Eduardo R. Mucciolo

Conductance quantization and transport gaps in disordered graphene nanoribbons

We study numerically the effects of edge and bulk disorder on the conductance of graphene nanoribbons. We compute the conductance suppression due to Anderson localization induced by edge scattering and find that even for weak edge roughness, conductance steps are suppressed and transport gaps are induced. These gaps are approximately inversely proportional to the nanoribbon width. On/off conductance ratios grow exponentially with the nanoribbon length. Our results impose severe limitations to the use of graphene in ballistic nanowires.

Adiabatic quantum pump of spin-polarized current.

We propose a mechanism by which an open quantum dot driven by two ac (radio frequency) gate voltages in the presence of a moderate in-plane magnetic field generates a spin-polarized, phase-coherent dc current. The idea combines adiabatic, nonquantized (but coherent) pumping through periodically modulated external parameters and the strong fluctuations of the electron wave function existent in chaotic cavities. We estimate that the spin polarization of the current can be observed for temperatures and Zeeman splitting energies of the order of the single-particle mean level spacing.

A Unified Description of the DC Conductivity of Monolayer and Bilayer Graphene Based on Resonant Scatterers

A.F. acknowledges FCT Grant No. SFRH/BPD/65600/2009. E.R.M. acknowledges partial financial support by NSF Grant No. DMR 1006230. A.H.C.N. acknowledges Grant No. DE-FG02-08ER46512. Discussions with A. K. Geim are acknowledged.

The recursive Green's function method for graphene

We describe how to apply the recursive Green’s function method to the computation of electronic transport properties of graphene sheets and nanoribbons in the linear response regime. This method allows for an amenable inclusion of several disorder mechanisms at the microscopic level, as well as inhomogeneous gating, finite temperature, and, to some extend, dephasing. We present algorithms for computing the conductance, density of states, and current densities for armchair and zigzag atomic edge alignments. Several numerical results are presented to illustrate the usefulness of the method.

Numerical studies of conductivity and Fano factor in disordered graphene

Using the recursive Greens function method, we study the problem of electron transport in a disordered single-layer graphene sheet. The conductivity is of order

A single-level tunnel model to account for electrical transport through single molecule-and self-assembled monolayer-based junctions

{e}^{2}∕h

Uncovering the dominant scatterer in graphene sheets on Si02

and its dependence on the carrier density has a scaling form that is controlled solely by the disorder strength and the ratio between the sample size and the correlation length of the disorder potential. The shot noise Fano factor is shown to be nearly density independent for sufficiently strong disorder, with a narrow structure appearing at the neutrality point only for weakly disordered samples. Our results are in good agreement with experiments and provide a way for extracting microscopic information about the magnitude of extrinsic disorder in graphene.

Kondo effect in single-molecule magnet transistors

We present a theoretical analysis aimed at understanding electrical conduction in molecular tunnel junctions. We focus on discussing the validity of coherent versus incoherent theoretical formulations for single-level tunneling to explain experimental results obtained under a wide range of experimental conditions, including measurements in individual molecules connecting the leads of electromigrated single-electron transistors and junctions of self-assembled monolayers (SAM) of molecules sandwiched between two macroscopic contacts. We show that the restriction of transport through a single level in solid state junctions (no solvent) makes coherent and incoherent tunneling formalisms indistinguishable when only one level participates in transport. Similar to Marcus relaxation processes in wet electrochemistry, the thermal broadening of the Fermi distribution describing the electronic occupation energies in the electrodes accounts for the exponential dependence of the tunneling current on temperature. We demonstrate that a single-level tunnel model satisfactorily explains experimental results obtained in three different molecular junctions (both single-molecule and SAM-based) formed by ferrocene-based molecules. Among other things, we use the model to map the electrostatic potential profile in EGaIn-based SAM junctions in which the ferrocene unit is placed at different positions within the molecule, and we find that electrical screening gives rise to a strongly non-linear profile across the junction.

Measuring the Lyapunov exponent using quantum mechanics.

We have measured the impact of atomic hydrogen adsorption on the electronic transport properties of graphene sheets as a function of hydrogen coverage and initial, pre-hydrogenation field-effect mobility. Our results are compatible with hydrogen adsorbates inducing intervalley mixing by exerting a short-range scattering potential. The saturation coverages for different devices are found to be proportional to their initial mobility, indicating that the number of native scatterers is proportional to the saturation coverage of hydrogen. By extrapolating this proportionality, we show that the field-effect mobility can reach

A tight-binding model for MoS2 monolayers

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