Accurate Measurement of the Gap of Graphene/hBN Moir\'e Superlattice through Photocurrent Spectroscopy

TL;DR

This study uses photocurrent spectroscopy to accurately measure the energy gap at the charge neutrality point in graphene/hBN superlattices, revealing a smaller gap than thermal methods but larger than theory predicts, influenced by lattice and interaction effects.

Contribution

It introduces two photocurrent spectroscopy methods for measuring the CNP gap in graphene/hBN superlattices, providing more precise and different insights compared to traditional electrical transport techniques.

Findings

Maximum gap of ~14 meV observed for twist angles < 1 degree

Measured gap is smaller than thermal activation estimates

Gap size exceeds theoretical single-particle predictions

Abstract

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.