Correlated Insulator and Chern Insulators in Pentalayer Rhombohedral Stacked Graphene

Rhombohedral stacked multilayer graphene is an ideal platform to search for correlated electron phenomena, due to its pair of flat bands touching at zero energy and further tunability by an electric field. Furthermore, its valley-dependent Berry phase at zero energy points to possible topological states when the pseudospin symmetry is broken by electron correlation. However, experimental explorations of these opportunities are very limited so far, due to a lack of devices with optimized layer numbers and configurations. Here we present electron transport measurements of hBN-encapsulated pentalayer graphene at down to 100 milli-Kelvin. We observed a correlated insulating state with >MOhm resistance at zero charge density and zero displacement field, where the tight-binding calculation predicts a metallic ground state. By increasing the displacement field, we observed a Chern insulator state with C = -5 and two other states with C = -3 at a low magnetic field of ~1 Tesla. At high displacement fields and charge densities, we observed isospin-polarized quarter- and half-metals. Therefore, rhombohedral-stacked pentalayer graphene is the first graphene system to exhibit two different types of Fermi-surface instabilities: driven by a pair of flat bands touching at zero energy, and by the Stoner mechanism in a single flat band. Our results demonstrate a new direction to explore intertwined electron correlation and topology phenomena in natural graphitic materials without the need of moir\'e superlattice engineering.