Last Friday, our annual institute excursion to the Wildenhof at the Rursee took place once again, and it was a great fun! The day kicked off with cycling (for a few of us) and various water sport activities in the morning. In the afternoon, we gathered on the terrace for a barbecue, with plenty of food and drinks to go around. A big thank you to everyone who participated and helped make this happen!

Nano Letters 24, 11454 (2024)
Cryogenic field-effect transistors (FETs) offer great potential for applications, the most notable example being classical control electronics for quantum information processors. For the latter, on-chip FETs with low power consumption are crucial. This requires operating voltages in the millivolt range, which are only achievable in devices with ultrasteep subthreshold slopes. However, in conventional cryogenic metal-oxide-semiconductor (MOS)FETs based on bulk material, the experimentally achieved inverse subthreshold slopes saturate around a few mV/dec due to disorder and charged defects at the MOS interface. FETs based on two-dimensional materials offer a promising alternative. Here, we show that FETs based on Bernal stacked bilayer graphene encapsulated in hexagonal boron nitride and graphite gates exhibit inverse subthreshold slopes of down to 250 μV/dec at 0.1 K, approaching the Boltzmann limit. This result indicates an effective suppression of band tailing in van der Waals heterostructures without bulk interfaces, leading to superior device performance at cryogenic temperature.

We are pleased to announce that Professor Slava Rotkin from Penn State University (US) has successfully completed a 3 month visit to our group. During this time, Slava worked closely with our team, contributing to several ongoing projects and sharing valuable insights from his extensive expertise in nanoscience and nanotechnology, with a particular focus on theoretical modelling of nanomaterials. We wish Slava continued success in his future endeavours and hope that this sabbatical experience has been as rewarding for him as it has been for our group.

From 29. April to 1. June, our Master student Jan-Lucas Uslu visited Prof. David Goldhaber-Gordon at Stanford University. During the research stay, Jan worked on the application of machine learning techniques to advanced image analysis and other topics. This visit helped with ongoing research in the field of 2D materials and twisted bilayer graphene. We support our students in gaining international research experience and academic collaborations.

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.

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.

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.

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.