  Interlayer excitons, comprising an electron in one material bound by Coulomb
attraction to a hole in an adjacent material, are composite bosons that can
assume a variety of many-body phases. The phase diagram of the bosonic system
is largely determined by the dispersion relation of the bosons, which itself
arises as a combination of the dispersion relations of the electron and hole
separately. Here I show that in situations where either the electron or the
hole has a non-monotonic, "Mexican hat-shaped" dispersion relation, the exciton
dispersion relation can have a range of qualitatively different forms, each
corresponding to a different many-body phase at low temperature. This diversity
suggests a novel platform for continuously tuning between different quantum
phases using an external field.
