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News 28.05.2024
2DMP Kickoff Meeting of the 2nd Funding Period of the DPG SPP 2244

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2DMP Kickoff Meeting of the 2nd Funding Period of the DPG SPP 2244

The kickoff meeting for the second funding period of the SPP 2244 program took place in Dresden. This event marked the beginning of a new phase for the SPP. At this meeting, the newly constituted Steering Committee for the current funding period was also elected. The Committee includes Ursula Wurstbauer from the University of Münster, Janina Maultzsch from FAU Erlangen-Nürnberg, Jaroslav Fabian from the University of Regensburg, Christoph Stampfer from our group, and Thomas Heine (Coordinator) from TU Dresden.

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New publication: Distance dependence of the energy transfer mechanism in WS2-graphene heterostructures


Phys. Rev. Lett. 132, 196902 (2024)
We report on the mechanism of energy transfer in van der Waals heterostructures of the twodimensional semiconductor WS2 and graphene with varying interlayer distances, achieved through spacer layers of hexagonal boron nitride (hBN). We record photoluminescence and reflection spectra at interlayer distances between 0.5 nm and 5.8 nm (0-16 hBN layers). We find that the energy transfer is dominated by states outside the light cone, indicative of a Förster transfer process, with an additional contribution from a Dexter process at 0.5 nm interlayer distance. We find that the measured dependence of the luminescence intensity on interlayer distances above 1 nm can be quantitatively described using recently reported values of the Förster transfer rates of thermalized charge carriers. At smaller interlayer distances, the experimentally observed transfer rates exceed the predictions and furthermore depend on excess energy as well as on excitation density. Since the transfer probability of the Förster mechanism depends on the momentum of electron-hole pairs, we conclude that at these distances, the transfer is driven by non-thermalized charge carrier distributions.

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New publication: Negative electronic compressibility in charge islands in twisted bilayer graphene


Phys. Rev. B 109, 155430 (2024)
We report on the observation of negative electronic compressibility in twisted bilayer graphene for Fermi energies close to insulating states. To observe this negative compressibility, we take advantage of naturally occurring twist angle domains that emerge during the fabrication of the samples, leading to the formation of charge islands. We accurately measure their capacitance using Coulomb oscillations, from which we infer the compressibility of the electron gas. Notably, we not only observe the negative electronic compressibility near correlated insulating states at integer filling, but also prominently near the band insulating state at full filling, located at the edges of both the flat- and remote bands. Furthermore, the individual twist angle domains yield a well-defined carrier density, enabling us to quantify the strength of electronic interactions and verify the theoretical prediction that the inverse negative capacitance contribution is proportional to the average distance between the charge carriers. A detailed analysis of our findings suggests that Wigner crystallization is the most likely explanation for the observed negative electronic compressibility.

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New publication: Band gap formation in commensurate twisted bilayer graphene/hBN moiré lattices


Phys. Rev. B 109, 155139 (2024)
We report on the investigation of periodic superstructures in twisted bilayer graphene (tBLG) van der Waals heterostructures, where one of the graphene layers is aligned to hexagonal boron nitride (hBN). Our theoretical simulations reveal that if the ratio of the resulting two moiré unit-cell areas is a simple fraction, the graphene/hBN moiré lattice acts as a staggered potential, breaking the degeneracy between tBLG AA sites. This leads to additional band gaps at energies where a subset of tBLG AA sites is fully occupied. These gaps manifest as Landau fans in magnetotransport, which we experimentally observe in an aligned tBLG/hBN heterostructure. Our study demonstrates the identification of commensurate tBLG/hBN van der Waals heterostructures by magnetotransport, highlights the persistence of moiré effects on length scales of tens of nanometers, and represents an interesting step forward in the ongoing effort to realize designed quantum materials with tailored properties.

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Election of Jan-Lucas Uslu as Chairperson of the Board for AI in Physics (AKPIK)


We are pleased to announce that our group member, Jan-Lucas Uslu, has been elected as Chairperson of the Board for AI in Physics, i.e. the DPG Working Group on Physics, Modern IT and Artificial Intelligence (AKPIK). This position is a testament to Jan's outstanding dedication to the field of AI in physics. We congratulate him for this achievement!

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New publication: An open-source robust machine learning platform for real-time detection and classification of 2D material flakes


Mach. Learn.: Sci. Technol. 5, 015027 (2024)
The most widely used method for obtaining high-quality two-dimensional (2D) materials is through mechanical exfoliation of bulk crystals. Manual identification of suitable flakes from the resulting random distribution of crystal thicknesses and sizes on a substrate is a time-consuming, tedious task. Here, we present a platform for fully automated scanning, detection, and classification of 2D materials, the source code of which we make openly available. Our platform is designed to be accurate, reliable, fast, and versatile in integrating new materials, making it suitable for everyday laboratory work. The implementation allows fully automated scanning and analysis of wafers with an average inference time of 100 ms for images of 2.3 Mpixels. The developed detection algorithm is based on a combination of the flakes’ optical contrast toward the substrate and their geometric shape. We demonstrate that it is able to detect the majority of exfoliated flakes of various materials, with an average recall (AR50) between 67% and 89%. We also show that the algorithm can be trained with as few as five flakes of a given material, which we demonstrate for the examples of few-layer graphene, WSe2, MoSe2, CrI3, 1T-TaS2 and hexagonal BN. Our platform has been tested over a two-year period, during which more than 106 images of multiple different materials were acquired by over 30 individual researchers

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New publication: Infrared Resonance Raman of Bilayer Graphene: Signatures of Massive Fermions and Band Structure on the 2D Peak


Nano Letters 24, 1867 (2024)
Few-layer graphene possesses low-energy carriers that behave as massive Fermions, exhibiting intriguing properties in both transport and light scattering experiments. Lowering the excitation energy of resonance Raman spectroscopy down to 1.17 eV, we target these massive quasiparticles in the split bands close to the K point. The low excitation energy weakens some of the Raman processes that are resonant in the visible, and induces a clearer frequency-separation of the substructures of the resonance 2D peak in bi- and trilayer samples. We follow the excitation-energy dependence of the intensity of each substructure, and comparing experimental measurements on bilayer graphene with ab initio theoretical calculations, we trace back such modifications on the joint effects of probing the electronic dispersion close to the band splitting and enhancement of electron–phonon matrix elements.

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Michael Schmitz successfully defended his PhD thesis

On 16.01.2024, Michael Schmitz successfully defended his PhD thesis. Congratulations to Dr. Michael Schmitz on this commendable accomplishment!

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