Temperature dependence of the ``0.7'' 2(e^2)/h quasi plateau in strongly confined quantum point contacts

We present new results of the ``0.7'' 2(e^2)/h structure or quasi plateau in some of the most strongly confined point contacts so far reported. This strong confinement is obtained by a combination of shallow etching and metal gate deposition on modulation doped GaAs/GaAlAs heterostructures. The resulting subband separations are up to 20 meV, and as a consequence the quantized conductance can be followed at temperatures up to 30 K, an order of magnitude higher than in conventional split gate devices. We observe pronounced quasi plateaus at several of the lowest conductance steps all the way from their formation around 1 K to 30 K, where the entire conductance quantization is smeared out thermally. We study the deviation of the conductance from ideal integer quantization as a function of temperature, and we find an activated behavior, exp(-T_a/T), with a density dependent activation temperature T_a of the order of 2 K. We analyze our results in terms of a simple theoretical model involving scattering against plasmons in the constriction.Comment: RevTex (4 pages) including 2 postscript figures. To appear in Physica B, 199

Fabry-Perot interference, Kondo effect and Coulomb blockade in carbon nanotubes

High quality single wall carbon nanotube quantum dots have been made showing both metallic and semiconducting behavior. Some of the devices are identified as small band gap semiconducting nanotubes with relatively high broad conductance oscillations for hole transport through the valence band and low conductance Coulomb blockade oscillations for electron transport through the conduction band. The transition between these regimes illustrates that transport evolves from being wave-like transmission known as Fabry-Perot interference to single particle-like tunneling of electrons or holes. In the intermediate regime four Coulomb blockade peaks appear in each Fabry-Perot resonance, which is interpreted as entering the SU(4) Kondo regime. A bias shift of opposite polarity for the Kondo resonances for one electron and one hole in a shell is in some cases observed.Comment: Proceeding for the International workshop on Quantum Coherence, Noise and decoherence in nanostructures (DECON'06). See published version for improved figure qualit

NON-EQUILIBRIUM EFFECTS IN CONSTRICTED SUPERCONDUCTORS

Des effets variés sont observés sur un supraconducteur hors-équilibre quand un voltage est appliqué aux bornes d'un conducteur de dimensions restreintes (longueur ℓ, largeur w, épaisseur d). La petite dimension est choisie inférieure à la longueur de cohérence ξ pour éviter les instabilités. Nous discutons des effets observés dans un système où deux supraconducteurs massifs sont reliés par un conducteur de petites dimensions soit d, w<ξ ("strips"), soit d, ℓ<ξ (pont large) soit enfin d, w, ℓ<ξ (micropont).Various non-equilibrium effects as a result of a voltage can be observed in superconductors where either the length ℓ, the width w or the thickness d is constricted. The dimensions of the constriction is normally chosen to be smaller than the coherence length ξ in order to avoid instabilities. We review some effects found in systems where two large superconductors are connected by a constriction with d, w<ξ (strips), d, ℓ<ξ (wide bridges) and d, w, ℓ<ξ (microbridges)