Abstract
Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The twisted double bilayer graphene (TDBG), where two Bernal bilayer graphene are stacked with a twist angle, is such a moiré system with tunable flat bands. Here, we use gate-tuned scanning tunneling spectroscopy to directly demonstrate the tunability of the band structure of TDBG with an electric field and to show spectroscopic signatures of electronic correlations and topology for its flat band. Our spectroscopic experiments are in agreement with a continuum model of TDBG band structure and reveal signatures of a correlated insulator gap at partial filling of its isolated flat band. The topological properties of this flat band are probed with the application of a magnetic field, which leads to valley polarization and the splitting of Chern bands with a large effective g-factor.
Highlights
Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states
Corroborating this picture, previous transport experiments have shown that a robust correlated insulator appears in a confined electric field range at densities that correspond to half filling of a flat band in TDBG22–26
scanning tunneling microscopy (STM) and gate-tuned scanning tunneling spectroscopy (GT-STS) have become a powerful tool to probe the properties of moiré systems, as its application to magic angle twisted bilayer graphene (MATBG) has provided critical information on the nature of flat bands in that system
Summary
Moiré superlattices created by the twisted stacking of two-dimensional crystals can host electronic bands with flat energy dispersion in which enhanced interactions promote correlated electron states. The electric field tunability makes it possible to realize this flat band in a wide range of twist angles (0.84°–1.53°) between the stacked bilayers, instead of at a specific angle as for the case of MATBG23,24 Corroborating this picture, previous transport experiments have shown that a robust correlated insulator appears in a confined electric field range at densities that correspond to half filling of a flat band in TDBG22–26. STM experiments have confirmed the theoretical picture of the single particle band structure of MATBG and have uncovered signatures of strong electronic correlation and topology at partial band filling of this system[7,8,9,10,19] Applying these techniques to TDBG demonstrates its tunability and provides spectroscopic signatures of correlation and topology of its flat bands that complements transport studies
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