Tunable moiré bands and strong correlations in small-twist-angle bilayer graphene

Abstract

According to electronic structure theory, bilayer graphene is expected to have anomalous electronic properties when it has long-period moiré patterns produced by small misalignments between its individual layer honeycomb lattices. We have realized bilayer graphene moiré crystals with accurately controlled twist angles smaller than 1° and studied their properties using scanning probe microscopy and electron transport. We observe conductivity minima at charge neutrality, satellite gaps that appear at anomalous carrier densities for twist angles smaller than 1°, and tunneling densities-of-states that are strongly dependent on carrier density. These features are robust up to large transverse electric fields. In perpendicular magnetic fields, we observe the emergence of a Hofstadter butterfly in the energy spectrum, with fourfold degenerate Landau levels, and broken symmetry quantum Hall states at filling factors ±1, 2, 3. These observations demonstrate that at small twist angles, the electronic properties of bilayer graphene moiré crystals are strongly altered by electron–electron interactions. Significance Accurately controlled, very long wavelength moiré patterns are realized in small-twist-angle bilayer graphene, and studied using electron transport and scanning probe microscopy. We observe gaps in electron transport at anomalous densities equal to ±8 electrons per moiré crystal unit cell, at variance with electronic structure theory, and the emergence of a Hofstadter butterfly in the energy spectrum in perpendicular magnetic fields. These findings open up an avenue to create artificial crystals by manipulating the relative angle between individual layers in a heterostructure.