  Monolayer graphene aligned with hexagonal boron nitride (hBN) develops a gap
at the charge neutrality point (CNP). This gap has previously been extensively
studied by electrical transport through thermal activation measurements. Here,
we report the determination of the gap size at the CNP of graphene/hBN
superlattice through photocurrent spectroscopy study. We demonstrate two
distinct measurement approaches to extract the gap size. A maximum of ~ 14 meV
gap is observed for devices with a twist angle of less than 1 degree. This
value is significantly smaller than that obtained from thermal activation
measurements, yet larger than the theoretically predicted single-particle gap.
Our results suggest that lattice relaxation and moderate electron-electron
interaction effects may enhance the CNP gap in hBN/graphene superlattice.
