Evidence for electron–hole crystals in a Mott insulator

The coexistence of correlated electron and hole crystals enables the realization of quantum excitonic states, capable of hosting counterflow superfluidity and topological orders with long-range quantum entanglement. Here we report evidence for imbalanced electron–hole crystals in a doped Mott insulator, namely, α-RuCl3, through gate-tunable non-invasive van der Waals doping from graphene. Real-space imaging via scanning tunnelling microscopy reveals two distinct charge orderings at the lower and upper Hubbard band energies, whose origin is attributed to the correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru–Ru bonds, respectively. Moreover, a gate-induced transition of electron–hole crystals is directly visualized, further corroborating their nature as correlation-driven charge crystals. The realization and atom-resolved visualization of imbalanced electron–hole crystals in a doped Mott insulator opens new doors in the search for correlated bosonic states within strongly correlated materials. Scanning tunnelling microscopy of doped RuCl3 shows distinct charge orderings at the lower and upper Hubbard bands, which can be attributed to a correlation-driven honeycomb hole crystal composed of hole-rich Ru sites and a rotational-symmetry-breaking paired electron crystal composed of electron-rich Ru–Ru bonds.