We report about investigations of time-dependent structural modifications in single-crystal graphene due to laser irradiation even at moderate power levels of 1 mW in a diffraction-limited spot. The modifications have been characterized by in situ scanning confocal Raman spectroscopy, atomic force height microscopy, and transport studies. The time evolution of the Raman spectrum reveals two different effects: on a short-time scale, dopants, initially present on the flake, are removed. The longer time scale behavior points to a laser induced gradual local decomposition of single-crystal graphene into a network of interconnected nanocrystallites with a characteristic length scale of approximately 10 nm due to bond breaking. The broken bonds offer additional docking sites for adsorbates as confirmed in transport and AFM height studies. These controlled structural modifications may for instance be valuable for enhancing the local reactivity, trimming graphene based gas sensors and generating spatially varying doping patterns.

We observe the total filling factor nuT=1 quantum Hall state in a bilayer two-dimensional electron system with virtually no tunneling. We find thermally activated transport in the balanced system with a monotonic increase of the activation energy with decreasing d/lB below 1.65. In the imbalanced system we find activated transport in each of the layers separately, yet the activation energies show a striking asymmetry around the balance point, implying a different excitation spectrum for the separate layers forming the condensed state.

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Edge magnetoplasmons propagating along the smooth boundary of a two-dimensional electron gas are studied by time-resolved magnetotransport experiments. The incident pulse splits up into several pulses due to modal dispersion. The observed delay times are well described by the predictions of Aleiner and Glazman for the fundamental and acoustic modes [Phys. Rev. Lett. 72, 2935], if the quantized Hall conductance is taken into account.[S0031-9007 (96)01655-9] PACS numbers: 73.20.Mf, 73.40.Hm, 73.50.JtPlasma oscillations of a two-dimensional electron gas (2DEG) acquire remarkable properties in the presence of a perpendicular magnetic field [1]. While the excitation of bulk plasmons requires a frequency equal to or larger than the cyclotron frequency v c , the spectrum of edge excitations remains gapless. The direction of allowed k vectors for these low energy excitations is determined by the confining potential and the direction of the Lorentz force acting on the moving carriers. For a given k vector, the edge magnetoplasmon (EMP) frequency decreases with increasing magnetic field.Experimentally, EMPs were discovered as low frequency eigenmodes of a confined 2DEG realized in a semiconductor heterostructure [2] or on the surface of liquid helium [3,4]. The EMP propagation direction was detected by phase sensitive measurements of the induced voltage on small electrodes around the perimeter of the 2DEG [5]. Time-domain measurements demonstrated the unidirectionality of the EMP motion in the quantum Hall effect (QHE) and the fractional QHE regime in a more intuitive way [6]. Using a 2DEG with a screening metallic electrode close by, the propagation of EMP wave packets confined to quantum Hall edge channels was identified in time-domain measurements [7].Recently, additional edge modes were observed, which were characterized by a resonance frequency below the frequency of the conventional EMP mode. An indication of a novel EMP mode was found in the nondegenerate 2DEG on the surface of liquid helium [8], which was explained in terms of oscillations of the boundary. A different set of EMP modes, for which the resonance frequency increased with increasing magnetic field, was observed for positive ions in a circular geometry [9]. These modes were identified with the edge modes predicted by Nazin and Shikin for a circular sheet of electrons on liquid helium with a smooth boundary [10].The theoretical description of plasma oscillations in a 2DEG starts with a seemingly simple set of elementary electrodynamic equations [1,11], which have to be solved self-consistently, in order to obtain the EMP dispersion relation. Since the electric fields are not restricted to the plane of the 2DEG, the problem is truly three dimensional. Analytic solutions could be obtained in a number of limiting cases (for a review, see [1]), and it was realized early that a smooth electron density profile permits additional (so called acoustic) EMP modes [1,10]. For a 2DEG in a semiconductor heterostructure, the dispersion relation for the fundamental mode...