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.
@article{arxiv.2306.00859,
title = {Imaging Moir\'e Excited States with Photocurrent Tunneling Microscopy},
author = {Hongyuan Li and Ziyu Xiang and Mit H. Naik and Woochang Kim and Zhenglu Li and Renee Sailus and Rounak Banerjee and Takashi Taniguchi and Kenji Watanabe and Sefaattin Tongay and Alex Zettl and Felipe H. da Jornada and Steven G. Louie1 and Michael F. Crommie and Feng Wang},
journal= {arXiv preprint arXiv:2306.00859},
year = {2023}
}