Imaging semiconductor-to-metal transition and topological flat bands of twisted bilayer MoTe2

Two-dimensional (2D) moiré materials have emerged as a highly tunable platform for investigating novel quantum states of matter arising from strong electronic correlations and nontrivial band topology1-4. Recently, topological flat bands formed in 2D semiconducting moiré superlattices have attracted great interests5-32. In particular, a series of topological quantum phases, including the long-sought fractional quantum anomalous Hall (FQAH) effect, have recently been experimentally observed14-22 in twisted bilayer MoTe2 (tMoTe2). However, the microscopic information of tMoTe2 moiré superlattice and its electronic structure is still lacking. Here, we present scanning tunneling microscopy and spectroscopy (STM/STS) studies of the tMoTe2 moiré superlattice, with twist angles ranging from about 2.3° to 2.8°. We developed a contact-STM mode to apply pressure on tMoTe2 and observed a phase transition from band insulator to metal of tMoTe2 under pressure at the charge neutrality point. STM imaging reveals a pronounced in-plane lattice reconstruction with periodic strain redistribution in the tMoTe2, which serves as gauge fields for generating topological moiré bands. Importantly, the electronic states of the low-energy moiré flat bands primarily concentrate at the XM and MX regions as revealed by STS imaging. Such spatial distributions are nicely reproduced by our first principal calculations with a large-scale basis, suggesting the low-energy moiré flat bands are formed through the hybridization of K valley bands of the top layer and K’ valley bands of the bottom layer. Overall, our findings provide compelling real-space evidence of electronic structure under pressure and topological flat bands of tMoTe2, paving the way for further STM/STS investigations of correlated topological states within the topological flat band in gate-tunable tMoTe2 devices.