Sébastien Michotte

Spin Transition Charted in a Fluorophore-Tagged Thermochromic Dinuclear Iron(II) Complex

The first crystal structures of a dinuclear iron(II) complex with three N1,N2-1,2,4-triazole bridges in the high-spin and low-spin states are reported. Its sharp spin transition, which was probed using X-ray, calorimetric, magnetic, and (57)Fe Mossbauer analyses, is also delineated in the crystalline state by variable-temperature fluorimetry for the first time.

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.

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

Superconducting properties of lead nanowires arrays

Mesoscopic superconducting lead nanowires with high aspect ratio and diameter ranging from 40 to 270 nm have been grown by electrodeposition inside nanoporous polycarbonate membranes. Nanowires with a diameter less than 50 nm were insulators due to a poor crystal structure. The others are shown to be type II superconductors because of their small electronic mean free path, instead of being type I which is usual for the bulk form of lead. An increase in the thermodynamic critical field H-c is observed and is attributed to the small transversal dimension leading to an incomplete Meissner effect. Finally, it is demonstrated that this enhancement agrees with numerical simulations based on the Ginzburg-Landau theory

Controlled growth of CoCu nanowires and application to multilayered CoCu/Cu nanowires with selected anisotropy

The effects of the solution pH and deposition potential on the structural and magnetic properties in arrays of electrodeposited CoCu nanowires with low Cu content have been studied combining ferromagnetic resonance, magnetometry and electron transmission microscopy. It is shown that, depending on the synthesis parameters, the average crystallographic structure can be controlled, giving rise to sensible changes in the effective crystal anisotropy field which varies from -4.05 to +2.75 kOe. It is also shown that in CoCu/Cu multilayered nanowires, the preferential structure of the CoCu-magnetic layer can also be controlled by both pH and deposition potential, opening an interesting route for designing multilayered CoCu/Cu nanowires with controlled and alternated crystal anisotropy.

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.

1D-transport properties of single superconducting lead nanowires

We report on the transport properties of single superconducting lead nanowires grown by an electrodeposition technique, embedded in a nanoporous track-etched polymer membrane. The nanowires are granular, have uniform diameter of similar to40 nm and a very large aspect ratio (similar to500). The diameter of the nanowire is small enough to ensure a I D superconducting regime in a wide temperature range below T-c. The non-zero resistance in the superconducting state and its variation caused by fluctuations of the superconducting order parameter were measured versus temperature, magnetic field, and applied DC current (or voltage). The current induced breakdowns in the V-1 characteristics may be explained by the formation of phase slip centers. Moreover, DC voltage driven measurements reveal the existence of a new S-shape behavior near the formation of these phase slip centers

Fabrication and physical properties of Pb∕Cu multilayered superconducting nanowires

Using nanoporous media as templates for electrodeposition, we have fabricated multilayered Pb/Cu nanowires 50 nm in diameter and 20 μ m long with Cu layer thicknesses as low as 10 nm. This was achieved by using a single-bath technique and a precise selection of the copper deposition potential to limit the redissolution of lead when the potential is raised to electrodeposit the more noble metal. Such superconductor/normal multilayered nanowires show interesting magnetoresistance properties at a low magnetic field that may be related to the proximity effect, which can therefore be investigated in a quasi-one-dimensional geometry

Multicontact measurements of a superconducting Sn nanowire

Multicontact transport measurements were performed on a superconducting tin nanowire to probe its local properties and especially the formation of phase-slip centers (PSCs). We find that normal metal contacts strongly disturb the behavior of the nanowire. In particular, they provide an efficient escape way for the energy dissipated at the PSC and thereby limit its size. We were able to observe PSC at 30 mK with an associated relaxation time typical to that of the inelastic scattering time for bulk normal tin