Lattice reconstruction induced multiple ultra-flat bands in twisted bilayer WSe2

TL;DR

This study demonstrates the emergence of multiple ultra-flat electronic bands in twisted bilayer WSe2 with twist angles above 57°, caused by lattice reconstruction, which could lead to novel correlated electronic phases.

Contribution

It provides experimental evidence of lattice-reconstruction-induced ultra-flat bands in twisted bilayer WSe2, confirming theoretical predictions with high spatial and spectral resolution.

Findings

Multiple ultra-flat bands observed above 57° twist angle

Bandwidth less than 10 meV, smaller than Coulomb energy

Wavefunction distribution matches theoretical models

Abstract

Moir\'e 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 moir\'e flat bands over a wide range of twist angles. For twist angle close to 60{\deg}, 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 larger than 57{\deg}. The bandwidth, manifested as narrow tunneling conductance peaks, is estimated less than 10meV, 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 the theoretical predictions incredibly well, 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.