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Applying current in type-II superconducting sub-micron size structures often leads to a discontinuity of the current-voltage characteristic as the instability current is reached. Also voltage-bias mode allows to avoid such an abrupt switch to the normal state, and gives a relevant outlook of the involved resistive phenomenons. Current and voltage-bias measurements on NbN microbridges with a constriction are reported here as a function of temperature and transverse magnetic field. Starting from a 75nm-thick sputtered film, 160nm-wide microstrips were patterned with an half-wide section constriction in their middle. From the current-bias curves it appears that the discontinuities are due to a flux flow instability that rapidly evolves in a hot-spot. Moreover applying the voltage reveals several dissipative states that leads to successive S-shaped current oscillations in a small current range. Formation of vortex channels can be considered to explain these features.

We performed a local spectroscopy of the Landau levels density of states using gated mesoscopic Hall bars placed at very low temperature in the integer quantum Hall regime. The transverse and longitudinal conductances were measured while scanning both the two-dimensional electron density and the applied magnetic field. We observe a succession of sharp peaks due to backscattering across the samples caused by tunneling effects. Using temperature as a parameter in the range of 0.1--1 K, we characterize those tunnel processes: a resonant double-barrier tunneling and a single-barrier tunneling which corresponds to the variable range hopping regime. We show that for vanishing temperature and noninteger filling factor $\ensuremath{\nu}$ the conductance $\ensuremath{\sigma}(T=0,\ensuremath{\nu})$ does not vanish unlike the case of wide samples: instead, it converges to a limit function ${\ensuremath{\sigma}}_{S}(\ensuremath{\nu})$ that is a noisy image of the Landau levels density of states.

International audience; We study a Fabry-Perot cavity formed from a ridge waveguide on a AlGaAs substrate. We experimentally determined the propagation losses in the waveguide at 780 nm, the wavelength of Rb atoms. We have also made a numerical and analytical estimate of the losses induced by the presence of the gap which would allow the interaction of cold atoms with the cavity field. We found that the intrinsic finesse of the gapped cavity can be on the order of F ~ 30, which, when one takes into account the losses due to mirror transmission, corresponds to a cooperativity parameter for our system C ~ 1.

We present an experiment where we tune the decoherence in a quantum interferometer using one of the simplest objects available in the physics of quantum conductors: an Ohmic contact. For that purpose, we designed an electronic Mach-Zehnder interferometer which has one of its two arms connected to an Ohmic contact through a quantum point contact. At low temperature, we observe quantum interference patterns with a visibility up to 57%. Increasing the connection between one arm of the interferometer to the floating Ohmic contact, the voltage probe, reduces quantum interference as it probes the electron trajectory. This unique experimental realization of a voltage probe works as a trivial which-path detector whose efficiency can be simply tuned by a gate voltage.

Quantum wires were fabricated by selective intermixing of a GaAs/GaAlAs quantum well through masked Ga+ implantation and rapid thermal annealing. The evolution of the luminescence spectra of the wires with the width of the implantation masks, enabled us to characterize the lateral selectivity of our process as well as the degree of one‐dimensional confinement. The lateral extent of the intermixing was estimated at 20 nm giving rise to an important penetration of aluminum into the wires. From numerical simulations of the spatial distribution of implantation‐induced damage, it was concluded that some lateral diffusion of the defects occurred during annealing. However it has been possible to assess the confinement energies to be around 4 meV. The linewidth of the wires’ emission turned out to increase with decreasing mask size, indicating the presence of some fluctuations of the confining potential along the wires. The roughness of the lateral definition of the wires was evaluated at 20 nm, of the same order of magnitude as the dimension of the intermixed region under the mask. Under these conditions optical excitation spectroscopy failed to detect the different one‐dimensional subbands. Finally the potentialities of this method of fabrication of quantum wires are inspected.

Arrays of tens of millions of identical 200-nm-wide particles made out of ultrathin epitaxial Fe(1 1 0) films with in-plane magnetization were studied using the microSQUID technique by putting a microSQUID chip upside down over the array of particles. The magnetization reversal of particles is very similar to the Stoner–Wohlfarth coherent reversal law and the mean reversal field Hr equals 0.75Ha (anisotropy field) in the easy axis direction. This model behavior is ascribed to the weak influence of the particles’ self-dipolar fields in the ultrathin geometry. Statistical reversal field distribution functions of single particles are deduced.

We report on the phase coherence time ?` in quasi one-dimensional Au/Fe Kondo wires and compare the temperature dependence of ?` with a recent theory of inelastic scattering from magnetic impurities. A very good agreement is obtained for temperatures down to 0.2TK. In particular, we show that the inverse of the phase coherence time varies basically linearly with temperature below TK over almost one decade in temperature.

We report on the size dependence of the amplitudes of Aharonov-Bohm (AB) as well as Altshuler-Aronov-Spivak (AAS) magnetoconductance oscillations in silver networks with anisotropic aspect ratio and for various sizes ranging from 10 to 10 6 plaquettes. We show that the amplitude of both AB and AAS oscillations exhibit an unexpected dependence as a function of number of plaquettes N when the smallest dimension of the network becomes smaller than the phase coherence length: in this case, the network can be considered as a fully coherent object (mesoscopic) in one direction, whereas macroscopic in the other.

We present phase coherence time measurements in quasi-one-dimensional mesoscopic wires made from high mobility two-dimensional electron gas. By implanting gallium ions into a GaAs/AlGaAs heterojunction we are able to vary the diffusion coefficient over 2 orders of magnitude. We show that in the diffusive limit, the decoherence time follows a power law as a function of diffusion coefficient as expected by theory. When the disorder is low enough so that the samples are semi-ballistic, we observe a new and unexpected regime in which the phase coherence time is independent of disorder. In addition, for all samples the temperature dependence of the phase coherence time follows a power law down to the lowest temperatures without any sign of saturation and strongly suggests that the frequently observed low temperature saturation is not intrinsic., 4 pages, 4 figures

An electronic Mach Zehnder interferometer is used in the integer quantum hall regime at filling factor 2, to study the dephasing of the interferences. This is found to be induced by the electrical noise existing in the edge states capacitively coupled to each others. Electrical shot noise created in one channel leads to phase randomization in the other, which destroys the interference pattern. These findings are extended to the dephasing induced by thermal noise instead of shot noise: it explains the underlying mechanism responsible for the finite temperature coherence time $\tau_\phi(T)$ of the edge states at filling factor 2, measured in a recent experiment. Finally, we present here a theory of the dephasing based on Gaussian noise, which is found in excellent agreement with our experimental results., Comment: ~4 pages, 4 figures

We present phase coherence time measurements in quasi-one-dimensional Ag wires implanted with Ag$^{+}$ ions with an energy of $100 keV$. The measurements have been carried out in the temperature range from $100 mK$ up to $10 K$; this has to be compared with the Kondo temperature of iron in silver, i.e. $T_{K}^{Ag/Fe} \approx 4 K$, used in recent experiments on dephasing in Kondo systems\cite{mallet_prl_06,birge_prl_06}. We show that the phase coherence time is not affected by the implantation procedure, clearly proving that ion implantation process by itself \emph{does not lead to any extra dephasing} at low temperature., 4 pages, 4figures

Transport properties of a superconducting NbN nanowire are studied experimentally and theoretically. Different attached leads (superconducting contacts) allowed us to measure current-voltage (I-V) characteristics of different segments of the wire independently. The experimental results show that with increasing the length of the segment the number of jumps in the I-V curve increases indicating an increasing number of phase-slip phenomena. The system shows a clear hysteresis in the direction of the current sweep, the size of which depends on the length of the superconducting segment. The interpretation of the experimental results is supported by theoretical simulations that are based on the time-dependent Ginzburg-Landau theory, the heat equation has been included in the Ginzbur-Landau theory.

In this paper we have studied the low frequency noise generated by sending voltage pulses in semiconductor wires. Two samples have been studied, one with strong disorder and one with weak disorder. The noise has been identified with the change of a single defect. The reduction of the noise by a small magnetic field definitively connects it to the universal conductance fluctuations.

We observed the recently predicted quantum suppression of dynamical Coulomb blockade on short coherent conductors by measuring the conductance of a quantum point contact embedded in a tunable on-chip circuit. Taking advantage of the circuit modularity we measured most parameters used by the theory. This allowed us to perform a reliable and quantitative experimental test of the theory. Dynamical Coulomb blockade corrections, probed up to the second conductance plateau of the quantum point contact, are found to be accurately normalized by the same Fano factor as quantum shot noise, in excellent agreement with the theoretical predictions., 4 pages, 4 figures, accepted for publication in Physical Review Letters

We have determined the finite temperature coherence length of edge states in the Integer Quantum Hall Effect (IQHE) regime. This was realized by measuring the visibility of electronic Mach-Zehnder interferometers of different sizes, at filling factor 2. The visibility shows an exponential decay with the temperature. The characteristic temperature scale is found inversely proportional to the length of the interferometer arm, allowing to define a coherence length $\l_\phi$. The variations of $\l_\phi$ with magnetic field are the same for all samples, with a maximum located at the upper end of the quantum hall plateau. Our results provide the first accurate determination of $\l_\phi$ in the quantum Hall regime., Comment: 4 pages, 4 figures

In a recent paper [B. A. Piot et al., Phys. Rev. B 72, 245325 (2005)], we have shown that the lifting of the electron spin degeneracy in the integer quantum Hall effect at high filling factors should be interpreted as a magnetic-field-induced Stoner transition. In this work, we extend the analysis to investigate the influence of the single-particle Zeeman energy on the quantum Hall ferromagnet at high filling factors. The single-particle Zeeman energy is tuned through the application of an additional in-plane magnetic field. Both the evolution of the spin polarization of the system and the critical magnetic field for spin splitting are well described as a function of the tilt angle of the sample in the magnetic field., Published in Phys. Rev. B

Lithography is the process of printing patterns onto a thin film called a resist, using a localised interaction between this layer and an engraving micro-tool or particle beam. The various techniques of lithography can be classified according to the micro-tool or the type of radiation used (detailed in Sect. 1.5). Hence, to print the pattern, photolithography uses photons, electron lithography uses electrons, and ion lithography uses ions. On the other hand, lithography by impression uses the mechanical interaction between a hard mould and a layer of soft resist, and near-field lithography uses various types of interaction (electrical, mechanical, thermal, optical) between a fine tip and the surface of the resist. The lithography itself does not therefore structure the active material which will constitute the core of the nanodevice. It simply sketches the outline of the future device in a sacrificial layer, the resist, and this is then used in a transfer stage to shape the active layer according to the dimensions of the pattern imposed in the lithography stage.

International audience; Mesoscopic transport measurements reveal a large effective phase coherence length in epitaxial GaMnAs ferromagnets, contrary to usual 3d-metal ferromagnets. Universal conductance fluctuations of single nanowires are compared for epilayers with a tailored anisotropy. At large magnetic fields, quantum interferences are due to structural disorder only, and an unusual behavior related to hole-induced ferromagnetism is evidenced, for both quantum interferences and decoherence. At small magnetic fields, phase coherence is shown to persist down to zero field, even in presence of magnons, and an additional spin disorder contribution to quantum interferences is observed under domain walls nucleation.

We use the resistance fluctuations (RFs) appearing in the integer quantum Hall regime to scan the density of states of a very thin Hall bar. By applying a dc voltage on a top gate, we analyze the correlation properties of the various resistances as a function of the magnetic field and the carrier density. In the gate voltage-magnetic field plane, these RFs follow lines with slopes quantized in unit of filling factor and the slope of these RFs depends on their correlation properties.

We present an original statistical method to measure the visibility of interferences in an electronic Mach-Zehnder interferometer in the presence of low frequency fluctuations. The visibility presents a single side lobe structure shown to result from a gaussian phase averaging whose variance is quadratic with the bias. To reinforce our approach and validate our statistical method, the same experiment is also realized with a stable sample. It exhibits the same visibility behavior as the fluctuating one, indicating the intrinsic character of finite bias phase averaging. In both samples, the dilution of the impinging current reduces the variance of the gaussian distribution., Comment: 4 pages, 5 figures

In a recent paper [Phys.Rev.B. 72,245325(2005)], we have shown that the lift of the electron spin degeneracy in the integer quantum Hall effect at high filling factors could be interpreted as a magnetic field induced Stoner transition. We provide here additional experimental results confirming these findings.

The Stoner-Wohlfarth model needs some modifications in order to take account of real anisotropy of monocrystal nanoparticles. Such a modified model predicts a qualitatively new behavior for the switching of the magnetization of such a particle. We provide the first experimental evidence of such a behavior.

Scanning $\mu$SQUID force microscopy is used to study magnetic flux structures in single crystals of the layered spin triplet superconductor Sr$\_{2}$RuO$\_{4}$. Images of the magnetic flux configuration above the $\vec{a}\vec{b}$-face of the cleaved crystal are acquired, mostly after field-cooling the sample. For low applied magnetic fields, individual vortices are observed, each carrying a single quantum of flux. Above 1 gauss, coalescence of vortices is discovered. The coalescing vortices may indicate the presence of domains of a chiral order parameter. When the applied field is tilted from the $\vec{c}$-axis, we observe a gradual transition from vortex domains to vortex chains. The in-plane component of the applied magnetic field transforms the vortex domains to vortex chains by aligning them along the field direction.This behavior and the inter-chain distance varies in qualitative agreement with the Ginzburg Landau theory of anisotropic 3D superconductors. The effective mass anisotropy of Sr$\_{2}$RuO$\_{4}$, $\gamma$=20, is the highest observed in three dimensional superconductors. When the applied field is closely in plane, the vortex form flux channels confined between the crystal-layers. Residual Abrikosov vortices are pinned preferentially on these channels. Thus the in-plane vortices are decorated by crossing Abrikosov vortices: two vortex orientations are apparent simultaneously, one along the layers and the other perpendicular to the layers., Comment: to appear in Physica C M2S conference Dresden

Magnetic flux structures in single crystals of the layered spin triplet superconductor Sr$\_{2}$RuO$\_{4}$ are studied by scanning micro SQUID Force microscopy. Vortex chains appear as the applied field is tilted along the in-plane direction of the superconductor. The vortex chains align along the direction of the in-plane component of the applied magnetic field. The decoration of in-plane vortices by crossing Abrikosov vortices is observed: two vortex orientations are apparent simultaneously, one along the layers and the other perpendicular to the layers. The crossing vortices appear preferentially on the in-plane vortices.

We present phase coherence time measurements in quasi-one-dimensional Ag wires doped with Fe Kondo impurities of different concentrations $n_s$. Due to the relatively high Kondo temperature $T_{K}\approx 4.3K$ of this system, we are able to explore a temperature range from above $T_{K}$ down to below $0.01 T_{K}$. We show that the magnetic contribution to the dephasing rate $��_m$ per impurity is described by a single, universal curve when plotted as a function of $(T/T_K)$. For $T>0.1 T_K$, the dephasing rate is remarkably well described by recent numerical results for spin $S=1/2$ impurities. At lower temperature, we observe deviations from this theory. Based on a comparison with theoretical calculations for $S>1/2$, we discuss possible explanations for the observed deviations., 4 pages, 3 figures, submitted to Phys. Rev. Lett

We report on magnetoconductance measurements of metallic networks of various sizes ranging from 10 to $10^{6}$ plaquettes, with anisotropic aspect ratio. Both Altshuler-Aronov-Spivak (AAS) $h/2e$ periodic oscillations and Aharonov-Bohm (AB) $h/e$ periodic oscillations are observed for all networks. For large samples, the amplitude of both oscillations results from the incoherent superposition of contributions of phase coherent regions. When the transverse size becomes smaller than the phase coherent length $L_\phi$, one enters a new regime which is phase coherent (mesoscopic) along one direction and macroscopic along the other, leading to a new size dependence of the quantum oscillations., Comment: 4 pages, to appear in Phys. Rev. Lett

28th International Conference on the Physics of Semiconductors (ICPS-28), Vienna, AUSTRIA, JUL 24-28, 2006; International audience; We present a low temperature magnetoconductance study of a submicron hall bar made from an AlGaAs/InGaAs/GaAs heterostructure. Experiments were performed at very low temperature (120 mK), with a magnetic field ranging from OT to 13.5 T. In the presence of a quantizing magnetic field, at the transition between two quantized Hall plateaus, a succession of sharp peaks is detected in the Hall signal RH and in the longitudinal resistance R-L. The peaks appearing on the high-v side of the R-L transition appear to be different from the peaks appearing on the low-v side. They mainly differ by their temperature dependence. On the high-v side of the RL transition, the temperature evolution of the peaks is typical for resonant tunneling through a single state in one of the antidots that are progressively formed when the initially occupied LL is emptied. On the low v of the transition, by contrast, the temperature dependence is different. This may be related to the asymmetry of the density of states, or to additional inelastic processes.

Magnetic nanodiscs are fabricated by depositing cobalt onto 10–30 nm diameter silicon nanopillars, which were prepatterned using gold colloids as etch masks. The magnetic anisotropy energy of individual nanodiscs is studied by measuring the angular dependence of switching fields using the micro-SQUID technique. The Stoner–Wohlfarth model, describing the magnetization reversal by unifom rotation, is used to analyse the data. The switching astroids of pure Co exhibit a cubic magnetocrystalline anisotropy indicating that the Co crystallites are fcc. After controlled oxidation of the nanoparticles, the anisotropy is dominated by a defect-induced uniaxial anisotropy, which means that the anisotropy can be used as a quality gauge.

Magnetization measurements on a crystal of ${\mathrm{Cr}}_{7}\mathrm{Ni}$ antiferromagnetic rings are presented. Irradiation with microwaves at frequencies between 1 and $10\phantom{\rule{0.3em}{0ex}}\mathrm{GHz}$ leads to observation of very narrow Gaussian resonant photon absorption lines which are mainly broadened by hyperfine interactions. The relaxation of magnetization after a nanosecond RF pulse is dominated by the phonon bottleneck effect. A two-pulse hole burning technique allowed us to estimate the characteristic energy diffusion time (spectral diffusion).

We present direct imaging of magnetic flux structures in the anisotropic, spin-triplet superconductor Sr$_{2}$RuO$_{4}$ using a scanning $\mu$SQUID microscope. Individual quantized vortices were seen at low magnetic fields. Coalescing vortices forming flux domains were revealed at intermediate fields. Based on our observations we suggest that a mechanism intrinsic to the material stabilizes the flux domains against the repulsive vortex-vortex interaction. Topological defects like domain walls can provide this, implying proof for unconventional chiral superconductivity.

We present measurements of the phase coherence time \tauphi in quasi one-dimensional Au/Fe Kondo wires and compare the temperature dependence of \tauphi with a recent theory of inelastic scattering from magnetic impurities (Phys. Rev. Lett. 93, 107204 (2004)). A very good agreement is obtained for temperatures down to 0.2 $T_K$. Below the Kondo temperature $T_K$, the inverse of the phase coherence time varies linearly with temperature over almost one decade in temperature., Comment: 5 pages, 3 figures

The observation of a novel type of commensurability resonance in two-dimensional, hexagonal antidot lattices is reported. These resonances have a classical character and occur at magnetic fields above the resonance that corresponds to the cyclotron motion around a single antidot. The resonances are visible only for antidots with effective diameters larger than 50 % of the lattice constant. Simulations reveal that they originate from quasi-stable electron trajectories that bounce between three neighboring antidots. This interpretation is backed by the observation of large-period Aharonov-Bohm type oscillations at low temperatures., Comment: 5 pages, 5 figures minor changes to text

A scanning micro-superconducting quantum interference-device microscope is used to directly image vortices in a superconducting Al thin film. We observe the temperature dependence of the vortex distribution in a regular defect (hole) array patterned into the Al film. The direct observation of the localized superconducting state around the holes is shown as well as the effect of the hole size on nucleation of the superconducting state.

The low temperature magnetoconductance of a large array of quantum coherent loops exhibits Altshuler-Aronov-Spivak oscillations with a periodicity corresponding to 1=2 flux quantum per loop. We show that the measurement of the harmonics content provides an accurate way to determine the electron phase-coherence length L’ in units of the lattice length with no adjustable parameters. We use this method to determine L’ in a square network realized from a 2D electron gas in a GaAs=GaAlAs heterojunction, with only a few conducting channels. The temperature dependence follows a power law T � 1=3 from 1.3 K to 25 mK with no saturation, as expected for 1D diffusive electronic motion and electron-electron scattering as the main decoherence mechanism.

International audience; We present a detailed model describing the effects of wire corrugation on the trapping potential experienced by a cloud of atoms above a current carrying micro wire. We calculate the distortion of the current distribution due to corrugation and then derive the corresponding roughness in the magnetic field above the wire. Scaling laws are derived for the roughness as a function of height above a ribbon shaped wire. We also present experimental data on micro wire traps using cold atoms which complement some previously published measurements and which demonstrate that wire corrugation can satisfactorily explain our observations of atom cloud fragmentation above electroplated gold wires. Finally, we present measurements of the corrugation of new wires fabricated by electron beam lithography and evaporation of gold. These wires appear to be substantially smoother than electroplated wires.

We report the first study of the micro-SQUID response of a molecular system to electromagnetic radiation. The advantages of our micro-SQUID technique in respect to pulsed electron paramagnetic resonance (EPR) techniques consist in the possibility to perform time-resolved experiments (below 1 ns) on submicrometer sizes samples (about 1000 spins) at low temperature (below 100 mK). Resonant photon absorption in the GHz range was observed via low temperature micro-SQUID magnetization measurements of the spin ground state S = 1/2 of the molecular complex V15. The line-width essentially results from intra-molecular hyperfine interaction. The results point out that observing Rabi oscillations in molecular nanomagnets requires well isolated low spin systems and high radiation power. Our first results open the way for time-resolved observations of quantum superposition of spin-up and spin-down states in SMMs., 7 pages, 5 figures

STM and nano-FIB have been used to modify InP(0 0 1) and related materials surfaces at nanoscale in order to create nucleation sites for quantum dots. STM has permitted the fabrication of holes as small as being 11.2±2.8 nm large and 3±0.8 nm deep on AlInAs surfaces. Nano-FIB has allowed the fabrication of hole arrays with hole diameter of 87±10 nm, hole depth of 1.5±1 nm, and array period of 100 nm. FIB-made holes are stable under annealing, but the surface exhibit pollution and structural degradations due to a direct exposure to Ga + ion beam. Holes obtained by both techniques could be suitable to be used as nucleation sites for quantum dots.

We have measured the in-plane angular variation of nucleation and annihilation fields of a multi-domain magnetic single dot with a microsquid. The dots are Fe/Mo(110) self-assembled in UHV, with sub-micron size and a hexagonal shape. The angular variations were quantitatively reproduced by micromagnetic simulations. Discontinuities in the variations are observed, and shown to result from bifurcations related to the interplay of the non-uniform magnetization state with the shape of the dot., 4 pages, 4 figures, for submission as a regular article

The low-temperature spin dynamics of a ${\mathrm{Fe}}_{8}$ single-molecule magnet was studied under circularly polarized electromagnetic radiation allowing us to establish clearly photon-assisted tunneling. This effect, while linear at low power, becomes highly nonlinear above a relatively low-power threshold. Heating due to phonon emission, spin-spin interactions, and coherent emission/absorption of photons might lead to the observed nonlinearity. These results are of importance if such systems are to be used as quantum computers.

7th International Symposium on Research in High Magnetic Fields, Inst Natl Sci Appl, Toulouse, FRANCE, JUL 20-23, 2003; We present preliminary experiments on Aharonov-Bohm interferometers. We have made mesoscopic semiconductor GaAs/GaAlAs rings in which the interference phenomenon still exists in presence of a high magnetic field. This electronic interferometer is an N waves interferometer like the optical Fabry-Perot system. (C) 2004 Elsevier B.V. All rights reserved.

Signature of phase coherence on the electric and magnetic response of non connected Aharonov-Bohm rings is measured by a resonant method at 350 MHz between 20 mK and 500 mK. The rings are etched in a GaAs-AlGaAs heterojunction. Both quantities exhibit a oscillating behaviour with a periodicity consistent with half a flux quantum $\Phi_0/2=h/2e$ in a ring. Electric screening is shown to be enhanced when time reversal symmetry is broken by magnetic field, leading to a positive magnetopolarisability, in agreement with theoretical predictions for isolated rings at finite frequency. Temperature and electronic density dependence are investigated. The dissipative part of the electric response, the electric absorption, is also measured and leads to a negative magnetoconductance. The magnetic orbital response of the very same rings is also investigated. It is consistent with diamagnetic persistent currents of 0.25 nA. This magnetic response is an order of magnitude smaller than the electric one, in qualitative agreement with theoretical expectations., Comment: 16 pages, 20 figures, revtex

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

Staircaselike hysteresis loops of the magnetization of a LiY0.998Ho0.002F4 single crystal are observed at subkelvin temperatures and low field sweep rates. This behavior results from quantum dynamics at avoided level crossings of the energy spectrum of single Ho3+ ions in the presence of hyperfine interactions. Enhanced quantum relaxation in constant transverse fields allows the study of the relative magnitude of tunnel splittings. At faster sweep rates, nonequilibrated spin-phonon and spin-spin transitions, mediated by weak dipolar interactions, lead to magnetization oscillations and additional steps.

We report measurements of the flux-dependent orbital magnetic susceptibility of an ensemble of 10^5 disconnected silver rings at 217 MHz. Because of the strong spin-orbit scattering rate in silver this experiment is a test of existing theories on orbital magnetism. Below 100 mK the rings exhibit a magnetic signal with a flux periodicity of h/2 e consistent with averaged persistent currents, whose amplitude is estimated to be of the order of 0.3 nA. The sign of the oscillations indicates diamagnetism in the vicinity of zero magnetic field. This sign is not consistent with theoretical predictions for average persistent currents unless considering attractive interactions in silver. We propose an alternative interpretation taking into account spin orbit scattering and finite frequency., Comment: 4 pages, 4 figures, revtex4, accepted for publication in Physical Review Letters