Electric-field control of the magnetic anisotropy in an ultrathin (Ga,Mn)As/(Ga,Mn)(As,P) bilayer

We report on the electric control of the magnetic anisotropy in an ultrathin ferromagnetic (Ga,Mn)As/(Ga,Mn)(As,P) bilayer with competing in-plane and out-of-plane anisotropies. The carrier distribution and therefore the strength of the effective anisotropy is controlled by the gate voltage of a field effect device. Anomalous Hall Effect measurements confirm that a depletion of carriers in the upper (Ga,Mn)As layer results in the decrease of the in-plane anisotropy. The uniaxial anisotropy field is found to decrease by a factor ~ 4 over the explored gate-voltage range, so that the transition to an out-of-plane easy-axis configuration is almost reached

Quasi one-dimensional transport in single GaAs/AlGaAs core-shell nanowires

We present an original approach to fabricate single GaAs/AlGaAs core-shell nanowire with robust and reproducible transport properties. The core-shell structure is buried in an insulating GaAs overlayer and connected as grown in a two probe set-up using the highly doped growth substrate and a top diffused contact. The measured conductance shows a non-ohmic behavior with temperature and voltage-bias dependences following power laws, as expected for a quasi-1D system

Fluxoid quantization in the critical current of a niobium superconducting loop far below the critical temperature

The critical current of a niobium type II superconducting loop has been measured far below the critical temperature using contacts applied symmetrically or asymmetrically around the loop. The magnetic field dependence of the critical current allows a direct observation of the circulating current resulting from the fluxoid quantization and presents a saw-tooth behavior. The periodicity is given by a flux quantum through the outer area of the loop. Despite the presence of many vortices at high magnetic fields, the circulating current finds its way in between them and still has an impact on the total critical current of the loop. Macroscopic quantum coherence effects in such a niobium loop allow observing single quanta changes of the fluxoid up to a magnetic field of 1.2 T

Propriétés de transport électronique de nanofils supraconducteurs électrodéposés

Dans le cas des supraconducteurs unidimensionnels (1D), la conservation du paramètre d'ordre supraconducteur sur de grandes distances n'est plus possible. Les fluctuations de ce paramètre d'ordre induisent alors des phénomènes résistifs hors équilibre qui se traduisent par la formation de "phase-slip center" (PSC). L'étude de tels systèmes supraconducteurs hors équilibre est très intéressante, tant sur le plan expérimental que théorique. Cela permet d'obtenir des informations sur la dynamique de la supraconductivité, et plus généralement sur les mécanismes de décohérence quantique. Dans ce contexte, les fils supraconducteur 1D suscitent un fort intérêt. Une voie élégante pour élaborer de tels nanofils consiste à utiliser des milieux nanoporeux dont les pores cylindriques sont remplis par dépôt électrolytique. Dans ce travail de thèse, nous avons étudié les propriétés de transport de ces nanofils supraconducteurs électrodéposés. La lithographie électronique a été utilisée pour poser plusieurs contacts électriques le long d un nanofil, permettant ainsi de sonder localement, sur quelques centaines de nanomètres, les propriétés électriques de ce fil. Des mesures jusqu'à 30 mK sur des nanofils d'étain de 50 nm de diamètre ont montrés des PSCs et un effet Meissner incomplet. Par ailleurs, nous avons montré l'impact de l'effet de proximité inverse induit par des électrodes normales sur le paramètre d'ordre supraconducteur du fil. Ce qui a permis de mettre en évidence deux régimes différents de relaxation des quasi-particules. De plus, ce couplage avec un environnement dissipatif semble permettre la stabilisation de la supraconductivité dans un fil 1D de longueur finie.In one-dimensional (1D) superconducting systems, any long-range order is impossible. Superconducting order parameter fluctuations destroy the zero resistance state and induce a non-equilibrium phenomenon which leads to the successive nucleation of phase-slip-center (PSC). Study of such superconducting systems is of importance not only for dynamical properties of 1D superconductivity, but also for understanding the decoherence mechanisms of quantum systems due to interaction with their environment. In this context, superconducting nanowires attract a lot of attention due to their fundamental properties as well as their potential applications in nanotechnologies. An elegant approach to fabricate those kind of nanowires, consists in the use of track-etched polymer membranes as a template for electrodeposition. In this work, we studied transport properties of single superconducting nanowire. Electronic lithography has been used to realise several electrical contacts along isolated nanowires spread over a substrate. This allows one to locally probe the electrochemical potential of wires, over a length of several hundreds of nanometers. Multiprobes measurements of a 50nm diameter tin wire, at 30 mK, show PSCs and incomplete Meissner effect. This geometry allows us to precisely study the evolution of PSCs under magnetic field. In addition, one could show the impact of the inverse proximity effect induced by normal electrodes on superconducting order parameter. We highlight two different modes of quasiparticle relaxation. Moreover one observed that coupling with a dissipative environment allowed the stabilization of superconductivity in a 1D finite wireREIMS-BU Sciences (514542101) / SudocSudocFranceF

Switching current modulations induced by vortices rearrangement in mesoscopic superconducting loops

Periodic modulations in the switching current of niobium mesoscopic thin loops have been observed as a function of a perpendicular magnetic field. These modulations reflect the formation of specific vortices configurations in the arms and in the contacts of the loops. The voltage measured just before the current-driven transition to the normal state takes either zero or nonzero values depending on whether the vortices are efficiently pinned or move under the action of the applied current. This switching voltage varies sharply in the vicinity of the regularly spaced matching fields at which the switching current is minimum. This reflects the rearrangement of the vortices pattern from an unstable configuration into a more stable one. In addition, multimodal switching current distributions obtained at a constant field highlight that the few vortices involved in the field-cooling process freeze into a limited number of configurations. Finally, the spacing between the matching fields allows us to extract the critical field for complete vortex expulsion of the samples. Electrical transport measurements appear to be an efficient tool to determine the superheating field of submicrometer-wide structures, which is not obvious when measured by scanning probe microscopy

Hyperbaric oxygen therapy in a case of vaginal and uterine necrosis after embolization for postpartum hemorrhage

International audienc

Si Incorporation in InP Nanowires Grown by Au-Assisted Molecular Beam Epitaxy

We report on the growth, structural characterization, and conductivity studies of Si-doped InP nanowires grown by Au-assisted molecular beam epitaxy. It is shown that Si doping reduces the mean diffusion length of adatoms on the lateral nanowire surface and consequently reduces the nanowire growth rate and promotes lateral growth. A resistivity as low as 5.1±0.3×10−5 Ω⋅cm is measured for highly doped nanowires. Two dopant incorporation mechanisms are discussed: incorporation via catalyst particle and direct incorporation on the nanowire sidewalls. The first mechanism is shown to be less efficient than the second one, resulting in inhomogeneous radial dopant distribution