Nanotechnology 34, 475703 (2023)
In this work, we report on the growth of hexagonal boron nitride (hBN) crystals from an iron flux at atmospheric pressure and high temperature and demonstrate that (i) the entire sheet of hBN crystals can be detached from the metal in a single step using hydrochloric acid and that (ii) these hBN crystals allow to fabricate high carrier mobility graphene-hBN devices. By combining spatially-resolved confocal Raman spectroscopy and electrical transport measurements, we confirm the excellent quality of these crystals for high-performance hBN-graphene-based van der Waals heterostructures. The full width at half maximum of the graphene Raman 2D peak is as low as 16 cm⁻¹, and the room temperature charge carrier mobilitiy is around 80000 cm²/(Vs) at a carrier density 1 × 10¹² cm⁻¹². This is fully comparable with devices of similar dimensions fabricated using crystalline hBN synthesized by the high pressure and high temperature method. Finally, we show that for exfoliated high-quality hBN flakes with a thickness between 20 and 40 nm the line width of the hBN Raman peak, in contrast to the graphene 2D line width, is not useful for benchmarking hBN in high mobility graphene devices.

Prof. Annika Kurzmann has been appointed ML4Q Professor for Experimental Solid-State Physics at the University of Cologne. In 2021 she joined our group with a RWTH Junior Principal Investigator (JPI) Fellowship, which she used as a steppingstone toward a tenure-track position. A real success story not only for Annika, but also for ML4Q, the RWTH and for the Aachen Graphene & 2D Materials Center. Congratulations!

npj 2D Mater. Appl. 7, 58 (2023)
Transition metal dichalcogenides (TMDs) have attracted much attention in the fields of valley- and spintronics due to their property of forming valley-polarized excitons when illuminated by circularly polarized light. In TMD-heterostructures it was shown that these electron-hole pairs can scatter into valley-polarized interlayer exciton states, which exhibit long lifetimes and a twist-angle dependence. However, the question how to create a valley polarization of free charge carriers in these heterostructures after a valley selective optical excitation is unexplored, despite its relevance for opto-electronic devices. Here, we identify an interlayer transfer mechanism in twisted WSe₂/MoSe₂ heterobilayers that transfers the valley polarization from excitons in WSe₂ to free charge carriers in MoSe₂ with valley lifetimes of up to 12 ns. This mechanism is most efficient at large twist angles, whereas the valley lifetimes of free charge carriers are surprisingly short for small twist angles, despite the occurrence of interlayer excitons.

Phys. Rev. Lett. 130, 256901 (2023)
We report on resonance Raman spectroscopy measurements with excitation photon energy down to 1.16 eV on graphene, to study how low-energy carriers interact with lattice vibrations. Thanks to the excitation energy close to the Dirac point at K, we unveil a giant increase of the intensity ratio between the double-resonant 2D and 2D′ peaks with respect to that measured in graphite. Comparing with fully ab initio theoretical calculations, we conclude that the observation is explained by an enhanced, momentum-dependent coupling between electrons and Brillouin zone-boundary optical phonons. This finding applies to two-dimensional Dirac systems and has important consequences for the modeling of transport in graphene devices operating at room temperature.

Nature 618, 51 (2023)
Particle–hole symmetry plays an important role in the characterization of topological phases in solid-state systems. It is found, for example, in free-fermion systems at half filling and it is closely related to the notion of antiparticles in relativistic field theories. In the low-energy limit, graphene is a prime example of a gapless particle–hole symmetric system described by an effective Dirac equation in which topological phases can be understood by studying ways to open a gap by preserving (or breaking) symmetries. An important example is the intrinsic Kane–Mele spin-orbit gap of graphene, which leads to a lifting of the spin-valley degeneracy and renders graphene a topological insulator in a quantum spin Hall phase while preserving particle–hole symmetry. Here we show that bilayer graphene allows the realization of electron–hole double quantum dots that exhibit near-perfect particle–hole symmetry, in which transport occurs via the creation and annihilation of single electron–hole pairs with opposite quantum numbers. Moreover, we show that particle–hole symmetric spin and valley textures lead to a protected single-particle spin-valley blockade. The latter will allow robust spin-to-charge and valley-to-charge conversion, which are essential for the operation of spin and valley qubits.

On 28 April, we hosted a symposium in honour of the 80th birthday of Prof. Dr. Gernot Güntherodt former director of the 2nd Institute of Physics A (1987-2011).

The symposium featured engaging talks on physics topics related to Prof. Dr. Güntherodt's work, as well as personal stories from the speakers about their connections with him. Ulrich Rüdiger, Rector of RWTH Aachen University, opened the symposium with a welcome address. Stuart Parkin from the Max Planck Institute for Microstructure Physics in Halle (Saale) then presented "Memory on the racetrack", followed by Werner Hanke from the University of Würzburg who talked about room temperature excitons in a topological insulator. After a short break, Burkard Hillebrands from the TU Kaiserslautern talked about spin waves and magnonics. Ioana Slabu from the Institute of Applied Medical Technology at RWTH Aachen University spoke about precision medicine with magnetic nanomaterials. Finally, Dieter Vollhardt from the University of Augsburg closed the symposium with a talk on theories in the natural sciences.

Phys. Rev. Materials 7, 044004 (2023)
Using microfocused angle-resolved photoemission spectroscopy we investigated microstructures containing regions of single-layer (SL) and bilayer (BL) WSe₂ on graphite substrates at different twist angles between SL WSe₂ and graphite and within the BL WSe₂. Fermi level electrons emitted from the graphite are sharply focused near their K gr points in the Brillouin zone, and, when passing through the WSe₂, get diffracted to form band replicas readily observed in experimental Fermi surface maps from twisted SL WSe₂/graphite. We investigated two twisted BL WSe₂ at twist angles ∼28° and ∼10° and found no evidence of hybridization gaps at the interlayer band-crossing points, that could be precursors of the flat bands at smaller twist angles. Similarly, no such gaps were found for SL WSe₂/graphite. Experimental results are complemented by theoretical density functional theory calculations, which suggest that a formation of hybridization gaps in the WSe₂/graphene (which approximates the experimental WSe₂/graphite system) sensitively depends on the WSe₂ band character at the crossing point with the graphene Dirac band.

npj 2D Mater. Appl. 7, 29 (2023)
The environment contributes to the screening of Coulomb interactions in two-dimensional semiconductors. This can potentially be exploited to tailor material properties as well as for sensing applications. Here, we investigate the tuning of the band gap and the exciton binding energy in the two-dimensional semiconductor WS₂ via the external dielectric screening. Embedding WS₂ in van der Waals heterostructures with graphene and hBN spacers of thicknesses between one and 16 atomic layers, we experimentally determine both energies as a function of the WS₂-to-graphene interlayer distance and the charge carrier density in graphene. We find that the modification to the band gap as well as the exciton binding energy are well described by a one-over-distance dependence, with a significant effect remaining at several nm distance, at which the two layers are electrically well isolated. This observation is explained by a screening arising from an image charge induced by the graphene layer. Furthermore, we find that the effectiveness of graphene to screen Coulomb interactions in nearby WS₂ depends on its doping level and can therefore be controlled via the electric field effect. We determine that, at room temperature, it is modified by approximately 20 % for charge carrier densities of 2×10¹² cm^-2.