Mott insulator of strongly interacting two-dimensional semiconductor excitons

In condensed-matter physics, Mott insulators are an important phase involving strongly interacting electrons because of their intricate relationship with high-temperature superconductors(1,2). Mott phases were recently observed for both bosonic and fermionic species in atomic systems(3-9). However, in the solid state, the fingerprint of a Mott insulator implemented with bosons has yet to be found. Here we demonstrate such signature by exploring the Bose-Hubbard model using semiconductor excitons confined in a two-dimensional lattice. We emphasize the regime where on-site interactions are comparable to the energy separation between lattice-confined states. We then observe that a Mott phase is accessible, with at most two excitons uniformly occupying each lattice site. The technology introduced here allows us to programme the geometry of the lattice that confines the excitons. This versatility, combined with the long-range nature of dipolar interactions between excitons, provides a route to explore many-body phases that spontaneously break the lattice symmetry(10,11).