Evanescent wave transport and shot noise in graphene: ballistic regime and effect of disorder

We have investigated electrical transport and shot noise in graphene field effect devices. In large width over length ratio $W/L$ graphene strips, we have measured shot noise at low frequency ($f$ = 600--850 MHz) in the temperature range of 4.2--30 K. We observe a minimum conductivity of $\frac{4e^{2}}{\pi h}$ and a finite and gate dependent Fano factor reaching the universal value of 1/3 at the Dirac point, i.e. where the density of states vanishes. These findings are in good agreement with the theory describing that transport at the Dirac point should occur via evanescent waves in perfect graphene samples with large $W/L$. Moreover, we show and discuss how disorder and non-parallel leads affect both conductivity and shot noise.Comment: Extended version (19 pages, 10 figures) of Phys. Rev. Lett. 100, 196802 (2008). Additional data on the effect of disorder and non-parallel leads. Submitted for publication in Journal of Low Temperature Physics for the Proceedings of the International Symposium on Quantum Phenomena and Devices at Low Temperatures (ULTI 2008), Espoo, Finlan

Quantum transport in graphene

After the experimental discovery of graphene -a single atomic layer of graphite- a scientific rush started to explore graphene’s electronic behaviour. Graphene is a fascinating two-dimensional electronic system, because its electrons behave as relativistic particles. Moreover, it is a promising material for future high-speed nano-electronic applications. In this thesis, several experiments are described to reveal graphene’s electronic transport properties. We have shown that we can control the bandstructure of bilayer and trilayer graphene. Simply by applying a perpendicular electric field in a graphene device, we could tune the bandgap in the bilayer and the bandoverlap in the trilayer. Furthermore, we have described transport measurements on graphene devices (length = 0.1-1 micrometer) showing that electronic transport in graphene is phase coherent at cryogenic temperatures (4 K or less). We have observed weak localization, bipolar supercurrents and the Aharonov-Bohm effect. We have also shown that in narrow graphene nanoribbons (width less than 100 nm) a transport gap appears, which can be well explained by strong localization of electronic states. Our experimental results provide a better understanding of electronic transport in graphene, and are also a first step towards the realization of graphene nano-electronic devices.Kavli Institute of Nanoscience DelftApplied Science

Manifestations of phase-coherent transport in graphene: The Josephson effect, weak localization, and aperiodic conductance fluctuations

The electronic transport properties of graphene exhibit pronounced differences from those of conventional two dimensional electron systems investigated in the past. As a consequence, well established phenomena such as the integer quantum Hall effect and weak localization manifest themselves differently in graphene. Here we present an overview of recent experiments that we have performed to probe phase coherent transport. In particular, we have investigated in great detail Josephson supercurrent and superconducting proximity effect in junctions consisting of a graphene layer in between superconducting electrodes. We have also used the same devices to measure aperiodic conductance fluctuations and weak localization. The experimental results clearly indicate that lowtemperature transport in graphene is phase coherent on a ?1 ?m length scale, irrespective of the position of the Fermi level. We discuss the different behavior of Josephson supercurrent and weak localization in terms of the unusual properties of the electronic states in graphene upon time reversal symmetry.kavli institute of nanoscienceApplied Science

Ballistic transport in bilayer nano-graphite ribbons under gate and magnetic fields

73.23.Ad Ballistic transport, 73.21.Ac Multilayers, 73.43.Qt Magnetoresistance,

Influence of disorder on the magnetism of graphene bilayers

73.22.-f Electronic structure of nanoscale materials: clusters, nanoparticles, nanotubes, and nanocrystals, 73.23.-b Electronic transport in mesoscopic systems, 75.30.-m Intrinsic properties of magnetically ordered materials,