Bilayer Wigner crystals in a transition metal dichalcogenide heterostructure

One of the first theoretically predicted manifestations of strong interactions in many-electron systems was the Wigner crystal(1-3), in which electrons crystallize into a regular lattice. The crystal can melt via either thermal or quantum fluctuations(4). Quantum melting of the Wigner crystal is predicted to produce exotic intermediate phases(5,6) and quantum magnetism(7,8) because of the intricate interplay of Coulomb interactions and kinetic energy. However, studying two-dimensional Wigner crystals in the quantum regime has often required a strong magnetic field(9-11) or a moire superlattice potential(12-15), thus limiting access to the full phase diagram of the interacting electron liquid. Here we report the observation of bilayer Wigner crystals without magnetic fields or moire potentials in an atomically thin transition metal dichalcogenide heterostructure, which consists of two MoSe2 monolayers separated by hexagonal boron nitride. We observe optical signatures of robust correlated insulating states at symmetric (1:1) and asymmetric (3:1, 4:1 and 7:1) electron doping of the two MoSe2 layers at cryogenic temperatures. We attribute these features to bilayer Wigner crystals composed of two interlocked commensurate triangular electron lattices, stabilized by inter-layer interaction(16). The Wigner crystal phases are remarkably stable, and undergo quantum and thermal melting transitions at electron densities of up to 6 x 10(12) per square centimetre and at temperatures of up to about 40 kelvin. Our results demonstrate that an atomically thin heterostructure is a highly tunable platform for realizing many-body electronic states and probing their liquid-solid and magnetic quantum phase transitions(4-8,17). Optical signatures reveal correlated insulating Wigner crystals-electron solids-in a bilayer of a two-dimensional transition metal dichalcogenide, MoSe2, with hexagonal boron nitride between the layers.