Correlated insulator and Chern insulators in pentalayer rhombohedral-stacked graphene

Abstract

Rhombohedral stacked multilayer graphene hosts a pair of flat bands touching at zero energy, which should give rise to correlated electron phenomena that can be further tuned by an electric field. Furthermore, when electron correlation breaks the isospin symmetry, the valley-dependent Berry phase at zero energy may give rise to topologically non-trivial states. Here, we measure electron transport through hBN-encapsulated pentalayer graphene down to 100 mK. We observed a correlated insulating state with resistance R>MΩ at charge density n=0 and displacement field D=0. Tight-binding calculations predict a metallic ground state under these conditions. By increasing D, we observed a Chern insulator state with C = -5 and two other states with C = -3 at magnetic field around 1 T. At high D and n, we observed isospin-polarized quarter- and half-metals. Hence, rhombohedral stacked pentalayer graphene exhibits two different types of Fermi-surface instabilities, one driven by a pair of flat bands touching at zero energy, and one induced by the Stoner mechanism in a single flat band. Our results establish rhombohedral stacked multilayer graphene as suitable system to explore intertwined electron correlation and topology phenomena in natural graphitic materials without the need for moiré superlattice engineering.