Stefan Mátéfi-Tempfli

Current-voltage characteristics of quasi-one-dimensional superconductors: An S-shaped curve in the constant voltage regime

Applying a constant voltage to superconducting nanowires we find that its I-V characteristic exhibits an unusual S behavior. This behavior is the direct consequence of the dynamics of the superconducting condensate and of the existence of two different critical currents: j(c2) at which the pure superconducting state becomes unstable and j(c1)<j(c2) at which the phase-slip state is realized in the system.

Conductance quantization in magnetic nanowires electrodeposited in nanopores

Magnetic nanocontacts have been prepared by a templating method that involves the electrodeposition of Ni within the pores of track-etched polymer membranes. The nanocontacts are made at the extremity of a single Ni nanowire either inside or outside the pores. The method is simple, flexible, and controllable as the width of the constriction can be varied reversibly by controlling the potential between the electrodeposited nanowire and a ferromagnetic electrode. At room temperature, the electrical conductance shows quantization steps in units of e(2)/h, as expected for ferromagnetic metals without spin degeneracy. Our fabrication method enables future investigation of ballistic spin transport phenomena in electrodeposited magnetic nanocontacts

Current-voltage characteristics of Pb and Sn granular superconducting nanowires

Arrays of granular superconducting Pb and Sn nanowires (40 - 55 nm in diameter and 22 or 50 mum long) have been prepared by electrodeposition in nanoporous membranes. A simple technique has been developed to perform electrical transport measurement on a single nanowire. By sweeping the dc current inside the nanowire, we observed the formation of phase-slip-centers far below the critical temperature. In contrast, in voltage-driven experiments, an interesting S-shaped behavior has been observed in the nucleation region of these phase-slip-centers

Electrodeposited L1(0) CoxPt1-x nanowires

Arrays of face-centred cubic (fcc) CoxPt1-x (0.45 < x < 0.55) nanowires were electrodeposited into thin film nanoporous alumina supported on a Si substrate. The heat treatment under specific conditions was then carried out in order to transform the fcc phase into the face-centred tetragonal or L1(0) ordered phase. The influence of both the phase transition and the temperature on the magnetic properties of CoxPt1-x nanowires has been studied. Coercive fields higher than 1 T (10 kOe) have been obtained at room temperature with ordered nanowires, 80 nm in diameter.

Guided transmission of slow Ne6+ ions through the nanochannels of highly ordered anodic alumina

A highly ordered hexagonally close-packed nanochannels array was prepared using the self-ordering phenomena during a two-step anodization process of a high purity aluminium foil. The anodized aluminium oxide, with pore diameters of about 280 nm and interpore distances of about 450 nm was prepared as a suspended membrane of about 15 mu m thickness on the aluminium frame to which it belongs. The Al2O3 capillaries were bombarded with 3 keV Ne6+ ions. The first results unambiguously show the existence of ion guiding observed at 5 degrees and 7.5 degrees tilt angles of the capillaries compared to the beam direction. To the best of our knowledge, such ion guiding effects of slow ions through hexagonally ordered nanochannels in alumina has not been reported previously.

Nanowire-decorated microscale metallic electrodes.

One of the challenging aspects of science and technology on a nanometer-scale is the precise three-dimensional control of nano-objects. Scanning probe microscopy manipulation, magneticor electric-field alignment and lithography-based techniques are only a few of the techniques that have been reported so far. Nevertheless, most of these techniques are still being developed and their integration for device fabrication represents a real challenge for the scientific community. Within this context, nanowires and nanotubes are of great interest because they lie between the macroscopic and atomic scales. The ability to fabricate andmanipulate such objects in a reliablemanner on a large scale will foster their use in electronic, photonic, and sensing applications. Templatebased methods have been successfully used for nanowire fabrication as they allow the realization of complex organic/ inorganic nanostructures. To date, nanoporous anodic alumina oxide (AAO) made by the electrochemical oxidation of aluminum has been extensively used because it provides a good platform for the development of various nanostructures. This interest originates from the fact that AAO membranes, having a high density of nanopores (up to 10 cm ), are easily produced over large areas with variable thicknesses. Moreover, a good chemical and mechanical stability combined with interesting electrical properties make AAO membranes good candidates for nanowire fabrication. However, the use of such nanostructures as passive or active components in emerging electronic devices requires smartly engineered arrays of nanowires with well defined position and pitch.

Functionalization of magnetic nanowires by charged biopolymers.

We report on a facile method for the preparation of biocompatible and bioactive magnetic nanowires. The method consists of the direct deposition of polysaccharides by layer-by-layer (LbL) assembly onto a brush of metallic nanowires obtained by electrodeposition of the metal within the nanopores of an alumina template supported on a silicon wafer. Carboxymethylpullulan (CMP) and chitosan (CHI) multilayers were grown on brushes of Ni nanowires; subsequent grafting of an enzyme was performed by conjugating free amine side groups of chitosan with carboxylic groups of the enzyme. The nanowires are finally released by a gentle ultrasonic treatment. Transmission electron microscopy, electron energy-dispersive loss spectroscopy, and x-ray photoelectron spectroscopy indicate the formation of an homogeneous coating onto the nickel nanowires when one, two, or three CMP/CHI bilayers are deposited. This easy and efficient route to the biochemical functionalization of magnetic nanowires could find widespread use for the preparation of a broad range of nanowires with tailored surface properties.

Controlled growth of single nanowires within a supported alumina template.

A simple technique for fabricating single nanowires with well-defined position is presented. The process implies the use of a silicon nitride mask for selective electrochemical growth of the nanowires in a porous alumina template. We show that this method allows the realization of complex nanowire patterns as well as arrays of single nanowires with a precise position and spacing.

Condition for the occurrence of phase slip centers in superconducting nanowires under applied current or voltage

Experimental results on the phase slip process in superconducting lead nanowires are presented under two different experimental conditions: constant applied current or constant voltage. Based on these experiments we established a simple model which gives us the condition of the appearance of phase slip centers in a quasi-one-dimensional wire. The competition between two relaxations times (relaxation time of the absolute value of the order parameter tau(psi) and relaxation time of the phase of the order parameter in the phase slip center tau(phi)) governs the phase slip process. Phase slips, as periodic oscillations in time of the order parameter, are only possible if the gradient of the phase grows faster than the value of the order parameter in the phase slip center, or equivalently if tau(phi)

Nanolithography based contacting method for electrical measurements on single template synthesized nanowires

A reliable method enabling electrical measurements on single nanowires prepared by electrodeposition in an alumina template is described. This technique is based on electrically controlled nanoindentation of a thin insulating resist deposited on the top face of the template filled by the nanowires. We show that this method is very flexible, allowing us to electrically address single nanowires of controlled length down to 100 nm and of desired composition. Using this approach, current densities as large as 10(9) A cm(-2) were successfully injected through a point contact on a single magnetic multilayered nanowire. This demonstrates that the technique is very promising for the exploration of electrical spin injection in magnetic nanostructures.