Ground-state properties of microcavity polariton condensates at arbitrary excitation density

The ground state of microcavity polariton Bose-Einstein condensates (BEC's) is determined as a function of experimentally tunable parameters (the excitation density and the detuning of cavity photons), and also a material parameter (the ultraviolet cutoff). To obtain the ground state at an arbitrary excitation density, an interpolation method for the BEC-BCS crossover of excitonic insulators is extended to microcavity polariton systems in two or three dimensions. The ground state of the condensate changes from excitonic to photonic with an increase in the excitation density. This change is accompanied by several interesting features: (i) A laserlike input (excitation density) and output (photon density) relation with a sharp onset for largely detuned systems, which changes to that with a smooth onset for slightly detuned systems. (ii) The origin of the binding force of electron-hole pairs changes from Coulomb attraction to photon-mediated interactions, resulting in the formation of strongly bound pairs with a small radius, such as 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-mentioned parameters, and is studied by plotting phase diagrams.