High Tc Josephson nanoJunctions made by ion irradiation : characteristics and reproducibility

Reproducible High Tc Josephson junctions have been made in a rather simple two-step process using ion irradiation. A microbridge 1 to 5 micrometers wide is firstly designed by ion irradiating a c-axis-oriented YBa2Cu3O7 film through a gold mask such as the unprotected part becomes insulating. A lower Tc part is then defined within the bridge by irradiating with a much lower dose through a 20 nm wide narrow slit opened in a standard electronic photoresist. These planar junctions, whose settings can be finely tuned, exhibit reproducible and nearly ideal Josephson characteristics. Non hysteretic Resistively Shunted Junction (RSJ) like behavior is observed, together with sinc Fraunhofer patterns for rectangular junctions. The IcRn product varies with temperature ; it can reach a few mV. The typical resistance ranges from 0.1 to a few ohms, and the critical current density can be as high as 30 kA/cm2. The dispersion in characteristics is very low, in the 5% to 10% range. Such nanojunctions have been used to make microSQUIDs (Superconducting Quantum Interference Device) operating at Liquid Nitrogen (LN2) temperature. They exhibit a very small asymmetry, a good sensitivity and a rather low noise. The process is easily scalable to make rather complex Josephson circuits.Comment: 4 pages, 5 figures, Applied Superconductivity Conference Seattle 200

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

High domain wall velocity at zero magnetic field induced by low current densities in spin-valve nanostripes

Current-induced magnetic domain wall motion at zero magnetic field is observed in the permalloy layer of a spin-valve-based nanostripe using photoemission electron microscopy. The domain wall movement is hampered by pinning sites, but in between them high domain wall velocities (exceeding 150 m/s) are obtained for current densities well below 10^{12} \unit{A/m^2}, suggesting that these trilayer systems are promising for applications in domain wall devices in case of well controlled pinning positions. Vertical spin currents in these structures provide a potential explanation for the increase in domain wall velocity at low current densities.Comment: Published version, Applied Physics Express 2, 023003 (2009) http://dx.doi.org/10.1143/APEX.2.02300

Etude de l'emboitement quantifie fractionnaire dans le metal organique (TMTSF)_2ClO_4

SIGLECNRS T Bordereau / INIST-CNRS - Institut de l'Information Scientifique et TechniqueFRFranc

Anisotropie de magnéto-résistance de diodes tunnel ferromagnétiques zener-esaki p-GaMnAs/n-GaAs (spectroscopie des anisotropies de bandes de GaMnAs)

GaMnAs est un semi-conducteur ferromagnétique dilué où l interaction d échange est induite par des porteurs délocalisés. Ces derniers sont souvent décrits par des états de Bloch dans la bande de valence, malgré une controverse sur l influence d une mini-bande d impuretés Mn. Une connaissance plus précise de la structure de bandes reste nécessaire pour déterminer à la fois la position du niveau de Fermi et la nature exacte des anisotropies magnétiques, encore mal connues. Nous avons étudié l anisotropie de magnétorésistance en régime tunnel (TAMR) de diodes Zener-Esaki p++-GaMnAs/n+-GaAs, reliée aux anisotropies de la structure de bandes de GaMnAs. Le transport tunnel inter-bandes permet de réaliser la spectroscopie électrique de la TAMR, résolue en énergie ou en impulsion. La comparaison de la dépendance en énergie des anisotropies cubique et uniaxiales montrent que le niveau de Fermi se trouve loin du haut de la bande de valence, et non piégé dans la minibande, et que les bandes de trous ont des contributions opposées à la TAMR, en accord qualitatif avec des calculs k.p. De plus, la dépendance en énergie de l anisotropie uniaxiale planaire précise son lien avec la mini-bande d impuretés Mn. La spectroscopie résolue en impulsion révèle aussi les effets de structure de bandes et de filtrage des états de Bloch, mais ne permet pas de déterminer les courbes de dispersion (transport multi-bandes et à 3D). Des résultats préliminaires offrent des perspectives d études sur la spectroscopie de TAMR réalisée par effet tunnel résonant via un niveau d énergie d un puits quantique, mais aussi sur l utilisation de la TAMR comme sonde du nanomagnétisme de plots individuels de GaMnAsFerromagnetism in highly-doped diluted magnetic semiconductor GaMnAs is mediated by delocalized carriers. Those are often described as Bloch states in the valence band of GaMnAs in presence of an exchange interaction. Nevertheless, the exact role of the Mn-impurity band overlapping the valence band is still under debate. A better knowledge of the exact band structure is still necessary to determine the actual Fermi level position, as well as to fully understand the true nature of bands anisotropies (valence bands, imurity band). We investigate the tunneling anisotropic magnetoresistance (TAMR) in ferromagnetic tunnel diodes p++-GaMnAs/n+-GaAs. Inter-band tunneling allows us to carry out the electrical spectroscopy of the TAMR, both in energy or impulse space. The comparison in the energy dependence of the cubic and uni-axial anisotropies suggests that the Fermi level is not pinned in the impurity band but lies deep into the valence band. The results further reveal the opposite contributions of different valence bands to TAMR, in qualitative agreement with k.p calculations, as well as an additional contribution to the in-plane uniaxial anisotropy due to the impurity band. Besides, spectroscopy in momentum space shows an enhancement of TAMR due to Bloch s states impulse filtering. Nevertheless, multi-bands tunnel spectroscopy of delocalized states in 3D space does not allow to determine the curves of dispersion. Preliminary results on the electrical spectroscopy of TAMR using resonant tunnelling through energy levels of a quantum well are also shown, as well as others on the use of TAMR to study nanomagnetism of an individual GaMnAs nanodot.ORSAY-PARIS 11-BU Sciences (914712101) / SudocSudocFranceF

Detection of current-induced resonance of geometrically confined domain walls

Magnetic domain walls are found to exhibit quasiparticle behavior when subjected to geometrical variations. Because of the spin torque effect such a quasiparticle in a potential well is excited by an ac current leading to a dip in the depinning field at resonance for current densities as low as 2 x 10 power 10 A/m power 2. Independently the resonance frequencies of transverse walls and vortex walls are determined from the dc voltage that develops due to a rectifying effect of the resonant domain wall oscillation. The dependence on the injected current density reveals a strongly nonharmonic oscillation