B. Pannetier

Josephson critical current in a long mesoscopic S-N-S junction

We carry out an extensive experimental and theoretical study of the Josephson effect in S-N-S junctions made of a diffusive normal metal ~N! embedded between two superconducting electrodes ~S!. Our experiments are performed on Nb-Cu-Nb junctions with highly transparent interfaces. We give the predictions of the quasiclassical theory in various regimes on a precise and quantitative level. We describe the crossover between the short- and the long-junction regimes and provide the temperature dependence of the critical current using dimensionless units eRNI c /e c and k BT/e c , where e c is the Thouless energy. Experimental and theoretical results are in excellent quantitative agreement.

Density of States in Superconductor-Normal Metal-Superconductor Junctions

We consider the χ0-dependence of the density of states inside the normal metal of a superconductor-normal metal-superconductor (SNS) junction. Here χ0is the phase difference of two superconductors of the junction. It is shown that in the absence of electron-electron interaction the energy dependence of the density of states has a gap which decreases as χ0increases and closes at χ0= π. Both the analytical expressions for the χ0-dependence of the density of states and the results of numerical simulations are presented.

Superconducting proximity effect in a mesoscopic ferromagnetic wire

We present an experimental study of the transport properties of a ferromagnetic metallic wire (Co) in metallic contact with a superconductor (Al). As the temperature is decreased below the Al superconducting transition, the Co resistance exhibits a significant dependence on both temperature and voltage. The differential resistance data show that the decay length for the proximity effect is much larger than we would simply expect from the exchange field of the ferromagnet.

Magnetization oscillations of a superconducting disk

We present magnetic measurements on micron-sized superconducting disks. The phase transition line Tc(H) and magnetization exhibit spectacular undamped oscillations as function of magnetic field. These results reflect the contribution of the edge states on the energy spectrum of electron pairs in the disk. We discuss their sensitivity to boundary conditions.

Reentrance of the Metallic Conductance in a Mesoscopic Proximity Superconductor

We present an experimental study of the diffusive transport in a normal metal near a superconducting interface, showing the reentrance of the metallic conductance at very low temperature. This new mesoscopic regime comes in when the thermal coherence length of the electron pairs exceeds the sample size. This reentrance is suppressed by a bias voltage given by the Thouless energy and can be strongly enhanced by an Aharonov-Bohm flux. Experimental results are well described by the linearized quasiclassical theory. {copyright} {ital 1996 The American Physical Society.}

A very low temperature scanning tunneling microscope 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. We achieved an efficient thermalization of the sample while maintaining a clean surface for STM imaging. Our spectroscopic data show unprecedented energy resolution. We present current–voltage characteristics and the deduced local density of states of hybrid superconductor–normal metal systems. This work is the first experimental realization of a local spectroscopy of mesoscopic structures at very low temperature.

Andreev Current-Induced Dissipation in a Hybrid Superconducting Tunnel Junction

We have studied hybrid superconducting microcoolers made of a double superconductor-insulator-normal metal tunnel junction. Under subgap conditions, the Andreev current is found to dominate the single-particle tunnel current. We show that the Andreev current introduces additional dissipation in the normal metal equivalent to Joule heating. By analyzing quantitatively the heat balance in the system, we provide a full description of the evolution of the electronic temperature with the voltage. The dissipation induced by the Andreev current is found to dominate the quasiparticle tunneling-based cooling over a large bias range.

Magnetic Field Induced Localization in a Two-Dimensional Superconducting Wire Network

We report transport measurements on superconducting wire networks which provide the first experimental evidence of a new localization phenomenon induced by magnetic field on a 2D periodic structure. In the case of a superconducting wave function this phenomenon manifests itself as a depression of the network critical current and of the superconducting transition temperature at a half magnetic flux quantum per tile. In addition, the strong broadening of the resistive transition observed at this field is consistent with enhanced phase fluctuations due to this localization mechanism.

Local spectroscopy of a proximity superconductor at very low temperature

We performed the local spectroscopy of a Normal-metal–Superconductor (N-S) junction with the help of a very low-temperature (60 mK) Scanning Tunneling Microscope (STM). The spatial dependence of the local density of states was probed locally in the vicinity of the N-S interface. We observed spectra with a fully developed gap in the regions where a thin normal-metal layer caps the superconductor dot. Close to the S metal edge, a clear pseudo-gap shows up, which is characteristic of the superconducting proximity effect in the case of a long normal metal. The experimental results are compared to the predictions of the quasiclassical theory.

Measurement of the effect of quantum phase slips in a Josephson junction chain

Coulomb interactions can cause a rapid change in the phase of the wavefunction along a very narrow superconducting system. Such a phase slip is now measured in a chain of Josephson junctions.