Imaging Bulk and Edge Transport near the Dirac Point in Graphene Moiré Superlattices

Van der Waals structures formed by aligning graphene monolayers with hexagonal boron nitride exhibit a moiré superlattice and broken sublattice symmetry that opens a gap at the Dirac point. The electrical conductivity is thermally activated at high temperature and saturates at low temperature indicating the existence of subgap states [1]. Transport via such states both in the bulk [2] and at the edges [3] have been suggested. We present a scanning gate microscopy study of moiré superlattice devices with similar gap size but different charge disorder. In the device with high charge impurity (~10¹⁰ cm^-2) and low saturated resistivity (~10 kΩ) at the Dirac point we observe a clear response at the edges. Combined with simulations, we interpret the response as a result of enhanced edge doping. In addition, a device with low charge impurity (~10⁹ cm^-2) and high resistivity (~100 kΩ) shows bulk response only, consistent with the absence of edge-state shunting [3]. Our results provide microscopic insight into edge conduction that can be helpful in understanding transport in gapped Dirac systems. [1] Hunt et al. Science. 21, 1427-1430 (2013) [2] Gorbachev et al. Science. 24, 448-451 (2014) [3] Zhu et al. Nat. Commun. 8, 14552 (2017)