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Etudes de transport quantique pour la mise en oeuvre de la spintronique : des systèmes de nanotubes de carbone supramoléculaires à l'isolant cristallin topologique

Abstract : Molecular electronics is one of the most intriguing fields of modern research, which could bring forth a modular and scalable building system for nanoscale spintronics applications. A particularly promising example are single-molecule magnets, which have already successfully shown to be suitable for spin valve or spin qubit operations. One of the biggest challenges of the field is the integration of these nanometer-sized objects in complex circuits in order to allow for detection and manipulation of moleculear spin states. As shown in recent years by the NanoSpin group, carbon nanotubes (CNTs) can serve as such type of carrier for the single-molecule magnets, combining features of both constituents.A corner stone of this thesis project was hence the development of a dependable fabrication technique for high-quality CNT devices, controllable by multiple local gate electrodes in order to enable local control of molecular hybrid systems. A process based on conventional one-chip fabrication was developed from scratch, for which optimization of sample design, lithography and deposition techniques as well as material choices had to be carefully incorporated, in order to accomodate the restrictions imposed by the CNT growth conditions on the prevention of leakage currents. We succeeded in producing clean CNT devices, which could support a double dot configuration, tunable from p- to n-type characteristics. The segments created in this way can be stabily controlled over the entire device length and should hence provide a suitable backbone to study molecular physics.Topological matter constitutes an enticing platform to investigate both fundamental principles as well as possible applications from spintronics to quantum computation. Topological crystalline insulators, with tin telluride ( SnTe ) as a prime example, represent a new state of matter within this zoo of 3D topological materials. Soon after first experimental realizations, suggestions were made about the possibility of an unconventional type of superconductivity hosted at the interface between topological matter and conventional superconductors. Possible implications of such systems include Cooper pairing with finite momentum, the FFLO phase, or topological quantum computing, based on peculiar excitations, called Majorana bound states.This thesis project aimed to participate in the investigation of signs of unconventional superconductivity in SnTe . Transport experiments on bare films in Hall bar geometries and superconducting hybrid devices, realized as both Josephson junctions and SQUIDs, are discussed. A surprisingly strong coupling of SnTe to Ta superconductor was found and dependencies of superconductivity on sample geometries, temperature and magnetic field were investigated. The current-phase relation was analyzed in the limit of strong kinetic effects. Electrostatic gating and rf exposure was explored, but predominant physics in such configurations turned out to be of purely conventional type, pointing out the importance of improvements on the material side.In-plane magnetic field measurements gave rise to the manifestation of ϕ0-SQUIDs with tunable 0−π-transitions, providing evidence for possible controlled transitions from trivial superconductivity to unconventional coupling regimes in SnTe.
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https://tel.archives-ouvertes.fr/tel-01895425
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Submitted on : Monday, October 15, 2018 - 11:19:11 AM
Last modification on : Wednesday, October 14, 2020 - 4:16:24 AM

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SCHONLE_2018_diffusion.pdf
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  • HAL Id : tel-01895425, version 1

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STAR | CNRS | UGA | NEEL

Citation

Joachim Schönle. Etudes de transport quantique pour la mise en oeuvre de la spintronique : des systèmes de nanotubes de carbone supramoléculaires à l'isolant cristallin topologique. Superconductivity [cond-mat.supr-con]. Université Grenoble Alpes, 2018. English. ⟨NNT : 2018GREAY022⟩. ⟨tel-01895425⟩

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