High-resolution Landau level spectroscopy in small-angle twisted double bilayer graphene

The wealth of correlated phases in twisted van der Waals multilayers has propelled the quest for novel many-body phenomena in tunable solid-state platforms. While the field recently started off with superconductivity and correlated insulating states in magic-angle twisted bilayer graphene, the attention has increasingly turned to systems containing more layers. The in-depth investigation of novel correlated and topological phases in these materials requires a nanoscale approach, forgoing the influence of twist-angle disorder. We present a combination of high-resolution gate-tuned STM, AFM, and KPFM measurements at temperatures down to 10 mK and at magnetic fields up to 15 T to locally investigate van der Waals materials. Here, we focus on small-angle twisted double bilayer graphene. A rich, layer-dependent local density of states featuring narrow bands is revealed, sensitively tunable by the electric and magnetic field. Landau levels emerge out of the valence and conduction narrow bands upon application of a perpendicular magnetic field. Comparing to theory, we show the Landau levels originate from different symmetry points in the band structure where electron and hole pockets reside and are tuned by the displacement field. Spatial Landau level spectroscopy measurements visualize the nanoscale localization of the non-uniform Landau levels in the moiré potential, including C₃-symmetry breaking at particular carrier densities. The large and tunable parameter space available for spatially resolved spectroscopy offers a detailed insight in the Landau level behavior and correlated physics in twisted double bilayer graphene.

In collaboration with Y. Maximenko, S. Kim, S. T. Le, E. M. Shih, S. R. Blankenship, F. Xue, Y. Barlas, P. Haney, N. B. Zhitenev, F. Ghahari and J. A. Stroscio.