The participation of our Institute at the “Lousberglauf” 2017 was once again. Despite the suffocating heat, Michael (21:29), Isabell (29:46), Marc (23:25), Markus (23:41) and Alex (25.09) had a lot of fun and improved their performance compared to last year. Congratulations!

Appl. Phys. Lett. 110, 263110 (2017)
We report on the fabrication and characterization of high-quality chemical vapor-deposited (CVD) bilayer graphene (BLG). In particular, we demonstrate that CVD-grown BLG can mechanically be detached from the copper foil by an hexagonal boron nitride (hBN) crystal after oxidation of the copper-to-BLG interface. Confocal Raman spectroscopy reveals an AB-stacking order of the BLG crystals and a high structural quality. From transport measurements on fully encapsulated hBN/BLG/hBN Hall bar devices we extract charge carrier mobilities up to 180,000 cm²/(Vs) at 2 K and up to 40,000 cm²/(Vs) at 300 K, outperforming state- of-the-art CVD bilayer graphene devices. Moreover, we show an on-off ratio of more than 10,000 and a band gap opening with values of up to 15 meV for a displacement field of 0.2 V/nm in such CVD grown BLG.

Nature Communications 8, 15783 (2017)
The electron microscope has been a powerful, highly versatile workhorse in the fields of material and surface science, micro and nanotechnology, biology and geology, for nearly 80 years. The advent of two-dimensional materials opens new possibilities for realizing an analogy to electron microscopy in the solid state. Here we provide a perspective view on how a two-dimensional (2D) Dirac fermion-based microscope can be realistically implemented and operated, using graphene as a vacuum chamber for ballistic electrons. We use semiclassical simulations to propose concrete architectures and design rules of 2D electron guns, deflectors, tunable lenses and various detectors. The simulations show how simple objects can be imaged with well-controlled and collimated in-plane beams consisting of relativistic charge carriers. Finally, we discuss the potential of such microscopes for investigating edges, terminations and defects, as well as interfaces, including external nanoscale structures such as adsorbed molecules, nanoparticles or quantum dots.

Phys. Rev. B 95, 235408 (2017)
We demonstrate long trion spin lifetimes in a WSe₂ monolayer of up to 150ns at 5K. Applying a transverse magnetic field in time-resolved Kerr-rotation measurements reveals a complex composition of the spin signal of up to four distinct components. The Kerr-rotation signal can be well described by a model which includes inhomogeneous spin dephasing and by setting the trion spin lifetimes to the measured excitonic recombination times extracted from time-resolved reflectivity measurements. We observe a continuous shift of the Kerr resonance with the probe energy, which can be explained by an adsorbate-induced, inhomogeneous potential landscape of the WSe₂ flake. A further indication of extrinsic effects on the spin dynamics is given by a change of both the trion spin lifetime and the distribution of g factors over laboratory time. Finally, we detect a Kerr-rotation signal from the trion's higher-energy triplet state when the lower-energy singlet state is optically pumped by circularly polarized light. We explain this by the formation of dark trion states, which are also responsible for the observed long trion spin lifetimes.

A team from the 2nd Institute of physics participated at 1st “RWTH Campuslauf”. Arne Hollmann, Thomas Descamps, Katarina Menne and Michael Schmitz made a fantastic second place in the relay 4 x 2,3 km. Congratulations!

We have published an article in the (German) "Physik in unserer Zeit" presenting our app phyphox with an emphasis on its abilities for remotely controlled smartphone experiments. This is presented using the example of tracking the speed of a wheel rolling down a hill. Physik in unserer Zeit 48, 148-149 (2017)

2D Mater. 4, 025030 (2017)
We report on a scanning confocal Raman spectroscopy study investigating the strain-uniformity and the overall strain and doping of high-quality chemical vapour deposited (CVD) graphenebased heterostuctures on a large number of different substrate materials, including hexagonal boron nitride (hBN), transition metal dichalcogenides, silicon, different oxides and nitrides, as well as polymers. By applying a hBN-assisted, contamination free, dry transfer process for CVD graphene, high-quality heterostructures with low doping densities and low strain variations are assembled. The Raman spectra of these pristine heterostructures are sensitive to substrate-induced doping and strain variations and are thus used to probe the suitability of the substrate material for potential highquality graphene devices. We find that the flatness of the substrate material is a key figure for gaining, or preserving high-quality graphene.