan. Special thanks go to my ol’ 6B10 office crew, Alexander ’Ali’ Kölker and Jon ’Nathan’ Parnell, for fun times, good music and serious target shooting competitions. I also highly enjoyed my “scientific“ roadtrips through Europe and the United States together with Daniel Rosenblatt and Christoph Grosse. I am glad that I met Sebastian Koslowski and that we share more than only our passion for running. Moreover, I want to thank Carola Straßer, Verena Schendel, Lukas Schlipf, Claudius Morchutt, Christian Dette, Ivan Pentegov, Matthias Muenks, Jacob Senkpiel, Dr. Tobias Herden, Benjamin Wurster, and Diana Hoetger for fun times at the MPI, on the Cannstatter Wasen and anywhere else. I want to express my deepest thankfulness to my parents, Dr. Herbert Jäck and Dorothea Jäck who, always and literally, unconditionally supported my path of life. My highest gratitude goes to my partner Silvia who cheered me up in hard times and enjoyed the happy times with me. Her permanent support was outstanding, such that it is not wrong to say: She did half of all the work! Stuttgart, 21st of May, 2015 B. J. ii Abstract The aim of this thesis is to characterize the properties of a Josephson junction in a Scanning Tunneling Microscope (STM) at millikelvin temperatures and to implement Josephson STM (JSTM) as a versatile probe at the atomic scale. To this end we investigate the I(VJ) tunnel- ing characteristics of the Josephson junction in our STM at a base temperature of 15 mK by means of current-biased and voltage-biased experiments. We find that in the tunneling regime, G G , the Josephson junction is operated in the dynamical Coulomb blockade N ¿ 0 (DCB) regime in which the sequential tunneling of Cooper pairs dominates the tunneling current. Employing P(E)-theory allows us to model experimental I(VJ) characteristics from voltage-biased experiments and determine experimental values of the Josephson critical current in agreement to theory. Moreover, we observe a breakdown of P(E)-theory for ex- periments at large values of the tunneling conductance G G , which could indicate that N ¼ 0 the coherent tunneling of Cooper pairs strongly contributes to the tunneling current in this limit. We also observe that the Josephson junction in an STM at temperatures well below 100 mK is highly sensitive to its electromagnetic environment that results from its tiny junc- tion capacitance of a few femtofarads. The combination of the experimental results with numerical simulations reveals that the immediate environment of the Josephson junction in an STM is frequency-dependent and, additionally, that a typical STM geometry shares the electromagnetic properties of a monopole antenna with the STM tip itself acting as the antenna rod. Comparing the I(VJ) curves of voltage-biased and current-biased experiments, we observe that the time evolution of the junction phase is strongly effected by dissipation due to quasiparticle excitations. From investigations on the retrapping current IR, we show first that the temporal evolution of the junction phase in our STM satisfies a classical equation of motion. Second, we can determine two different channels for energy dissipation of the junction phase. For tunneling resistance values of R 150k­ the junction dissipates via N · Andreev reflections, whereas for larger values of R 150k­ energy dissipation is dominated N ¸ by the lifetime effects of Cooper pairs. Moreover, from comparing both experiments we also observe that the quantum-mechanical nature of the junction phase manifests in quantum- mechanical phenomena, such as phase tunneling, which strongly alter our experimental I(VJ) characteristics for G G . N · 0 To conclude, within this thesis we characterized the properties of a Josephson junction in an STM that is operated at millikelvin temperatures. Hence, this work represents necessary and fundamental steps that allows us to employ the Josephson effect as a versatile probe on the atomic scale. iii Acknowledgements Key words: Josephson Effect, Scanning Tunneling Microscopy (STM), Josephson Scanning Tunneling Microscopy (JSTM), Dynamical Coulomb Blockade, Quasiparticle Dissipation iv Zusammenfassung Das Ziel der vorliegenden Dissertation ist zunächst die Eigenschaften eines Josephson Tunnel- kontakts in einem Rastertunnelmikroskop (RTM) bei Millikelvin-Temperaturen zu bestimmen und dadurch Josephson RTM (JRTM) als eine vielseitig einsetzbare Sonde auf atomarer Ebene zu etablieren. Hierfür untersuchten wir die Eigenschaften der I(VJ)-Spektren des Josephson Tunnelkontaktes in unserem RTM bei einer Basistemperatur von 15 mK mittels strom- und spanunnungsgetriebener Experimente. Für den Tunnelbereich, G G , ergaben die Unter- N ¿ 0 suchungen, dass sich der Josephson Tunnelkontakt innerhalb des dynamischen Coulomb- Blockade (DCB) Regimes befindet, in welchem das sequentielle Tunneln von Cooper-Paaren den Tunnelstrom dominiert. Mit Hilfe der sogenannten P(E)-Theorie konnten wir sowohl die experimentellen I(VJ)-Spektren aus spannungsgetriebenen Experimenten fitten als auch ex- perimentelle Werte des kritischen Josephson Stromes bestimmen. Darüberhinaus stellten wir fest, dass diese experimentellen Werte mit thereotisch berechneten Werten übereinstimmen. Des Weiteren ergaben unsere Untersuchungen, dass die bei großen Leitfähigkeitswerten des Tunnelkontakts, G G , gemessenen Cooper-Paar Tunnelspektren nicht mehr mit Hilfe der N ¼ 0 P(E)-Theorie beschrieben werden können. Dies könnte darauf hinweisen, dass das kohärente Tunneln der Copper-Paare wesentlich zu dem Tunnelstrom unter dieser Bedingung beiträgt. Wir beobachteten auch, dass der Josephson
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