We studied magnetotransport in graphene nanoribbons in pulsed magnetic fields up to 60 Tesla. Close to the charge neutrality point, our samples show a high resistance which drops by about an order of magnitude at fields up to 20 Tesla, and then approaches a high field induced insulating state. At higher carrier densities we observe clear quantum Hall features. We can explain our data by assuming at least two different scattering mechanisms located at the sample edge and in the bulk. This is confirmed by corresponding transport simulations based on a tight binding model.
Looking at phase-coherent effects in nanoribbons at millikelvin temperatures, in single nanoribbons we observe strong universal conductance fluctuations, while in arrays of nanoribbons, ensemble averaging suppresses the UCFs and allows us to study weak localization. Both effects show that at the lowest temperatures the phase coherence length approaches 1 micrometer, clearly exceeding the ribbon width.
We also studied spin injection and detection in single and bilayer
graphene flakes with Co electrodes, with and without a tunnel barrier.
The evaluation of the Hanle effect allows us to determine the injection
efficiency, the spin lifetime and spin diffusion constant. Our results
confirm that spin and charge diffusion constant are identical and the
dependence of the spin lifetimes on diffusion constants indicates a
dominant Dyakonov-Perel mechanism in BLG and Elliot-Yafet mechanism in
SLG respectively, in line with experiments reported in the literature.
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