Imaging moir excited states with photocurrent tunnelling microscopy

Moire superlattices provide a highly tuneable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators to moire excitons. Scanning tunnelling microscopy has played a key role in probing microscopic behaviours of the moire correlated ground states at the atomic scale. However, imaging of quantum excited states in moire heterostructures remains an outstanding challenge. Here we develop a photocurrent tunnelling microscopy technique that combines laser excitation and scanning tunnelling spectroscopy to directly visualize the electron and hole distribution within the photoexcited moire exciton in twisted bilayer WS2. The tunnelling photocurrent alternates between positive and negative polarities at different locations within a single moire unit cell. This alternating photocurrent originates from the in-plane charge transfer moire exciton in twisted bilayer WS2, predicted by our GW-Bethe-Salpeter equation calculations, that emerges from the competition between the electron-hole Coulomb interaction and the moire potential landscape. Our technique enables the exploration of photoexcited non-equilibrium moire phenomena at the atomic scale. The authors combine laser excitation and scanning tunnelling spectroscopy to visualize the electron and hole distributions in photoexcited moire excitons in twisted bilayer WS2. This photocurrent tunnelling microscopy approach enables the study of photoexcited non-equilibrium moire phenomena at atomic scales.