Interaction-Induced Gap and Gate Tunable Magnetism in Suspended Rhombohedral Few-Layer Graphene

The flat dispersion in rhombohedral stacked N-layer graphene, where E~k^N, gives rise to diverging (for N>2) density of states that are unstable to enormous electronic interactions, leading to the formation of electronic states with spontaneous broken symmetries. In free-standing samples, the electronic interactions are further strengthened due to the absence of screening. Using transport measurements on suspended dual-gated devices, we observe an insulating ground state with a large interaction-induced gap up to 80 meV at half filling. This gapped state can be enhanced by a perpendicular magnetic field, and suppressed by an interlayer potential, carrier density, or a critical temperature of ~40 K, and is most likely a layer antiferromagnet. Upon small doping, we observe prominent conductance hysteresis and giant magnetoconductance that exceeds 1000% as a function of magnetic fields. Both phenomena are tunable by density and temperature, and disappear at n>10¹² cm^-2 or T>5K. These results are confirmed by first principles calculations, which indicate the formation of a half-metallic state in doped r-FLG, in which the magnetization is tunable by electric field. Our results demonstrate that magnetism and spin polarization, arising from the strong electronic interactions in flat bands, emerge in a system composed entirely of carbon atoms. In the future, different ground states are expected to be stabilized by substrate engineering. Finally I will discuss a technique to make more robust suspended gates.