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Transitions de phase quantiques dans les systèmes désordonnés de basse dimension

Abstract : Superconductivity is established by the collective organization of electrons, then described within the BCS theory through a single macroscopic wave function. In the presence of strong disorder, the situation becomes more complex: disorder enhances Coulomb interactions and induces the localization of electrons. These two phenomena act against superconductivity. For a critical disorder, the superconducting state is destroyed and the system becomes either metallic or insulating. In 2D, in the absence of strong Coulomb interactions, the theory of Anderson localization prevents the existence of a metallic state. Disorder thus induces a Superconductor-Insulator Transition (SIT). We have studied the transport properties of amorphous NbxSi1-x thin films at very low temperature. We have performed resistance measurements at low frequencies through the SIT and initiated measurements of the complex impedance at higher frequencies (a few GHz) in order to probe the dynamics of the system through this quantum phase transition. The study of the static properties of NbxSi1-x films have focused on the effect of annealing. This parameter induces a gradual variation of the microscopic disorder of this system, which allowed us a very fine tuning of the SIT. We have thus evidenced two dissipative states, non-predicted by the current theories of the SIT, which separate the superconducting and insulating ground states. In parallel, we have set up a broadband microwave reflectometry experiment. In particular, we have developed a calibration procedure based on hypotheses on the electrodynamic response of the samples and not on the measure of external references as it is usual. This method allows us to measure the sample’s response independently from the experimental setup. The results we have obtained provide a first validation of this approach and therefore constitute a first step towards the determination of the absolute dynamical response of the system through the SIT.
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Submitted on : Wednesday, May 14, 2014 - 10:22:10 AM
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François Couëdo. Transitions de phase quantiques dans les systèmes désordonnés de basse dimension. Autre [cond-mat.other]. Université Paris Sud - Paris XI, 2014. Français. ⟨NNT : 2014PA112065⟩. ⟨tel-00990782⟩

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