Galletti, Luca (2014) coherence effects in superconducting hybrid devices. [Tesi di dottorato]

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This PhD project is focused on the study of superconducting hybrid devices and nanostructures. This study includes a fairly broad range of problems of condensed matter physics, as for instance Josephson and proximity effect in conjunction with the study of phase dynamics, or the topological state of matter, covering topologically edge states and Majorana bound states. The interest in the field of weak superconductivity in hybrid structures has been recently renewed by the introduction of unconventional barriers including semiconducting nanowires, or quasi bi-dimensional system, like graphene flakes or the edge states of topological insulators (TI). These materials are also characterized by the presence of a Dirac cone in the energy spectrum. These junctions require advanced nano-lithographic techniques, which make these devices state-of-the-art samples. On the basis of a strong background on grain boundary HTS Josephson junctions, their phase dynamics and quantum properties, a special focus of my thesis is dedicated to the study of coplanar superconductor-topological insulator-superconductor (S-TI-S) devices. The barriers used in this work are mainly Bi2Se3 and Bi2Te3. A detailed analysis performed on Al/Bi2Se3/Al devices was combined with the study of Bi2Se3 Hall bars at high magnetic field (14 T) and STM. We have observed a superconductive ballistic transport trough the topologically protected edge state of the material. The ballistic nature of the transport is proved by the analysis of the IV curved as a function of the temperature, down to the temperature of 20 mK. A ballistic transport regime was also observed on Al/Bi2Te3/Al devices. In these devices we performed a careful characterization of coherence and dissipation, dedicating a special attention to the study of the escape dynamics. Here evidence of a moderate damping regime is obtained, through a comparative analysis performed also on other junctions, such as YBCO bi-epitaxial junctions, and a comparison with a numerical code, based on Monte Carlo simulations. This study was performed for the first time on the ballistic transport channel, provided by the edge state of the TI. These studies are of great relevance for determining possible structures to be used for the investigation of Majorana fermions and to understand the propagation of superconducting coherence in extreme conditions, including nano-constrictions and uni-dimensional transport channels, or topologically protected edge states in topological insulators.

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