Physica Status Solidi B (online)
We present a thermal annealing study on single-layer and bilayer (BLG) graphene encapsulated in hexagonal boron nitride. The samples are characterized by electron transport and Raman spectroscopy measurements before and after each annealing step. While extracted material properties such as charge carrier mobility, overall doping, and strain are not influenced by the annealing, an initial annealing step lowers doping and strain variations and thus results in a more homogeneous sample. Additionally, the narrow 2D-sub-peak widths of the Raman spectrum of BLG, allow us to extract information about strain and doping values from the correlation of the 2D-peak and the G-peak positions.

Phys. Rev. Lett. 113, 126801 (2014)
We present transport measurements on high-mobility bilayer graphene fully encapsulated in hexagonal boron nitride. We show two terminal quantum Hall effect measurements which exhibit full symmetry broken Landau levels at low magnetic fields. From weak localization measurements, we extract gate-tunable phase coherence times τ_φ as well as the inter- and intra-valley scattering times τ_i and τ_∗. While τ_φ is in qualitative agreement with an electron-electron interaction mediated dephasing mechanism, electron spin-flip scattering processes are limiting τ_φ at low temperatures. The analysis of τ_i and τ_∗ points to local strain fluctuation as the most probable mechanism for limiting the mobility in high-quality bilayer graphene.

Phys. Status Solidi B (2014)
The front cover schematically shows three important aspects of the experiments. First, the lattice mismatch between ZnO and Al₂O₃ is reduced by rotating the ZnO lattice by 30°. Second, dilute magnetic doping with cobalt results in phase segregation and the formation of nanometer-sized metallic clusters, as indicated via the bright red contrast of the energy-filtered TEM image (background). Third, the sketched device allows for studying spin transport properties. A spin-polarized current from a magnetic Co electrode is injected into ZnO, and the spin polarization is determined optically via the polarization of reflected photons. The source files for the front cover image were prepared by Matthias Althammer.

Christoph Stampfer has been selected and honoured as one of the “30 exceptional scientists under the age of 40” in the 2014 World Economic Forum.

Professor Christoph Stampfer has been selected as a Young Scientist by the World Economic Forum, recognizing his “expertise and thought leadership” in the field of Experimental Physics. The appointment comes with an exclusive invitation to this year’s Annual New Champions meeting in Tianjin, China, alongside leading figures in business and politics.

Each year, the World Economic Forum selects 30 exceptional scientists below the age of 40 to participate alongside business and political leaders in the Annual Meeting of the New Champions in China. The winners are scientists chosen world-wide, representing a wide range of disciplines. The selected “Young Scientists” bring value to the Annual Meeting by “contributing their scientific perspective and delivering the most up-to-date trends from various fields of science.”

For more information please see http://www.weforum.org/news/world-economic-forum-honours-its-2014-young-scientists-community-annual-meeting-new-champions.

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.