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A theoretical physicist who, with Pilkyung Moon, calculated optical absorption in twisted bilayer graphene. Rotating one sheet changes the electronic electronic band structure and shifts absorption peaks associated with Van Hove singularities. Comparing a tight-binding model with a continuum model showed how an optical spectrum can identify the relative rotation of two atomically thin sheets.^[ref]

Two layers need not have the same electronic structure everywhere. Different rotational alignments create domains of twisted bilayer graphene; AB-stacked regions instead have the usual coupled low-energy bilayer bands. In a twisted domain, displaced Dirac cones hybridize where they meet, producing avoided crossings and saddle points in the two-dimensional dispersion relation. The resulting Van Hove singularities enhance optical transitions at energies that depend on the twist angle. Different domains therefore absorb different parts of the visible spectrum and can show different colours despite having the same layer count.^[ref]

On a silicon/silicon-dioxide substrate, reflected beams from the film stack also interfere. The wavelength-dependent optical response of the bilayer graphene changes this reflected contrast, making the domain differences visible. The observation is consistent with stacking domains; colour alone does not uniquely prove a twist angle. Variations in oxide thickness, trapped contamination, folds or local doping can also change contrast and should be checked independently, for example by Raman spectroscopy.

A theoretical physicist who coauthored a study of optical absorption in twisted bilayer graphene with Mikito Koshino. The calculation linked the rotation angle to peaks in the optical spectrum and derived selection rules from the model's symmetry. It provides a way to read structural information from light absorption while testing simplified descriptions of coupled electronic layers.^[ref]