Exciton condensation is collective quantum coherence of electron-hole pairs or excitonic degrees of freedom. A finite exciton population alone does not establish it; the central issue is macroscopic phase coherence or the corresponding ordered state.
The physical setting matters. Long-lived excitons in a semiconductor, electron-hole bilayers and driven exciton-polariton systems have different conservation laws, decay channels and interactions. A spectral feature or an increase in emitted intensity therefore needs supporting coherence and dynamical evidence before being interpreted as condensation.[ref]
Bose-Einstein Condensation of Excitons in Cu2O: Progress Over Thirty Years by David Snoke and G. M. Kavoulakis. review of experimental criteria and exciton systems
New to topics? Read the docs here!