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Dissipation par effet Joule en régime hyperfréquence dans les supraconducteurs à haute température critique

Abstract : This work treats a subject of interest to both fundamental and applied physics. The strategy of the work is to use high temperature superconducting materials (HTSC), destined for applications in microwave telecommunications devices, as well as the device prototypes themselves, as samples in fundamental studies on microwave power dissipation in superconductors. In this manner, we could progress in the understanding of the nonlinear response of superconductors in that part of the electromagnetic spectrum relevant for telecommunications, and, at the same time, develop strategies for the improvement of the devices. In effect, the energy dissipation in superconductors leads to the degradation of the device quality factor (when one is dealing with filters or resonators), and the severe limitation of the number of telecommunications channels that can benefit from a given bandwidth. The degradation of the qualtity factor of YBa2Cu3O7 thin film superconducting filters when these are exposed to the high intensity electromagnetic fields used in telecommunications (GDM, UMTS,...) is due to the increase, with microwave power, of the surface resistance, a phenomenon for which a great many explanations have been proposed : excitation of quasi-particles by the rf field, hysteretic losses due to the motiuon of quantized magnetic flux vortices, the penetration of the rf magnetic field through grain boundaries in the film, and, last but not least, local heating of the film. Using the measurement of the resonant frequency of the same TiO2 tutile resonator placed on top of the YBa2Cu3O7 film under study, and thereby, that of the dielectric constant of the the resonator, it is shown that in the frequency range around 10 GHz, it is the local heating of the superconducting film by the time-varying electromagnetic field, and the ensuing temperature rise, that is responsible for all the observed nonlinearities. No intrinsic nonilnear response of the superconducting film therefore comes into play. Joule heating, together with the correct appraisal of the hat link to the bath, allows one to account for all the nonlinear phenomena. This result opens the path foir new strategies for the improvement of HTSC thin film-based devices. Where linear losses are due to nanometer-scale oscillations of flux lines present due to Earth's magnetic field, the inclusion of nanometric holes or second phase precipitates will have a benificial effect. In all other cases, magnetic screening of the device imposes itself. If the losses are due to the non-conventionnal nature of the superconducting order parameter and quasi-particle excitation close to the nodal directions of the superconducting gap, one is faced with a more fundamental problem. An extension of this study is concerned with superconducting planar resonators. The characterization of a series of YBa2Cu3O7 resonators with eigenfrequencies between 500 MHz and 12 GHz shows that in this case, too, the device performance was limited by heating of the superconductin g thin film. In order to demonstrate this effect, a technique is developped where a dielectric resonator ( rutile phase TiO2 ) is placed on top of the YBa2Cu3O7 reosnator in order to measure the latter's temperature. The YBa2Cu3O7 and TiO2 resonance frequencies are measured using two distinct network analyzers, operating in widely different frequency ranges. By modelling, as well as by the application of the magneto-optical imaging technique for the visualisation of magnetic flux penetration in the superconducting resonator, it is shown that heating is most pronounced close to the acute corners of the device, where the flux dnesity as well as the rf current density are exacerbated. A second version of the YBa2Cu3O7 resonator device, in which all acute corners are replaced with smooth rounded corners has allowed for a tenfold improvement of the power range in which the reonator can be applied. This improvement has allowed the device to be included in an application for the French Directorate General for Armement; a patent is to be deposited.
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Julien Kermorvant. Dissipation par effet Joule en régime hyperfréquence dans les supraconducteurs à haute température critique. Physique [physics]. Ecole Polytechnique X, 2010. Français. ⟨tel-00604160⟩

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