Keyan Bennaceur

Transport and elastic scattering times as probes of the nature of impurity scattering in single-layer and bilayer graphene.

Transport and elastic scattering times, tau{tr} and tau{e}, are experimentally determined from the carrier density dependence of the magnetoconductance of monolayer and bilayer graphene. Both times and their dependences on carrier density are found to be very different in the monolayer and the bilayer. However, their ratio tau{tr}/tau{e} is found to be close to 1.8 in the two systems and nearly independent of the carrier density. These measurements give insight on the nature (neutral or charged) and range of the scatterers. Comparison with theoretical predictions suggests that the main scattering mechanism in our samples is due to strong (resonant) scatterers of a range shorter than the Fermi wavelength, likely candidates being vacancies, voids, adatoms or short-range ripples.

Carrier drift velocity and edge magnetoplasmons in graphene.

We investigate electron dynamics at the graphene edge by studying the propagation of collective edge magnetoplasmon excitations. By timing the travel of narrow wave packets on picosecond time scales around exfoliated samples, we find chiral propagation with low attenuation at a velocity that is quantized on Hall plateaus. We extract the carrier drift contribution from the edge magnetoplasmon propagation and find it to be slightly less than the Fermi velocity, as expected for an abrupt edge. We also extract the characteristic length for Coulomb interaction at the edge and find it to be smaller than that for soft depletion-edge systems.

Unveiling quantum Hall transport by Efros-Shklovskii to Mott variable-range hopping transition in graphene

The quantum localization in the quantum Hall regime is revisited using Graphene monolayers with accurate measurements of the longitudinal resistivity as a function of temperature and current. We experimentally show for the first time a cross-over from Efros-Shklovskii Variable Range Hopping (VRH) conduction regime with Coulomb interactions to a Mott VRH regime without interaction. This occurs at Hall plateau transitions for localization lengths larger than the interaction screening length set by the nearby gate. Measurements of the scaling exponents of the conductance peak widths with both temperature and current give the first validation of the Polyakov-Shklovskii scenario that VRH alone is sufficient to describe conductance in the Quantum Hall regime and that the usual assumption of a metallic conduction regime on conductance peaks is unnecessary.

Quantum hall effect in hydrogenated graphene

The quantum Hall effect is observed in a two-dimensional electron gas formed in millimeter-scale hydrogenated graphene, with a mobility less than 10  cm2/V·s and corresponding Ioffe-Regel disorder parameter (k(F)λ)(-1) ≫ 1. In a zero magnetic field and low temperatures, the hydrogenated graphene is insulating with a two-point resistance of the order of 250h/e2. The application of a strong magnetic field generates a negative colossal magnetoresistance, with the two-point resistance saturating within 0.5% of h/2e2 at 45 T. Our observations are consistent with the opening of an impurity-induced gap in the density of states of graphene. The interplay between electron localization by defect scattering and magnetic confinement in two-dimensional atomic crystals is discussed.

Mechanical flip-chip for ultra-high electron mobility devices

Electrostatic gates are of paramount importance for the physics of devices based on high-mobility two-dimensional electron gas (2DEG) since they allow depletion of electrons in selected areas. This field-effect gating enables the fabrication of a wide range of devices such as, for example, quantum point contacts (QPC), electron interferometers and quantum dots. To fabricate these gates, processing is usually performed on the 2DEG material, which is in many cases detrimental to its electron mobility. Here we propose an alternative process which does not require any processing of the 2DEG material other than for the ohmic contacts. This approach relies on processing a separate wafer that is then mechanically mounted on the 2DEG material in a flip-chip fashion. This technique proved successful to fabricate quantum point contacts on both GaAs/AlGaAs materials with both moderate and ultra-high electron mobility.

Second Landau level fractional quantum Hall effects in the Corbino geometry

Abstract For certain measurements, the Corbino geometry has a distinct advantage over the Hall and van der Pauw geometries, in that it provides a direct probe of the bulk 2DEG without complications due to edge effects. This may be important in enabling detection of the non-Abelian entropy of the 5/2 fractional quantum Hall state via bulk thermodynamic measurements. We report the successful fabrication and measurement of a Corbino-geometry sample in an ultra-high mobility GaAs heterostructure, with a focus on transport in the second and higher Landau levels. In particular, we report activation energy gaps of fractional quantum Hall states, with all edge effects ruled out, and extrapolate σ 0 from the Arrhenius fits. Our results show that activated transport in the second Landau level remains poorly understood. The development of this Corbino device opens the possibility to study the bulk of the 5/2 state using techniques not possible in other geometries.

Measurement of topological Berry phase in highly disordered graphene

We have observed the quantum Hall effect (QHE) and Shubnikov-de Haas (SdH) oscillations in highly disordered graphene at magnetic fields up to 65 T. Disorder was introduced by hydrogenation of graphene up to a ratio H/C

Specific heat and entropy of fractional quantum Hall states in the second Landau level

\approx 0.1\%

Competing Charge Density Waves Probed by Nonlinear Transport and Noise in the Second and Third Landau Levels

. The analysis of SdH oscillations and QHE indicates that the topological part of the Berry phase, proportional to the pseudo-spin winding number, is robust against introduction of disorder by hydrogenation in large scale graphene.

Mechanical Flip-Chip for Ultra-High Electron Mobility Devices

Specific heat has had an important role in the study of superfluidity and superconductivity, and could provide important information on the fractional quantum Hall effect as well. However, tra- ditional measurements of the specific heat of a two-dimensional electron gas are difficult due to the large background contribution of the phonon bath, even at very low temperatures. Here, we report measurements of the specific heat per electron of a single two-dimensional electron gas with no contribution from the phonon bath or other addenda. This is possible by measuring the ther- malization time between the electrons and phonons. Furthermore, by using the Corbino geometry we ensure that we measure the properties of the bulk rather than spurious edge effects. We ob- serve activated behaviour of the specific heat, and infer quasiparticle effective masses of 1.3m_e and 2.2m_e for {\nu} = 5/2 and 7/3, respectively. We also see evidence for a peak in the specific heat at around 100 mK. Finally, extending the technique to lower temperatures could lead to detection of the non-Abelian entropy predicted to occur in the bulk quasiparticles at 5/2 half-filling.