Nano Lett. 14, 6050 (2014)
We present a new fabrication method of graphene spin-valve devices that yields enhanced spin and charge transport properties by improving both the electrode-to-graphene and graphene-to-substrate interface. First, we prepare Co/MgO spin injection electrodes onto Si⁺⁺/SiO₂. Thereafter, we mechanically transfer a graphene–hBN heterostructure onto the prepatterned electrodes. We show that room temperature spin transport in single-, bi-, and trilayer graphene devices exhibit nanosecond spin lifetimes with spin diffusion lengths reaching 10 μm combined with carrier mobilities exceeding 20,000 cm²/(Vs).

Phys. Rev. B 90, 165403 (2014)
By successive oxygen treatments of graphene nonlocal spin-valve devices we achieve a gradual increase of the contact-resistance–area products (R_cA) of Co/MgO spin injection and detection electrodes and a transition from linear to nonlinear characteristics in the respective differential dV−dI curves. With this manipulation of the contacts, both spin lifetime and the amplitude of the spin signal can significantly be increased by a factor of seven in the same device. This demonstrates that contact-induced spin dephasing is the bottleneck for spin transport in graphene devices with small R_cA values. With increasing R_cA values, we furthermore observe the appearance of a second charge neutrality point (CNP) in gate-dependent resistance measurements. Simultaneously, we observe a decrease of the gate voltage separation between the two CNPs. The strong enhancement of the spin-transport properties as well as the changes in charge transport are explained by a gradual suppression of a Co-graphene interaction by improving the oxide barrier during oxygen treatment.

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.