Abstract
Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergent properties that are absent in the natural crystal form. Twisted bilayer transition metal dichalcogenides (TB-TMDs) can host moiré flat bands over a wide range of twist angles. For twist angle close to 60°, it was predicted that TB-TMDs undergo a lattice reconstruction which causes the formation of ultra-flat bands. Here, by using scanning tunneling microscopy and spectroscopy, we show the emergence of multiple ultra-flat bands in twisted bilayer WSe2 when the twist angle is within 3° of 60°. The ultra-flat bands are manifested as narrow tunneling conductance peaks with estimated bandwidth less than 10 meV, which is only a fraction of the estimated on-site Coulomb repulsion energy. The number of these ultra-flat bands and spatial distribution of the wavefunctions match well with the theoretical predictions, strongly evidencing that the observed ultra-flat bands are induced by lattice reconstruction. Our work provides a foundation for further study of the exotic correlated phases in TB-TMDs.
Highlights
Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergent properties that are absent in the natural crystal form
We conclude that in addition to the well-known basic moiré bands caused by interlayer hybridization, the ultra-flat bands mainly arise from atomic reconstruction of the moiré superlattices when the twist angle is within 3° of 60° due to the enhanced interlayer interaction
Different from twisted bilayer graphene (TBG), twisted bilayer TMDs form two distinct moiré structures for the twist angle (θ) close to 0°and 60°14,16, owing to sublattice symmetry breaking in the TMDs
Summary
Moiré superlattices in van der Waals heterostructures provide a tunable platform to study emergent properties that are absent in the natural crystal form. These ultra-flat bands, with bandwidths estimated to be a few meV only, appear when the twist angle is within 3° of 60°.
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