Exciton condensation evidence and competing optical signatures

A condensation claim needs macroscopic low-energy occupation and spontaneous coherence together with a model of cooling, lifetime and pumping. Spectral narrowing, a nonlinear intensity threshold or long-range transport alone can have competing explanations.

A population of excitons can approach a thermal distribution only if redistribution and cooling occur before too many excitons disappear. A useful necessary scale comparison is ${\tau }_{\mathrm{therm}}\ll {\tau }_{\mathrm{life}}$, where the lifetime includes radiative and nonradiative recombination and the thermalization time describes the relevant energy redistribution.

The inequality is not sufficient to establish exciton condensation: density, disorder, pumping and the stability of the excitons also matter. A continuously pumped short-lived population can have a stationary distribution without satisfying equilibrium statistical mechanics. Spectral narrowing or a nonlinear emission threshold can therefore support several interpretations. Relating such observations to condensation requires a model of creation, cooling, interactions and losses, as emphasized in exciton condensation evidence and competing optical signatures.