New publication: A two-dimensional Dirac fermion microscope
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.
New publication: Inter-valley dark trion states with spin lifetimes of 150 ns in WSe2
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.
phyphox in "Physik in unserer Zeit"
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)
New publication: Identifying suitable substrates for high-quality graphene-based heterostructures
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.
Fellowship for Digital Innovations in Academic Teaching
Spotlight: Luca Banszerus
The Graphene Flagship’s new Spotlight series aims to promote the contributions of the younger members of the Graphene Flagship. First up: Luca Banszerus discusses his excellent work in developing a reliable transfer process for high-quality CVD graphene (more).
Crossing the Atlantic with Graphene Flagship Mobility Grant
Luca Banszerus, PhD Student in our group is the first receiver of a Graphene Flagship Mobility Grant to the US. He will stay for six months at the lab of Professor Philip Kim at Harvard University. For more information please see: graphene-flagship.eu/mobility-grant-to-us
New publication: Growth, characterization, and transport properties of ternary (Bi1−x Sbx)2Te3 topological insulator layers
J. Phys.: Condens. Matter 28, 495501 (2016) Ternary (Bi1−xSbx)2Te3 films with an Sb content between 0 and 100% were deposited on a Si(1 1 1) substrate by means of molecular beam epitaxy. X-ray diffraction measurements confirm single crystal growth in all cases. The Sb content is determined by x-ray photoelectron spectroscopy. Consistent values of the Sb content are obtained from Raman spectroscopy. Scanning Raman spectroscopy reveals that the (Bi1−xSbx)2Te3 layers with an intermediate Sb content show spatial composition inhomogeneities. The observed spectra broadening in angular-resolved photoemission spectroscopy (ARPES) is also attributed to this phenomena. Upon increasing the Sb content from x = 0 to 1 the ARPES measurements show a shift of the Fermi level from the conduction band to the valence band. This shift is also confirmed by corresponding magnetotransport measurements where the conductance changes from n- to p-type. In this transition region, an increase of the resistivity is found, indicating a location of the Fermi level within the band gap region. More detailed measurements in the transition region reveals that the transport takes place in two independent channels. By means of a gate electrode the transport can be changed from n- to p-type, thus allowing a tuning of the Fermi level within the topologically protected surface states.
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