We measure weak quasiparticle tunneling across a constriction in the second Landau level. At ν = 7/3, 8/3 and 5/2, comparison of temperature and DC bias dependence to weak tunneling theory allows extracting parameters that describe the edges' quasiparticle excitations. At ν = 8/3, our results are well described by a particle-hole conjugate Laughlin state, but not compatible with proposed non-Abelian quasiparticle excitations. For ν = 5/2, our measurements are in good agreement with previous experiments and favor the Abelian (3,3,1) or (1,1,3)-states. At these filling factors, we further investigate the influence of the backscattering strength on the extracted scaling parameters. For ν = 7/3, the backscattering strength strongly affects the scaling parameters, whereas quasiparticle tunneling at ν = 8/3 and 5/2 appears more robust. Our results provide important additional insight about the physics in the second Landau level and contribute to the understanding of the physics underlying the fractional quantum Hall states at ν = 7/3, 8/3 and 5/2.

Quantum point contacts are fundamental building blocks for mesoscopic transport experiments and play an important role in recent interference-and fractional quantum Hall experiments. However, it is not clear how electron-electron interactions and the random disorder potential influence the confinement potential and give rise to phenomena like the mysterious 0.7 anomaly. Novel growth techniques of Al X Ga 1−X As heterostructures for high-mobility two-dimensional electron gases enable us to investigate quantum point contacts with a strongly suppressed disorder potential. These clean quantum point contacts indeed show transport features that are obscured by disorder in standard samples. From this transport data, we are able to extract the parameters of the confinement potential which describe its shape in longitudinal and transverse direction. Knowing the shape (and hence the slope) of the confinement arXiv:1106.2982v2 [cond-mat.mes-hall]

The coupling between a two-level system and its environment leads to decoherence. Within the context of coherent manipulation of electronic or quasiparticle states in nanostructures, it is crucial to understand the sources of decoherence. Here, we study the effect of electronphonon coupling in a graphene and an InAs nanowire double quantum dot. Our measurements reveal oscillations of the double quantum dot current periodic in energy detuning between the two levels. These periodic peaks are more pronounced in the nanowire than in graphene, and disappear when the temperature is increased. We attribute the oscillations to an interference effect between two alternative inelastic decay paths involving acoustic phonons present in these materials. This interpretation predicts the oscillations to wash out when temperature is increased, as observed experimentally.Coherent spin manipulation has already been accomplished in AlGaAs/GaAs double quantum dots (DQDs) 1, 2 and, more recently, also in InAs nanowires (NWs) 3 . While the coherence times are usually limited by random nuclear fields 4 , also electron-phonon coupling can be a source of decoherence 5 . InAs nanowires (NW) and graphene are two alternative and promising materials for achieving coherent spin manipulation. In InAs NW DQDs, spin-orbit interactions (SOI) are very strong and enable a more efficient electron spin resonance driven by SOI compared to AlGaAs/GaAs DQDs 3 . In graphene, it is expected that hyperfine coupling as a source of decoherence is very weak compared to AlGaAs/GaAs. While electron-phonon interaction effects have been observed in carbon nanotube 6, 7 , AlGaAs/GaAs 8 , or silicon quantum dots (QDs) 9 and in AlGaAs/GaAs DQDs 10, 11 , only little is known about electron-phonon interaction in graphene and InAs nanowires.Almost 60 years ago, Dicke predicted superradiant and subradiant spontaneous emission 12 , which was observed 40 years later with two trapped ions 13 . In this experiment, the spontaneous emission rate Γ(R) of a two-ion crystal excited by a short laser pulse was studied as a function of the ion-ion separation R. Superradiant (subradiant) spontaneous emission was observed with Γ(R) > Γ 0 (Γ(R) < Γ 0 ), where Γ 0 is the emission rate of a single ion. In analogy to the Dicke subradiance phenomenon, Brandes et al. 14 later proposed an interference effect due to electronphonon interactions in a solid-state two-level system (DQD). Our experimental observations are interpreted in this framework.

The Dresselhaus spin-orbit interaction ͑SOI͒ of a series of two-dimensional electron gases hosted in GaAs/ AlGaAs and InGaAs/GaAs ͑001͒ quantum wells ͑QWs͒ is measured by monitoring the precession frequency of the spins as a function of an in-plane electric field. The measured spin-orbit-induced spin splitting is linear in the drift velocity, even in the regime where the cubic Dresselhaus SOI is important. We relate the measured splitting to the Dresselhaus coupling parameter ␥, the QW confinement, the Fermi wave number k F , and strain effects. From this, ␥ is determined quantitatively, including its sign.

The fractional quantized Hall state at the filling factor ν = 5/2 is of special interest due to its possible application for quantum computing. Here we report on the optimization of growth parameters that allowed us to produce twodimensional electron gases (2DEGs) with a 5/2 gap energy up to 135 mK. We concentrated on optimizing the molecular beam epitaxy (MBE) growth to provide high 5/2 gap energies in 'as-grown' samples, without the need to enhance the 2DEGs properties by illumination or gating techniques. Our findings allow us to analyse the impact of doping in narrow quantum wells with respect to conventional DX-doping in Al x Ga 1−x As. The impact of the setback distance between doping layer and 2DEG was investigated as well. Additionally, we found a considerable increase in gap energy by reducing the amount of background impurities. To this end growth techniques like temperature reductions for substrate and effusion cells and the reduction of the Al mole fraction in the 2DEG region were applied.