Quantum pumping in graphene nanoribbons at resonant transmission

Adiabatic quantum charge pumping in graphene nanoribbon double barrier structures with armchair and zigzag edges in the resonant transmission regime is analyzed. Using recursive Green's function method we numerically calculate the pumped charge for pumping contours encircling a resonance. We find that for armchair ribbons the whole resonance line contributes to the pumping of a single electron (ignoring double spin degeneracy) per cycle through the device. The case of zigzag ribbons is more interesting due to zero-conductance resonances. These resonances separate the whole resonance line into several parts, each of which corresponds to the pumping of a single electron through the device. Moreover, in contrast to armchair ribbons, one electron can be pumped from the left lead to the right one or backwards. The current direction depends on the particular part of the resonance line encircled by the pumping contour.Comment: 6 pages, 5 figures. This is an author-created, un-copyedited version of an article accepted for publication in EPL. IOP Publishing Ltd is not responsible for any errors or omissions in this version of the manuscript or any version derived from it. The definitive publisher authenticated version is available online at 10.1209/0295-5075/92/4701

Probing interactions in mesoscopic gold wires

We have measured in gold wires the energy exchange rate between quasiparticles, the phase coherence time of quasiparticles and the resistance vs. temperature, in order to probe the interaction processes which are relevant at low temperatures. We find that the energy exchange rate is higher than expected from the theory of electron-electron interactions, and that it has a different energy dependence. The dephasing time is constant at temperatures between 8 K and 0.5 K, and it increases below 0.5 K. The magnetoresistance is negative at large field scales, and the resistance decreases logarithmically with increasing temperatures, indicating the presence of magnetic impurities, probably Fe. Whereas resistivity and phase coherence measurements can be attributed to magnetic impurities, the question is raised whether these magnetic impurities could also mediate energy exchanges between quasiparticles.Comment: latex pothier.tex, 12 files, 15 pages in: Proceedings of the NATO Advanced Research Workshop on Size Dependent Magnetic Scattering, Pesc, Hungary, May 28 - June 1st, 2000 Chandrasekhar V., Van Haesendonck C. eds (Kluwer, 2001) [SPEC-S00/083

Occupational lead neurotoxicity: Improvement in behavioural effects after reduction of exposure.

To evaluate critical exposure levels and the reversibility of lead neurotoxicity a group of lead exposed foundry workers and an unexposed reference population were followed up for three years. During this period, tests designed to monitor neurobehavioural function and lead dose were administered. Evaluations of 160 workers during the first year showed dose dependent decrements in mood, visual/motor performance, memory, and verbal concept formation. Subsequently, an improvement in the hygienic conditions at the plant resulted in striking reductions in blood lead concentrations over the following two years. Attendant improvement in indices of tension (20% reduction), anger (18%), depression (26%), fatigue (27%), and confusion (13%) was observed. Performance on neurobehavioural testing generally correlated best with integrated dose estimates derived from blood lead concentrations measured periodically over the study period; zinc protoporphyrin levels were less well correlated with function. This investigation confirms the importance of compliance with workplace standards designed to lower exposures to ensure that individual blood lead concentrations remain below 50 micrograms/dl

Nonequilibrium Electron Distribution in Presence of Kondo Impurities

We study the energy relaxation of quasiparticles in voltage biased mesoscopic wires in presence of magnetic impurities. The renormalization of the exchange interaction of Kondo impurities coupled to conduction electrons is extended to the case of a nonequilibrium electron distribution, which is determined self-consistently from a Boltzmann equation with a collision term due to Kondo impurity mediated electron-electron scattering. The approach leads to predictions in quantitative agreement with recent experiments by Pothier et al. [Phys. Rev. Lett. 79, 3490 (1997)].Comment: 4 pages, 3 figure

Andreev Reflections in Micrometer-Scale Normal-Insulator-Superconductor Tunnel Junctions

Understanding the subgap behavior of Normal-Insulator-Superconductor (NIS) tunnel junctions is important in order to be able to accurately model the thermal properties of the junctions. Hekking and Nazarov developed a theory in which NIS subgap current in thin-film structures can be modeled by multiple Andreev reflections. In their theory, the current due to Andreev reflections depends on the junction area and the junction resistance area product. We have measured the current due to Andreev reflections in NIS tunnel junctions for various junction sizes and junction resistance area products and found that the multiple reflection theory is in agreement with our data

Magnetic field effects in energy relaxation mediated by Kondo impurities

We study the energy distribution function of quasiparticles in voltage biased mesoscopic wires in presence of magnetic impurities and applied magnetic field. The system is described by a Boltzmann equation where the collision integral is determined by coupling to spin 1/2 impurities. We derive an effective coupling to a dissipative spin system which is valid well above Kondo temperature in equilibrium or for sufficiently smeared distribution functions in non-equilibrium. For low magnetic field an enhancement of energy relaxation is found whereas for larger magnetic fields the energy relaxation decreases again meeting qualitatively the experimental findings by Anthore et al. (cond-mat/0109297). This gives a strong indication that magnetic impurities are in fact responsible for the enhanced energy relaxation in copper wires. The quantitative comparison, however, shows strong deviations for energy relaxation with small energy transfer whereas the large energy transfer regime is in agreement with our findings.Comment: 14 pages, 8 figure