Mesoscopic transition in the shot noise of diffusive S/N/S junctions

We experimentally investigated the current noise in diffusive Superconductor/Normal metal/Superconductor junctions with lengths between the superconducting coherence length xi_Delta and the phase coherence length L_Phi of the normal metal (xi_Delta < L < L_Phi). We measured the shot noise over a large range of energy covering both the regimes of coherent and incoherent multiple Andreev reflections. The transition between these two regimes occurs at the Thouless energy where a pronounced minimum in the current noise density is observed. Above the Thouless energy, in the regime of incoherent multiple Andreev reflections, the noise is strongly enhanced compared to a normal junction and grows linearly with the bias voltage. Semi-classical theory describes the experimental results accurately, when taking into account the voltage dependence of the resistance which reflects the proximity effect. Below the Thouless energy, the shot noise diverges with decreasing voltage which may indicate the coherent transfer of multiple charges.Comment: 5 pages, 5 figures, accepted for publication in Phys. Rev. B, Rapid Communicatio

Localization Effect in a 2D Superconducting Network without Disorder

The superconducting properties of a two-dimensional superconducting wire network with a new geometry have been measured as a function of the external magnetic field. The extreme localization effect recently predicted for this periodic lattice is revealed as a suppression of the critical current when the applied magnetic field corresponds to half a flux quantum per unit cell. For this particular magnetic field, the observed vortex state configuration is highly disordered.Comment: 6 pages, 2 eps figures, submitted to Physica C. Title change

A very low temperature STM for the local spectroscopy of mesoscopic structures

We present the design and operation of a very-low temperature Scanning Tunneling Microscope (STM) working at $60 mK$ in a dilution refrigerator. The STM features both atomic resolution and micron-sized scanning range at low temperature. This work is the first experimental realization of a local spectroscopy of mesoscopic structures at very low temperature. We present high-resolution current-voltage characteristics of tunnel contacts and the deduced local density of states of hybrid Superconductor-Normal metal systems.Comment: 5 pages, 5 figures, slightly corrected versio

Voltage-induced Shapiro steps in a superconducting multi-terminal structure

When a superconducting tunnel junction at a finite voltage is irradiated with microwaves, the interplay between the alternating Josephson current and the ac field gives rise to steps in the dc current known as Shapiro steps. In this work we predict that in a mesoscopic structure connected to several superconducting terminals one can induce Shapiro-like steps in the absence of any external radiation simply by tuning the voltages of the leads. To illustrate this effect we make quantitative predictions for a three-terminal structure which comprises a diffusive superconductor-normal metal-superconductor junction and a tunneling probe, a set-up which can be realized experimentally.Comment: revtex4, 5 pages, 5 figures, to appear in Phys. Rev.

Embryogenèse somatique du cocotier (Cocos nucifera L.) à partir d'explants foliaires : étude histologique

Une étude histologique a été réalisée aux différentes étapes de l'embryogenèse somatique du cocotier (Cocos nucifera L.) à partir d'explants foliaires. Elle a permis de localiser les premières divisions cellulaires au niveau des cellules périvasculaires des explants. La croissance des cals nodulaires obtenus est assurée par la mise en place d'une assise méristématique à fonctionnement de type cambial. La nature racinaire de certaines formations a été observée. Des structures embryogènes peuvent apparaître sur les cals nodulaires; entièrement méristématiques au départ, elles s'épidermisent et se multiplient pour la plupart. Leur évolution peut conduire à des structures de type embryon après fragmentation et contagion mitotique au niveau de l'assise de type cambial. Ce schéma général d'obtention de structures embryonnaires peut cependant varier : (i) soit il y a obtention rapide d'embryons à partir des cals nodulaires primaires. Cette voie a permis la régénération, par embryogenèse secondaire, d'un massif d'embryons (bourgeonnement au pôle racinaire). L'un d'entre eux a développé une pousse feuillée et des racines. La plante régénérée a été transférée en champ. (ii) Soit il y a formation de cals granuleux secondaires et production de façon plus systématique de structures embryogènes. Ces derniers devraient permettre une meilleure maîtrise du développement des structures embryonnaire

Quasiparticle diffusion based heating in superconductor tunneling micro-coolers

In a hybrid Superconductor - Insulator - Normal metal tunnel junction biased just below the gap, the extraction of hot electrons out of the normal metal results in electronic cooling effect. The quasiparticles injected in the superconductor accumulate near the tunnel interface, thus increasing the effective superconductor temperature. We propose a simple model for the diffusion of excess quasiparticles in a superconducting strip with an additional trap junction. This diffusion model has a complete analytic solution, which depends on experimentally accessible parameters. We find that the accumulated quasiparticles near the junction reduce the efficiency of the device. This study is also relevant to more general situations making use of superconducting tunnel junctions, as low temperature detectors.Comment: 4 pages, 3 figure