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Etude de la coherence quantique dans les systemes supraconducteur - metal normal par microscopie et spectroscopie a effet tunnel

Walter Escoffier 1
1 LCP - Laboratoire de Cryophysique
SPSMS - UMR 9001 - Service de Physique Statistique, Magnétisme et Supraconductivité : DSM/DRFMC/SPSMS
Abstract : When a normal metal (N) is in electrical contact with a superconductor (S), it acquires superconducting properties close to its interface : a phenomen known as "the proximity effect". The objective of the present thesis was to study quantum phase coherence in N by scanning tunelling microscopy and spectroscopy (STMS) at 1.6 K and 50 mK. The advantage of the experimental method is that it allows a simultaneous visualisation of the topography of the [S-N] system and its local electronic properties with outstanding spatial and energy resolution. The proximity effect has previously been experimentaly studied for cases where the size of the [S-N] system could be considered to be infinite. In the diffusive limit, the local density of states shows a pseudo-gap in N which continuously vanishes as the probe is moved further from the SN interface. However, when the typical dimensions of N are smaller than the coherence length, a mini-gap related to the Thouless energy is predicted, implying the presence of superconduting correlations in the whole volume of N. In the ballistic regime, similar conclusions apply depending on the integrable (pseudo-gap) or chaotic (minigap) dynamics of the metallic structure in contact with the superconductor. To experimentaly observe such effects with the STM, we used electronic lithography to make samples where normal metal islands of mesoscopic sizes and different geometries are close to a superconductor. We sometimes observed sharp singularities in the local differential tunnel conductance spectra. We believe they are consequences of Andreev bound states, implying the existence of semi-classical electronic trajectories which satisfy the De Gennes St James quantification condition. In the scope of this thesis, we also caught the opportunity to study the electronic properties of thin disordered sperconducting films of titanium nitride. As their thickness is getting smaller, the reduction of electron effective attraction potential together with localization effects give rise to a superconductor insulator transition, observed by magneto-transport measurements. Thanks to STMS studies, spectroscopic measurements combined with topographic images revealed the co-existence of superconducting and normal metallic domains as well as an atypical evolution of the densiy of states at the SN boundary. Thus, this material shows inhomogeneous superconducting properties which are believed to play a crucial role in the S.I.T. mechanisms.
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https://tel.archives-ouvertes.fr/tel-00007327
Contributor : Walter Escoffier <>
Submitted on : Monday, November 8, 2004 - 3:33:05 PM
Last modification on : Wednesday, November 4, 2020 - 2:48:49 PM
Long-term archiving on: : Thursday, September 13, 2012 - 12:40:18 PM

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Walter Escoffier. Etude de la coherence quantique dans les systemes supraconducteur - metal normal par microscopie et spectroscopie a effet tunnel. Matière Condensée [cond-mat]. Université Joseph-Fourier - Grenoble I, 2004. Français. ⟨tel-00007327⟩

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