Imaging Moiré Excited States with Photocurrent Tunneling Microscopy

Abstract: Moir\'e superlattices provide a highly tunable and versatile platform to explore novel quantum phases and exotic excited states ranging from correlated insulators1-17 to moir\'e excitons7-10,18. Scanning tunneling microscopy has played a key role in probing microscopic behaviors of the moir\'e correlated ground states at the atomic scale1,11-15,19. Atomic-resolution imaging of quantum excited state in moir\'e heterostructures, however, has been an outstanding experimental challenge. Here we develop a novel photocurrent tunneling microscopy by combining laser excitation and scanning tunneling spectroscopy (laser-STM) to directly visualize the electron and hole distribution within the photoexcited moir\'e exciton in a twisted bilayer WS2 (t-WS2). We observe that the tunneling photocurrent alternates between positive and negative polarities at different locations within a single moir\'e unit cell. This alternating photocurrent originates from the exotic in-plane charge-transfer (ICT) moir\'e exciton in the t-WS2 that emerges from the competition between the electron-hole Coulomb interaction and the moir\'e potential landscape. Our photocurrent maps are in excellent agreement with our GW-BSE calculations for excitonic states in t-WS2. The photocurrent tunneling microscopy creates new opportunities for exploring photoexcited non-equilibrium moir\'e phenomena at the atomic scale.