  The ground state of a microcavity polariton Bose-Einstein condensate is
determined by considering experimentally tunable parameters such as excitation
density, detuning, and ultraviolet cutoff. During a change in the ground state
of Bose-Einstein condensate from excitonic to photonic, which occurs as
increasing the excitation density, the origin of the binding force of
electron-hole pairs changes from Coulomb to photon-mediated interactions. The
change in the origin gives rise to the strongly bound pairs with a small
radius, like Frenkel excitons, in the photonic regime. The change in the ground
state can be a crossover or a first-order transition, depending on the
above-mentionsed parameters, and is outlined by a phase diagram. Our result
provides valuable information that can be used to build theoretical models for
each regime.
