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International audience; We present low-temperature electronic transport properties of superconducting nanowires obtained by nanolithography of 4-nm-thick niobium nitride (NbN) films epitaxially grown on sapphire substrate. Below 6 K, clear evidence of phase slippages is observed in the transport measurements. Upon lowering the temperature, we observe the signatures of a crossover between a thermal and a quantum behavior in the phase slip regimes. We find that phase slips are stable even at the lowest temperatures and that no hotspot is formed. The photoresponse of these nanowires is measured as a function of the light irradiation wavelength and temperature and exhibits a behavior comparable with previous results obtained on thicker films.

In contact with a superconductor, the Andreev reflection of the electrons locally modifies the N metal electronic properties, including the local density of states (LDOS). We investigated the LDOS in superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60 mK) STM on the normal metal side. High resolution tunneling spectra measured on the Au surface show a clear proximity effect with an energy gap of reduced amplitude compared to the bulk Nb gap. The dependence of this mini-gap width with the normal metal thickness is discussed in terms of the Thouless energy. Within the mini-gap, the density of states does not reach zero and shows clear sub-gap features. We compare the experimental spectra with the well-established quasi-classical theory.

At the contact with a superconductor, the Andreev reflection of the electrons from a normal metal locally modifies the normal metal electronic properties, including the local density of states (LDOS). Depending on the sample geometry, a mini-gap or a pseudo-gap is expected. We performed the local spectroscopy of normal metal–superconductor bilayers with the help of a very low temperature (60 mK) scanning tunnelling microscope (STM). A non-zero density of states appears within the expected mini-gap for samples with a large normal metal length. The experimental spectra are compared with the well-established quasi-classical theory.

We report here two processes for fabricating quantum devices based on local anodization induced by Atomic Force Microscope (AFM). The first process involves ultra-thin films of doped silicon-on-insulator (SOI) passivated with hydrogen. AFM-drawn oxide lines create a chemical contrast that is used as a mask for silicon wet etch. Etching is performed down to the buried silicon oxide layer, thus leading to silicon nanowires supported on insulator. We show that this process appears well suited to obtain SOI nanostructures and investigate electrical transport though silicon wires with sub-1000 nm 2 cross-section. In the second process, diffusion of oxygen species is performed through the whole layer of metallic ultra-thin films, which provides a technique for direct writing of insulating regions. This technique was applied to ultra-thin niobium films for fabricating mesoscopic structures. A superconducting quantum interferometer useful for nanomagnetism is demonstrated.

We have investigated the vortex state in a superconducting dice network using the Bitter decoration technique at several magnetic frustrations f=1/2 and 1/3. In contrast to other regular network geometries where the existence of a commensurate state was previouly demonstrated, no ordered state was observed in the dice network at f=1/2 and the observed vortex-vortex correlation length is close to one lattice cell., Comment: 7 pages, 6 figures, submitted to Europhysics Letter

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.

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.

We have studied the charge-density-wave (CDW) current conversion process in NbSe3 wire structures of mesoscopic dimensions. A significant reduction of the phase-slip voltage associated with this conversion is observed if the spacing between current contacts is smaller than a few mm. This reduction cannot be explained with existing models of CDW current conversion. We suggest that single phase-slip events play a central role in micron-sized systems. The removal and addition of wave fronts may then become correlated in time.

The Andreev Reflection is the key mechanism for thesuperconducting proximity effect. It provides phasecorrelations in a system of non-interacting electrons atmesoscopic scales, i.e. over distances much larger than themicroscopic lengths: Fermi wavelength and elastic electronmean free path. This field of research has attracted anincreasing interest in the recent years in part because of thetremendous development of nanofabrication technologies, andalso because of the richness of the involved quantum effects.In this paper we review some recently achieved advances. Wealso discuss new open questions, in particular non-equilibriumeffects and proximity effect in systems with ferromagneticelements.

The transition line of superconducting arrays of holes exhibits a rich field structure due to the interference of superconducting states nucleated at the holes edges. We studied by means of resistance measurements their effect on the T*c(H) line as a function of transverse magnetic field using regular arrays of nanofabricated micron size holes. The arrays transition fields are higher than for the bulk. Moreover we found a nontrivial field modulation of the T*c(H) line with an inversion, with increasing field, of the modulation concavity which we assigned to a crossover from a collective to an isolated edge state regime. The high field regime is well described by the nucleation at a single hole in an infinite film. The modulation at low fields was found to be dominated by the interference of neighbor edge states when the inter-hole distance w becomes comparable to the coherence length ξ(T*c). A comparison between arrays of different hole shape shows the influence of geometry on the type of interaction established, which can described either as a superconducting wire network or as a weak link array.

We have fabricated wire structures with (sub)micron sizes in the charge-density wave conductor Nbse 3- Electrical transport measurements include complete mode-locking on Shapiro steps and show that the patterning has not affected the CDW material. Our mesoscopic wires show strong fluctuation and hysteresis effects in the low-temperature current-voltage characteristics, as well as a strong reduction of the phase-slip voltage. This reduction can not be explained with existing models. We suggest that single phase-slip events are responsible for a substantial reduction of the CDW strain in micron-sized systems.

We review the mesoscopic transport in a diffusive proximity superconductor made of a normal metal (N) in metallic contact with a superconductor (S). The Andreev reflection of electrons on the N-S interface is responsible for the diffusion of electron pairs in N. Superconducting-like properties are induced in the normal metal. In particular, the conductivity of the N metal is locally enhanced by the proximity effect. A re-entrance of the metallic conductance occurs when all the energies involved (e.g. temperature and voltage) are small. The relevant characteristic energy is the Thouless energy which is $\hbar$ divided by the diffusion time for an electron travelling throughout the sample. In loop-shaped devices, a 1/T temperature-dependent oscillation of the magnetoresistance arises with a large amplitude from the long-range coherence of low-energy pairs., Review paper, 13 pages with 5 included epsf figures, to appear in Superlattices and Microstructures, minor changes

In a mesoscopic metal in proximity with a superconductor, the electronic conductance is enhanced in a very energy-sensitive way. In this paper, we discuss the spectral conductance of a proximity superconductor from both the theoretical and experimental point of view. The dependence of the spectral conductance on the phase-breaking length, gap of the superconductor and interface transparency is theoretically investigated. We present experimental data on the reentrance of the normal-state conductance at very low temperature and bias voltage. A complete description of the experimental data needs taking into account heating of the reservoirs by the bias current. In addition, we show that the energy sensitivity of the proximity effect enables one to access the energy distribution of the conduction electrons inside a mesoscopic sample.

Surface electromagnetic modes of lamellar metallic gratings made of gold are analyzed both theoretically and experimentally in the 0.21.0 eV spectral range for p-polarized light. For deep enough grooves, we show how waveguide resonances that show no dispersion with parallel momentum appear in the photonic band structure of these surface plasmons. In these resonances the electric field is highly localized inside the grooves and is almost zero in all other regions. We also illustrate the existence of hybrid modes, combination of standing waves with propagating surface plasmon polaritons. Experimental evidence of the excitation of both kind of localized surface plasmons is given for lamellar gratings of period 3.5 /spl mu/m with grooves 0.5 /spl mu/m wide and 0.6 /spl mu/m deep.

We demonstrate for the first time that a periodic array of submicrometer holes (antidots) can be patterned into thin single NbSe3 crystals. We report on the study of Charge Density Wave (CDW) transport of the network of mesoscopic units between antidots. Size of the elementary unit can be as small as 0.5 μm along the chain axis and $$0.2\mu mX0.3\mu m$$ in cross-section. We observe size effects for Ohmic residual resistance and in CDW transport current-voltage characteristics in submicronic networks.

We consider the chi_0 dependence of the density of states inside the normal metal of a superconductor - normal metal - superconductor (SNS) junction.Here chi_0 is 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 chi_0 increases and closes at chi_0= pi. Both the analytical expressions for the chi_0 dependence of the density of states and the results of numerical simulations are presented.

We review transport measurements in a normal metal (N) in contact with one or two superconducting (S) islands. From the experiment, we distinguish the Josephson coupling, the mesoscopic fluctuations and the proximity effect. In a loop-shaped N conductor, we observe large h/2e-periodic magnetoresistance oscillations that decay with temperature T with a 1/T power-law. This behaviour is the signature of the long-range coherence of the low-energy electron pairs induced by the Andreev reflection at the S interface. At temperature and voltage below the Thouless energy $\hbar D / L^2$, we observe the re-entrance of the metallic resistance. Experimental results agree with the linearized quasiclassical theory., Comment: 8 pages, 6 included epsf figures, Invited paper at the LT21 Conference, Praha, August 1996. To appear in Czech. J. of Phys. 46, Part S6 (1996)

The magnetostatic effect of a ferromagnetic particle array on an underlying two‐dimensional superconducting Nb thin film was investigated in terms of flux penetration processes by means of high sensitive magnetometry. For bare Nb film, a large drop in magnetization due to a flux rearrangement appeared in the low field region below 2.5 mT. However, once magnetic particles were fabricated on the surface of the Nb film, the drop was not observed. In this configuration, the ferromagnetic particles seem to behave as flux guides for the external magnetic field, whereby the flux distribution inside the superconductor is well regulated. Furthermore, a periodical oscillation was observed in the hysteresis loops. This behavior is associated with a flux quantization in the regions where the superconductivity is suppressed by the stray field of the ferromagnetic particles. The local flux density distribution around the ferromagnetic particles was evaluated by using the London equation, and the magnetization curve of the Nb film with periodically modulated superconducting order parameter was calculated.

The critical field of a thin superconducting film with a blind circular hole is found theoretically. It is shown that the value of the critical field is sensitive to the bottom thickness, but the orbital momentum, i.e., the number of vortices which nucleate inside the hole, is not sensitive. A simple boundary condition for a steplike thin film is derived and used for comparative numerical analysis of the superconductivity nucleation in a microdisk and near the hole. By increasing the thickness of the bottom of a blind hole one can transform the hole into a disk of the same radius which rests on top of the film. We show that such transformation leads to a jump in the number of vortices which nucleate at the critical magnetic field inside the perimeter of the hole (the disk). We report also the results of the Bitter decoration experiments of a thin superconducting film with a lattice of open or blind holes. It is found (in accordance with the calculation) that the bottom thickness has only a weak influence on the number of vortices captured by a hole during the cooling of the sample at a constant perpendicular magnetic field. All the experimental results are explained under the assumption that the vortices nucleated inside a hole rest inside during the cooling process and no additional vortices enter the hole. \textcopyright{} 1996 The American Physical Society.

Single‐electron devices are now very promising for high‐accuracy and high‐sensitivity electrical measurements, based on the quantification of the macroscopic charge in metallic nanostructures. The ultimate performance of these devices can only be attained by a careful filtering of the measurement circuit at very low temperature (20 mK). A new kind of wide‐band high‐frequency filter integrated on a copper block has been designed and studied. This original substrate provides a massive and well‐thermalized electric ground plane in the vicinity of the filter. Attenuation measurements show that one has to carefully design the filter box in order to avoid the transmission of electromagnetic modes through the filter box. An improved filter showed −95 dB attenuation at 20 GHz, without any increase of the transmission at high frequency.

We have studied the diffusion of excess quasiparticles in a current-biased superconductor strip in proximity to a metallic trap junction. In particular, we have measured accurately the superconductor temperature at a near-gap injection voltage. By analyzing our data quantitatively, we provide a full description of the spatial distribution of excess quasiparticles in the superconductor. We show that a metallic trap junction contributes significantly to the evacuation of excess quasiparticles., Comment: 4 pages, 4 figures

The Hall effect associated with non-equilibrium magnetization was investigated using multiple Al Hall bars connected to a Co/Al ohmic wire junction. A significant change in the Hall voltage due to the spin injection was clearly detected below 100 K when the Hall bars are placed within the distance of 10 μm from the interface. Experimentally determined extraordinary Hall coefficient seems to reflect a spatial distribution of non-equilibrium magnetization induced by spin-injection.

We report on a combined structural and electronic analysis of niobium ultrathin films (from 2 to 10 nm) deposited in ultra-high vacuum on atomically flat R-plane sapphire wafers. A textured polycrystalline morphology is observed for the thinnest films showing that hetero-epitaxy is not achieved under a thickness of 3.3nm, which almost coincides with the first measurement of a superconducting state. The superconducting critical temperature rise takes place on a very narrow thickness range, of the order of a single monolayer (ML). The thinnest superconducting sample (3 nm/9ML) has an offset critical temperature above 4.2K and can be processed by standard nanofabrication techniques to generate air- and time-stable superconducting nanostructures, useful for quantum devices., Comment: 22 pages, 8 figures, to appear in Physical Review B (2011)

We report on a combined structural and electronic analysis of niobium ultrathin films (from 2.5 to 10 nm) epitaxially grown in ultra-high vacuum on atomically flat sapphire wafers. We demonstrate a structural transition in the early stages of Nb growth, which coincides with the onset of a superconducting-metallic transition (SMT). The SMT takes place on a very narrow thickness range (1 ML). The thinnest superconducting sample (3 nm/ 9ML) has an offset critical temperature above 4.2K and allows to be processed by standard nanofabrication techniques to generate air and time stable superconducting nanostructures.

Two-dimensional arrays of micrometer-scale thin square slabs of amorphous Sm 0.2 Co 0.8 were prepared by means of e-beam lithography and dc sputtering. When a laser beam is Bragg-diffracted by this structure, under transverse magneto-optical-Kerr effect geometry, the intensities of the various diffracted beams have very different variations as a function of applied magnetic field. The diffraction hysteresis loops provided by the various Bragg-diffracted beams yield information about the average domain structure of the individual particles.

Two-dimensional arrays of amorphous R-Co (R = Sm and Gd) square particles were prepared on 200Athick niobium films. Demagnetizing processes studied as a function of spatial periodicity are very different from those of uniform films with no pattern. Calculations show that the inter-particle dipolar interactions are negligibly small compared to self dipolar interactions. Interesting periodic oscillations were observed in the magnetoresistance measured around the superconducting transition temperature of the niobium film ( T c =7.1 K).

The authors studied the resistive transition of several 2-D superconducting-wire networks of various coupling strengths, which they characterize in terms of the Kosterlitz-Thouless transition temperature and the ratio {xi}/a of the coherence length to the array period. In the extreme strong-coupling limit where the mesh size is of the order of the zero-temperature coherence length, the superconducting behavior is well described by the mean-field properties of the superconducting wave function. Extending to 2-D array, the 1-D phase-slippage model explains the dissipative regime observed above the Ginzburg-Landau depairing critical current. On the other hand, when the coupling is weak, phase fluctuations below the Ginzburg-Landau transition and vortex depinning dominate the resistive behavior. An activated dissipation is observed even below the depairing critical current. Results obtained in this regime for critical temperature, magnetoresistance, or critical current versus temperature, and magnetic field are shown; their periodic oscillations are discussed in terms of depinning of vortices on the array. A simple periodic pinning potential for a vortex in a wire network is calculated, and compared with the case of pinning in Josephson junction arrays. It is shown that this model explains qualitatively the experimental results observed for small {xi}/a.

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., 4 pages, 3 figures

We investigate experimentally the physics of quantum phase slips in one-dimensional Josephson Junction chains. These quantum phase-slips are induced by quantum phase fluctuations occurring on single junctions of the chain. In our experiment we can tune the strength of these fluctuations as each chain junction is realized in form of a SQUID leading to tunable Josephson coupling. We determine the ground state of the chain via switching current measurements of the chain shunted by a large Josephson junction. Our results can be well fitted with a tight binding Hamiltonian taking into account quantum phase-slips., 5 pages, 5 figures

International audience; We have measured the transport properties of Ferromagnet - Superconductor nanostructures, where two superconducting aluminum (Al) electrodes are connected through two ferromagnetic iron (Fe) ellipsoids in parallel. We find that, below the superconducting critical temperature of Al, the resistance depends on the relative alignment of the ferromagnets' magnetization. This spin-valve effect is analyzed in terms of spin accumulation in the superconducting electrode submitted to inverse proximity effect.

We present low temperature transport measurements in one dimensional Josephson junctions rhombi chains. We have measured the current phase relation of a chain of 8 rhombi. The junctions are either in the classical phase regime with the Josephson energy much larger than the charging energy, $E_{J}\gg E_{C}$, or in the quantum phase regime where $E_{J}/E_{C}\approx 2$. In the strong Josephson coupling regime ($E_{J}\gg E_{C} \gg k_{B}T$) we observe a sawtooth-like supercurrent as a function of the phase difference over the chain. The period of the supercurrent oscillations changes abruptly from one flux quantum $\Phi_{0}$ to half the flux quantum $\Phi_{0}/2$ as the rhombi are tuned in the vicinity of full frustration. The main observed features can be understood from the complex energy ground state of the chain. For $E_{J}/E_{C}\approx 2$ we do observe a dramatic suppression and rounding of the switching current dependence which we found to be consistent with the model developed by Matveev et al.(Phys. Rev. Lett. {\bf 89}, 096802(2002)) for long Josephson junctions chains., Comment: to appear in Phys. Rev. B

International audience; We have studied hybrid superconducting micro-coolers 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.

International audience; We discuss very low temperature experiments on superconducting micro-coolers made of a double Normal metal - Insulator - Superconductor junction. We investigate with a high resolution the differential conductance of the micro-cooler as well as of additional probe junctions. There is an explicit crossover between the single quasi-particle current and the phase-coherent Andreev current. We establish a thermal model by considering the thermal contribution due to the Andreev current. The related increase of the electron temperature is discussed, including the influence of several parameters like the phase-coherence length or the tunnel junction transparency.

International audience; We discuss inherent thermometry in a Superconductor - Normal metal - Superconductor tunnel junction. In this configuration, the energy selectivity of single-particle tunneling can provide a significant electron cooling, depending on the bias voltage. The usual approach for measuring the electron temperature consists in using an additional pair of superconducting tunnel junctions as probes. In this paper, we discuss our experiment performed on a different design with no such thermometer. The quasi-equilibrium in the central metallic island is discussed in terms of a kinetic equation including injection and relaxation terms. We determine the electron temperature by comparing the micro-cooler experimental current-voltage characteristic with isothermal theoretical predictions. The limits of validity of this approach, due to the junctions asymmetry, the Andreev reflection or the presence of sub-gap states are discussed.

We report the first detailed measurements on the critical current of periodic superconducting wire networks as a function of an external magnetic field. Sharp spikes are observed at integer and fractional reduced magnetic fluxes /0, where is the magnetic flux per unit cell of the network. The experimental results are discussed in terms of the depairing current for the 2D array. An approach based upon the band properties of the paired Electrons on the underlying lattice is proposed. This model explains the main features of the field and temperature dependence of the critical current without referring to pinning effects of the vortex lattice.

Superconductivity offers an unique example in condensed matter physics where a quantum state is occupied coherently over macroscopic distances. We review some recent studies on artificial 2D superconducting microstructures including periodic and nonperiodic submicron wire networks and small superconducting dots. By measuring the phase boundary between the superconducting and normal state, the equilibrium magnetization, the magnetoresistance or the critical current, one can trace back to the spectral properties of a quantum particle on the underlying 2D lattice. Through a few examples of such superconducting model systems we illustrate some remarkable properties of Landau levels in periodic systems, as well as quantization effects in singly connected dots.

The authors present a fabrication technique of superconducting single photon detectors made by local oxidation of niobium nitride ultrathin films. Narrow superconducting meander lines are obtained by direct writing of insulating niobium oxynitride lines through the films using voltage-biased tip of an atomic force microscope. Due to the 30nm resolution of the lithographic technique, the filling factor of the meander line can be made substantially higher than detector of similar geometry made by electron beam lithography, thus leading to increased quantum efficiency. Single photon detection regime of these devices is demonstrated at 4.2K.

We present evidence for the cooling of normal metal phonons by electron tunneling in a Superconductor - Normal metal - Superconductor tunnel junction. The normal metal electron temperature is extracted by comparing the device current-voltage characteristics to the theoretical prediction. We use a quantitative model for the phonon cooling that includes the electron-phonon coupling in the normal metal and the Kapitza resistance between the substrate and the metal. It gives an excellent fit to the data and enables us to extract an effective phonon temperature in the normal metal., 4 pages, 5 figures

Current noise measurements enligthen the transport mechanisms in mesoscopic samples in a complementary way to conductance measurements. The noise gives directly access to the charge of the current carriers and is modified by electronic interactions. This is very instructive in the context of hybrid superconductor ‐ normal metal (S‐N) structures where charge pairs generated by Andreev reflections compete with quasiparticles in the current transport. In S‐N‐S junctions, the shot noise is very much enhanced compared to the normal case because of the confinement of normal quasiparticles between the superconducting electrodes. The relevant time depends on the number of Andreev reflections (NAR) necessary for quasiparticles to reach the superconducting gap Δ and escape the normal metal. It can therefore be tuned by the bias voltage because NAR goes like 1/eV. Diffusive junctions of intermediate size (ξΔ < L < LΦ) are particularly interesting because various transport regimes can then be studied from coherent (w...

We experimentally investigated the current noise in diffusive superconductor-normal-metal-superconductor junctions with lengths between the superconducting coherence length ${\ensuremath{\xi}}_{\ensuremath{\Delta}}$ and the phase coherence length ${L}_{\ensuremath{\Phi}}$ of the normal metal $({\ensuremath{\xi}}_{\ensuremath{\Delta}}lLl{L}_{\ensuremath{\Phi}})$. 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. Semiclassical 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.

We investigated the local electronic density of states in superconductor-normal metal (Nb-Au) bilayers using a very low temperature (60 mK) STM. High resolution tunneling spectra measured on the normal metal (Au) surface show a clear proximity effect with an energy gap of reduced amplitude compared to the bulk superconductor (Nb) gap. Within this mini-gap, the density of states does not reach zero and shows clear sub-gap features. We show that the experimental spectra cannot be described with the well-established Usadel equations from the quasi-classical theory., 4 pages, 5 figures

We present electrical transport experiments performed on submicron hybrid devices made of a ferromagnetic conductor (Co) and a superconducting (Al) electrode. The sample was patterned in order to separate the contributions of the Co conductor and of the Co-Al interface. We observed a strong influence of the Al electrode superconductivity on the resistance of the Co conductor. This effect is large only when the interface is highly transparent. We characterized the dependence of the observed resistance decrease on temperature, bias current and magnetic field. As the differential resistance of the ferromagnet exhibits a non-trivial asymmetry, we claim that the magnetic domain structure plays an important role in the electron transport properties of superconducting / ferromagnetic conductors., Comment: 7 pages, 6 figures, final version

We have studied the current voltage characteristics of diffusive mesoscopic Nb-Cu-Nb Josephson junctions with highly-transparent Nb-Cu interfaces. We consider the low-voltage and high-temperature regime eV, Comment: 4 pages, 4 figures, final version

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., 7 pages, 3 figures

We present a method for fabricating Josephson junctions and superconducting quantum interference devices (SQUIDs) which is based on the local anodization of niobium strip lines 3 to 6.5 nm-thick under the voltage-biased tip of an Atomic Force Microscope. Microbridge junctions and SQUID loops are obtained either by partial or total oxidation of the niobium layer. Two types of weak link geometries are fabricated : lateral constriction (Dayem bridges) and variable thickness bridges. SQUIDs based on both geometries show a modulation of the maximum Josephson current with a magnetic flux periodic with respect to the superconducting flux quantum h/2e. They persist up to 4K. The modulation shape and depth for SQUIDs based on variable thickness bridges indicate that the weak link size becomes comparable to the superconducting film coherence length which is of the order of 10nm., Comment: 12 pages

We present a fabrication method of superconducting quantum interference devices (SQUIDs) based on direct write lithography with an Atomic Force Microscope (AFM). This technique involves maskless local anodization of Nb or NbN ultrathin films using the voltage biased tip of the AFM. The SQUIDs are of weak-link type, for which two geometries have been tested: Dayem and variable thickness nanobridges. The magnetic field dependence of the maximum supercurrent Ic(flux) in resulting SQUIDs is thoroughly measured for different weak link geometries and for both tested materials. It is found that the modulation shape and depth of Ic(flux) curves are greatly dependent on the weak link size. We analyze the results taking into account the kinetic inductance of nanobridges and using the Likharev-Yakobson model. Finally we show that the present resolution reached by this technique (20nm) enables us to fabricate Nb weak-links which behavior approaches those of ideal Josephson junctions., Comment: 22 pages, 8 figures

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 $eR_{N}I_{c}/\epsilon_{c}$ and $k_{B}T/\epsilon_{c}$ where $\epsilon_{c}$ is the Thouless energy. Experimental and theoretical results are in excellent quantitative agreement., Comment: 5 pages, 4 figures, slighly modified version, published