Hyperbolic enhancement of photocurrent patterns in minimally twisted bilayer graphene
Sunku S.S., Halbertal D., Stauber T., Chen S., McLeod A.S., Rikhter A., Berkowitz M.E., Lo C.F.B., Gonzalez-Acevedo D.E., Hone J.C., Dean C.R., Fogler M.M., Basov D.N.
Quasi-periodic moiré patterns and their effect on electronic properties of twisted bilayer graphene have been intensely studied. At small twist angle θ, due to atomic reconstruction, the moiré superlattice morphs into a network of narrow domain walls separating micron-scale AB and BA stacking regions. We use scanning probe photocurrent imaging to resolve nanoscale variations of the Seebeck coefficient occurring at these domain walls. The observed features become enhanced in a range of mid-infrared frequencies where the hexagonal boron nitride substrate is optically hyperbolic. Our results illustrate the capabilities of the nano-photocurrent technique for probing nanoscale electronic inhomogeneities in two-dimensional materials.
Top: Seebeck coefficient S profile across a domain wall calculated from first principles (Supplementary Note 3). The DW is located at x = 0. Middle: perspective view of the experiment showing photocurrent generation at the domain wall. The green background represents the Seebeck coefficient profile and the blue dots represent carriers generated by thermoelectric effect. Bottom: schematic of the BLG band structure across the DW for three different stackings AB, BA, and saddle point (SP).
