Phys. Status Solidi B (2014)
Time-resolved magneto-optics is a well-established optical pump–probe technique to generate and to probe spin coherence in semiconductors. By this method, spin dephasing times T₂* can easily be determined if their values are comparable to the available pump–probe delays. If T₂* exceeds the laser repetition time, however, resonant spin amplification (RSA) can equally be used to extract T₂* image. We demonstrate that in ZnO these techniques have several tripping hazards resulting in deceptive values for T₂* and show how to avoid them. We show that the temperature dependence of the amplitude ratio of two separate spin species can easily be misinterpreted as a strongly temperature-dependent T₂* of a single spin ensemble, while the two spin species have T₂* values, which are nearly independent of temperature. Additionally, consecutive pump pulses can significantly diminish the spin polarization, which remains from previous pump pulses. While this barely affects T₂* values extracted from delay line scans, it results in seemingly shorter inline image values in RSA.

Appl. Phys. Lett. 104, 083105 (2014)
We present electronic transport measurements on etched graphene nanoribbons on silicon dioxide before and after a short hydrofluoric acid (HF) treatment. We report on changes in the transport properties, in particular, in terms of a decreasing transport gap and a reduced doping level after HF dipping. Interestingly, the effective energy gap is nearly unaffected by the HF treatment. Additional measurements on a graphene nanoribbon with lateral graphene gates support strong indications that the HF significantly modifies the edges of the investigated nanoribbons leading to a significantly reduced disorder potential in these graphene nanostructures.

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Phys. Rev. B 89, 115406 (2014)
We present numerical simulations of the capacitive coupling between graphene nanoribbons of various widths and gate electrodes in different configurations. We compare the influence of lateral metallic or graphene side gate structures on the overall back gate capacitive coupling. Most interestingly, we find a complex interplay between quantum capacitance effects in the graphene nanoribbon and the lateral graphene side gates, giving rise to an unconventional negative quantum capacitance. The emerging nonlinear capacitive couplings are investigated in detail. The experimentally relevant relative lever arm, the ratio between the coupling of the different gate structures, is discussed.

The Söllerhaus-Workshop 2014 of the 2^nd Institute of Physics was a great success.

The Master College Physics in Aachen has now a facebook site:
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Appl. Phys. Lett. 104, 062406 (2014)
We demonstrate all-electrical spin generation and subsequent manipulation by two successive electric field pulses in an n-InGaAs heterostructure in a time-resolved experiment at zero external magnetic field. The first electric field pulse along the [110] crystal axis creates a current-induced spin polarization (CISP) which is oriented in the plane of the sample. The subsequent electric field pulse along [110] generates a perpendicular magnetic field pulse leading to a coherent precession of this spin polarization with 2-dimensional electrical control over the final spin orientation. Spin precession is probed by time-resolved Faraday rotation. We determine the build-up time of CISP during the first field pulse and extract the spin dephasing time and internal magnetic field strength during the spin manipulation pulse.