Partners and International Organizations
(English)
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University of Göteborg, University of Catania, University of Delft, CNRS/CRTBT Grenoble, University of Karlsruhe, University of Neuchâtel
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Abstract
(English)
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In this project we investigate disorder, pinning, frustration, and localization phenomena (mostly related to phase transitions, both classical and quantum) occurring in micro- and nanofabriacted two-dimensional (2D) arrays of Josephson junctions. Allowing an accurate control and a detailed knowledge of their microscopic parameters, these systems provide ideal testing grounds to explore fundamental concepts of statistical mechanics and condensed matter physics. In the period covered by the present report we addressed issues related to (i) the magnetic field dependence of the energy barrier opposing vortex motion in frustrated arrays with strong percolative disorder, (ii) the origin of the low-temperature 1/f flux noise in regular arrays, (iii) the magnetic-field-induced localization effect in arrays with a dice lattice structure, and (iv) the superconductor-insulator (SI) transition of arrays in the quantum regime. In good agreement with theoretical predictions in (i) we found that the vortex energy barrier decreases with increasing frustration and reaches a minimum at full frustration. Concerning (ii), the observed 1/f frequency dependence of the low-temperature flux noise spectra strongly contrasts with predictions of the Kosterlitz-Thouless theory for pure 2D systems. We found that this anomalous behavior, reminiscent of the glass-like dynamics of disordered systems, can be unambiguosly attributed to 'hidden' disorder in the coupling energies of the junctions. This conjecture is strongly supported by the good quantitative agreement between the measured spectra and the predictions of a simple model, in which single vortices are assumed to be fluctuating between neighboring pairs of metastable states in the random potential described by a uniform distribution of coupling energies. Unlike truly disordered systems, in the arrays studied in (iii) localization phenomena arise from the subtle interplay between the dice (or T3) lattice (a periodic structure with hexagonal symmetry consisting of 3- fold and 6-fold coordinated sites) and the magnetic field. Experimentally, this novel localization phenomenon became clearly manifest in the strong depression of the superfluid density (and the related peak in dissipation) we observed, at full frustration, in ac magnetoconductance measurements. The investigation of this very interesting effect is now being pursued withtin the framework of a PhD thesis. The study of the quantum SI transition at mK temperatures of the all-aluminum arrays of submicron tunnel junctions provided by the Göteborg group required the solution of complex experimental problems. Because of the extremely weak diamagnetic response of these systems, an entirely new set of microcoils was designed and fabricated to efficiently suppress flux noise in the conductance measurements, whereas hysteresis effects inherent to the geometry of the samples were eliminated. The new coil-sample configuration was successfully tested in experiments probing the quantum critical behavior of the arrays at zero frustration. The very good collaboration with the Göteborg group will continue to explore the field-tuned SI transition.
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