Nat. Mater. (2025)
Moiré materials represent strongly interacting electron systems bridging topological and correlated physics. Despite notable advances, decoding wavefunction properties underlying the quantum geometry remains challenging. Here we utilize polarization-resolved photocurrent measurements to probe magic-angle twisted bilayer graphene, leveraging its sensitivity to the Berry connection that encompasses quantum ‘textures’ of electron wavefunctions. Using terahertz light resonant with optical transitions of its flat bands, we observe bulk photocurrents driven by broken symmetries and reveal the interplay between electron interactions and quantum geometry. We observe inversion-breaking gapped states undetectable through quantum transport, sharp changes in the polarization axes caused by interaction-induced band renormalization and recurring photocurrent patterns at integer filling factors of the moiré unit cell that track the evolution of quantum geometry through the cascade of phase transitions. The large and tunable terahertz response intrinsic to flat-band systems offers direct insights into the quantum geometry of interacting electrons and paves the way for innovative terahertz quantum technologies.

There is a new short film about the physics programme at RWTH Aachen University (unfortunately only in German) which can be found here on youtube: https://www.youtube.com/watch?v=G7N0rrBHsWw

Please forward to anyone who might be interested.

Phys. Rev. B 111, 165416 (2025)
We report on an experimental study of spin and valley blockade in two-electron bilayer graphene (BLG) double quantum dots (DQDs) and explore the limits set by asymmetric orbitals and electron-electron interactions. The results obtained from magnetotransport measurements on two-electron BLG DQDs, where the resonant tunneling transport involves both orbital symmetric and antisymmetric two-particle states, show a rich level spectrum. We observe a magnetic field tunable spin and valley blockade, which is limited by the orbital splitting, the strength of the electron-electron interaction and the difference in the valley g-factors between the symmetric and antisymmetric two-particle orbital states. Our conclusions are supported by simulations based on rate equations, which allow the identification of prominent interdot transitions associated with the transition from single- to two-particle states observed in the experiment.

Nano Lett. 25, 6429 (2025)
Twisted bilayer graphene (tBLG) near the magic angle is a unique platform where the combination of topology and strong correlations gives rise to exotic electronic phases. These phases are gate-tunable and related to the presence of flat electronic bands, isolated by single-particle band gaps. This enables gate-controlled charge confinements, essential for the operation of single-electron transistors (SETs), and allows one to explore the interplay of confinement, electron interactions, band renormalization, and the moiré superlattice, potentially revealing key paradigms of strong correlations. Here, we present gate-defined SETs in tBLG with well-tunable Coulomb blockade resonances. These SETs allow us to study magnetic field-induced quantum oscillations in the density of states of the source-drain reservoirs, providing insight into gate-tunable Fermi surfaces of tBLG. Comparison with tight-binding calculations highlights the importance of displacement-field-induced band renormalization crucial for future advanced gate-tunable quantum devices and circuits in tBLG including, e.g., quantum dots and Josephson junction arrays.

Nat. rev. electr. eng. 2, 277 (2025)
Universal cryogenic computing, encompassing von Neumann, neuromorphic and quantum computing, paves the way for future big-data processing with high energy efficiency. Complementary metal oxide semiconductor (CMOS) technology operating at cryogenic temperatures with ultra-low power consumption is a key component of this advancement. However, classical CMOS technology, designed for room temperature applications, suffers from band-tail effects at cryogenic levels, leading to an increased subthreshold swing and decreased mobility values. In addition, threshold voltages are enlarged. Thus, classical CMOS technology fails to meet the low power requirements when cooled close to zero Kelvin. In this Perspective, we show that steep slope cryogenic devices can be realized by screening the band tails with the use of high-k dielectrics and wrap-gate architectures and/or reducing them through the optimization of the surfaces and interfaces within the transistors. Cryogenic device functionality also strongly benefits from appropriate source/drain engineering employing dopant segregation from silicides. Furthermore, the threshold voltage control can be realized with back-gating, work-function engineering and dipole formation. As a major implication, future research and development towards cryogenic CMOS technology requires a combination of these approaches to enable universal cryogenic computing at the necessary ultra-low power levels.

The Söllerhaus-Workshop 2025 from the 15th to the 22nd of March of the 2nd Institute of Physics A , B and C focusing on interesting (partly cross-sectional) topics was great fun with many interesting talks and discussions, good skiing and slippery snow hiking with torches.

Alexander Rothstein won one of the three poster prizes at the IWEPNM Winterschool in Kirchberg with his poster on "Gate-tunable Josephson diods in magic angle twisted bilayer graphene". Congratulations!