Correlated insulating states at fractional fillings of the WS2/WSe2 moire lattice

The strong electron interactions in the minibands formed in moire superlattices of van der Waals materials, such as twisted graphene and transition metal dichalcogenides, make such systems a fascinating platform with which to study strongly correlated states(1-19). In most systems, the correlated states appear when the moire lattice is filled by an integer number of electrons per moire unit cell. Recently, correlated states at fractional fillings of 1/3 and 2/3 holes per moire unit cell have been reported in the WS2/WSe2 hetero-bilayer, hinting at the long-range nature of the electron interaction(16). Here we observe a series of correlated insulating states at fractional fillings of the moire minibands on both electron- and hole-doped sides in angle-aligned WS2/WSe2 hetero-bilayers, with certain states persisting at temperatures up to 120 K. Simulations reveal that these insulating states correspond to ordering of electrons in the moire lattice with a periodicity much larger than the moire unit cell, indicating a surprisingly strong and long-range interaction beyond the nearest neighbours.