We describe measurements of spin-polarized tunnelling via discrete energy levels of single Aluminum grains. In high resistance samples (∼ GΩ), the spin-polarized tunnelling current rapidly saturates as a function of the bias voltage. This indicates that spin-polarized current is carried only via the ground state and the few lowest in energy excited states of the grain. At the saturation voltage, the spin-relaxation rate T to the electron-phonon relaxation rate is in agreement with the Elliot-Yafet scaling, an evidence that spin-relaxation in Al grains is governed by the spin-orbit interaction.

We present transfer-length-method measurements of the contact resistance between Cu and graphene, and a method to significantly reduce the contact resistance by vacuum annealing. Even in samples with heavily contaminated contacts, the contacts display very low contact resistance post annealing. Due to the common use of Cu, and its low chemical reactivity with graphene, thermal annealing will be important for future graphene devices requiring non-perturbing contacts with low contact resistance.

We present a technique to fabricate tunnel junctions between graphene and Al and Cu, with a Si back gate, as well as a simple theory of tunneling between a metal and graphene. We map the differential conductance of our junctions versus probe and back gate voltage, and observe fluctuations in the conductance that are directly related to the graphene density of states. The conventional strong-suppression of the conductance at the graphene Dirac point can not be clearly demonstrated, but a more robust signature of the Dirac point is found: the inflection in the conductance map caused by the electrostatic gating of graphene by the tunnel probe. We present numerical simulations of our conductance maps, confirming the measurement results.In addition, Al causes strong n-doping of graphene, Cu causes a moderate p-doping, and in high resistance junctions, phonon resonances are observed, as in STM studies. 1 arXiv:1011.5067v1 [cond-mat.mes-hall]

A model describing spin-polarized current via discrete energy levels of a metallic nanoparticle, which has strongly asymmetric tunnel contacts to two ferromagnetic leads, is presented. In absence of spin relaxation, the model leads to a spin accumulation in the nanoparticle, a difference ͑⌬͒ between the chemical potentials of spin-up and spin-down electrons, proportional to the current and the Julliere tunnel magnetoresistance. Taking into account an energy dependent spin-relaxation rate ⍀͑͒, ⌬ as a function of bias voltage ͑V͒ exhibits a crossover from linear to a much weaker dependence, when ͉e͉⍀͑⌬͒ equals the spin-polarized current through the nanoparticle. Assuming that the spin relaxation takes place via electron-phonon emission and Elliot-Yafet mechanism, the model leads to a crossover from linear to V 1/5 dependence. The crossover explains recent measurements of the saturation of the spin-polarized current with V in aluminum nanoparticles, and leads to the spin-relaxation rate of Ϸ1.6 MHz in an aluminum nanoparticle of diameter 6 nm, for a transition with an energy difference of one level spacing.

The effects of Au grains on graphene conduction and doping are investigated in this report. To obtain a clean Au-graphene contact, Au grains are deposited over graphene at elevated temperature and in high vacuum, before any chemical processing. The bulk and the effective contact resistance versus gate voltage demonstrate that Au grains cause p-doping in graphene. The Fermi level shift is in agreement with first principles calculations, but the equilibrium separation we find between the graphene and the top-most Au layer is larger than predicted. Nonequilibrium electron transport displays giant-phonon thresholds observed in graphene tunnel junctions, demonstrating the tunneling nature of the contact, even though there are no dielectrics involved.