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.

Sebastian Staacks and Christoph Stampfer have been awarded the DPG Georg-Kerschensteiner Prize for the development of the free app phyphox. The award ceremony took place during the spring meeting of the Condensed Matter Division of the German Physical Society (DPG) in Dresden on the 28th of March.

The DPG's Georg-Kerschensteiner Prize honours outstanding achievements in physics didactics and/or physics teaching. More background and previous award winners can be found here (German only).

Alexander Rothstein just returned from a 3 weeks visit of the Quantum Nanoelectronics Group of Prof. Pablo Jarillo-Herrero at the Massachusetts Institute of Technology (MIT) in Boston (USA) for kicking-off a joined collaboration and for learning more about the fabrication of making large twisted bilayer graphene. Our 2D Materials and Quantum Devices Group at the RWTH thanks Dr. Aviram Uri (left) und Liqiao Xia (right) for sharing their knowledge.

Phys. Rev. B 107, 075420 (2023)
We present low-temperature Raman measurements on gate-tunable graphene encapsulated in hexagonal boron nitride, which allows us to study in detail the Raman G and 2D mode frequencies and linewidths as a function of the charge carrier density. We observe a clear softening of the Raman G mode (of up to 2.5 cm⁻¹) at low carrier density due to the phonon anomaly and a residual G mode linewidth of ≈3.5 cm⁻¹ at high doping. By analyzing the G mode dependence on doping and laser power we extract an electron-phonon-coupling constant of ≈4.4×10⁻³ (for the G mode phonon). The ultraflat nature of encapsulated graphene results in a minimum Raman 2D peak linewidth of 14.5 cm⁻¹ and allows us to observe intrinsic electron-electron scattering-induced broadening of the 2D peak of up to 18 cm⁻¹ for an electron density of 5×10¹²cm⁻² (laser excitation energy of 2.33 eV). Our findings not only provide insights into electron-phonon coupling and the role of electron-electron scattering in the broadening of the 2D peak but also crucially show the limitations when it comes to the use of Raman spectroscopy (i.e., the use of the frequencies and the linewidths of the G and 2D modes) to benchmark graphene in terms of charge carrier density, strain, and strain inhomogeneities. This is particularly relevant when utilizing spatially resolved 2D Raman linewidth maps to assess substrate-induced nanometer-scale strain variations.