Cryogenic near-field imaging of twist angle disorder in twisted bilayer graphene

Twisted bilayer graphene has emerged as a versatile platform to study strongly correlated phenomena and superconductivity. These properties only occur near the magic angle of 1.1 degree, and allow for little twist angle inhomogeneity. Recent works have shown the widespread twist angle disorder and strain in such samples, which are found to be a limiting factor for the existence of strongly correlated states. Yet, these works required the use of highly specialized experimental setups. Here we report on cryogenic (10 K) near-field photovoltage measurements, in which the photoresponse turns out to be a sensitive probe to map the twist angle within our device. Strengthened by nanoscale absorption measurements, we interpret our observations using the photothermoelectric effect, in which the resistive states at the full-filling carrier density act as sensitive probes of small twist angle variations. Our work demonstrates that cryogenic near-field photovoltage mapping is a powerful and accessible technique for studying the optical and optoelectronic properties of twisted bilayer graphene on the nanoscale.