Tunable bands and correlations in twisted double bilayer graphene in magnetic fields

Place: Conference room and online. 

Zoom link: https://nanoscience-imdea.zoom.us/j/96503133235

Abstract: 

Artificial superlattices in 2D materials proved to be versatile platforms for band engineering and studying correlated quantum phases. Due to the additional periodic potential, flat-band phase can be induced in a crystal leading to strong electron-electron interactions. Specifically in twisted moiré heterostructures, this periodicity is controlled by tuning the angular misalignment between layers. In magic angle twisted bilayer graphene, flat bands and correlated quantum phases appear only at a few precise values of the twist angle. In contrast, small-angle twisted double bilayer graphene (TDBG) can be electrostatically tuned into the correlated regime for a range of twist angles. Local probe measurements are key to avoid the common complication of twist angle disorder in global transport studies. Here we employ scanning tunneling and atomic force microscopy together with electrostatic gating to study TDBG with atomic spatial and high energy resolution in magnetic fields up to 15 T. We observe magnetic-field-induced quantization and use it to explore the dramatic band structure changes in response to electric field. We use theoretical modeling of the effects of displacement fields, Berry curvature, and magnetic fields to support our experimental findings. We also observe a cascade of correlated insulating states in a range of fillings and image them spatially with atomic resolution. In this talk, I will also address how small natural strain affects the measurements in this system including the presence of stripe phases.