Nanoscale Optical Conductivity Imaging of Double-Moiré Twisted Bilayer Graphene.

A central paradigm of moiré materials relies on the formation of superlattices that yield enlarged effective crystal unit cells. While a critical consequence of this phenomenon is the celebrated flat electronic bands that foster strong interaction effects, the presence of superlattices has further implications. Here we explore the advantages of moiré superlattices in twisted bilayer graphene (TBG) aligned with hexagonal boron nitride (hBN) for passively enhancing optical conductivity in the low-energy regime. To probe the local optical response of TBG/hBN double-moiré lattices, we use infrared (IR) nano-imaging in conjunction with nanocurrent imaging to examine local optical conductivity over a wide range of TBG twist angles. We show that interband transitions associated with the multiple moiré flat and dispersive bands produce tunable transparent IR responses even at finite carrier densities, which is in stark contrast to the previously limited metallic near transparency observed only in undoped pristine graphene.