Local, global, and nonlinear screening in twisted double layer graphene
Significance Using scanning tunneling microscopy and numerical simulations we demonstrate that a graphene buffer sheet placed underneath an atomically thin layer is an extremely effective method to increase the quality of 2D electron systems without modifying their electronic structure and without compromising the gating capabilities. We show that owing to its remarkable nonlinear screening capability the graphene buffer layer provides better shielding from substrate induced perturbations than either increasing the distance to the substrate or doubling the carrier density, or even than using a graphene bilayer. One atom thick crystalline layers and their vertical heterostructures carry the promise of designer electronic materials that are unattainable by standard growth techniques. To realize their potential it is necessary to isolate them from environmental disturbances, in particular those introduced by the substrate. However, finding and characterizing suitable substrates, and minimizing the random potential fluctuations they introduce, has been a persistent challenge in this emerging field. Here we show that Landau level (LL) spectroscopy offers the unique capability to quantify both the reduction of the quasiparticles' lifetime and the long range inhomogeneity due to random potential fluctuations. Harnessing this technique together with direct scanning tunneling microscopy and numerical simulations we demonstrate that the insertion of a graphene buffer layer with a large twist angle is a very effective method to shield a 2D system from substrate interference that has the additional desirable property of preserving the electronic structure of the system under study. We further show that owing to its remarkable nonlinear screening capability a single graphene buffer layer provides better shielding than either increasing the distance to the substrate or doubling the carrier density and reduces the amplitude of the potential fluctuations in graphene to values even lower than the ones in AB stacked bilayer graphene.
Proceedings of the National Academy of Sciences