Large tunable intrinsic gap in rhombohedral-stacked tetralayer graphene at half filling

Rhombohedral-stacked tetralayer (r-4LG) has a highly unusual energy dispersion, which can be approximated as E~k⁴, where k is the wave vector. At half filling, the very flat energy bands in r-4LG are unstable to electronic interactions, giving rise to electronic states with spontaneous broken symmetries. Using transport measurements on suspended dual-gated devices, we observe an insulating ground state with a large interaction-induced transport gap up to 80 meV at the charge neutrality point. The energy gap is enhanced further with a perpendicular magnetic field, but closed or suppressed upon the application of an out-of-lane electric field of either polarity, increasing charge density or a critical temperature of ~40 K. This insulating gapped state is consistent to that observed in bilayer graphene (BLG) and rhombohedral-stacked trilayer graphene (r-TLG), and therefore proposed to be a layer antiferromagnet with broken time reversal symmetry. The large magnitude of the gap also suggests that, at lease in r-4LG, the band flattening effect prevails over the increasing screening and charge de-confinement in thicker graphene, in agreement with a first principle calculation.