During the course of Toponanop, twisted graphene was discovered as a fascinating topological material with interacting electrons and numerous control knobs. For this reason, we focussed a portion of the project on this material system and applied our unique experimental techniques. Since twisted graphene is gate-tunable, we were able to switch the electron interaction effects on and off while probing the local response of the system. This has provided us with unique insights into the quantum metric and topological properties of the system, as well as the interplay of these properties with the electron interaction effects.
Several studies led to often surprising results that were clearly beyond the state-of-the-art and demonstrated novel capabilities and physical insights:
• The first photocurrent mapping of a moiré system with 20-nm spatial resolution has been achieved. This technical capability has been adopted by other research groups as it can be applied to materials that are encapsulated, where other scanning-probe techniques are no longer applicable. In particular, photocurrent nanoscopy has emerged as a versatile probe for sensing a combination of properties, including correlated electron states, Bloch band quantum geometry, quantum kinetic processes, and device characteristics of quantum materials.
• By applying the novel cryo-SNOM tool to a novel topological material system, it was discovered that the degree of interaction-induced valley polarization can be probe directly AND locally. The capability for creating spatial map enables the connection of local variations in material properties (e.g. twist angle and strain) with electronic correlations.
• By twisting two layers of bilayer graphene, we demonstrated a giant ultra-broadband photoconductivity in twisted double bilayer graphene heterostructures spanning a spectral range of 2-100 μm with internal quantum efficiencies ~ 40 % at speeds of 100 kHz. The giant response originates from unique properties of twist-decoupled heterostructures including pristine, crystal field induced terahertz band gaps, parallel photoactive channels, and strong photoconductivity enhancements caused by interlayer screening of electronic interactions by respective layers acting as sub-atomic spaced proximity screening gates. The achievements introduce twist-decoupled graphene heterostructures as a viable route for engineering gapped graphene photodetectors with 3D scalability.
• Patterning of ultrasharp gold structures on hBN enabled the creation of ultra-small nanocavities with relatively high quality factor and the formation of nanophotonic lattices. By combining these two techniques, a novel nanophotonic topological state has been realised, which exhibits robustness, strong confinement and tunability. This achievement marked the first instance of extending topological nanophotonics into the deep subwavelength regime. This discovery has the potential to revolutionize the field, as it can be directly extended and hybridized with other Van der Waals materials, enabling expanded spectral coverage and compatibility with diverse electronic and excitonic systems.
