Exciton-polaritons constitute a unique realization of a quantum fluid interacting with its environment. Using Selenide based microcavities, we exploit this feature to warm up a polariton condensate in a controlled way and monitor its spatial coherence. We determine directly the amount of heat picked up by the condensate by measuring the phonon-polariton scattering rate and comparing it with the loss rate. We find that upon increasing the heating rate, the spatial coherence length decreases markedly, while localized phase structures vanish, in good agreement with a stochastic mean field theory. From the thermodynamical point-of-view, this regime is unique as it involves a nonequilibrium quantum fluid with no well-defined temperature, but which is nevertheless able to pick up heat with dramatic effects on the order parameter. For ultra-cold atoms kept in a magneto-optical trap, Bose-Einstein condensation is a phase transition entirely driven by thermal equilibration [1,2]. In other bosonic many-body systems this conclusion is sometime harder to reach and requires a careful examination. This is for example the case of photons stored in a cavity filled with dye molecules, for which condensation has been demonstrated [3]. In spite of the intrinsic driven-dissipative (DD) character of the system, it has been found eventually that thermalization happens at a much faster rate than the photon loss rate [4,5], via grand-canonical interaction with the molecules rovibronic degrees of freedom [6].Exciton-polaritons (polaritons) in semiconductor microcavities [7] constitute another example of a nontrivial open system, in which condensation has been reported [8,9] and studied. In this case, the DD dynamics usually has a strong influence on the phenomenon [10][11][12][13][14][15], which thus differs from its textbook thermal-equilibrium counterpart in many respects: condensation can for example take place in more than one state [16], and have a non-zero momentum as a result of broken time reversal symmetry [17,18], and a diffusive Goldstone mode is expected as the long wavelength excitations [19,20]. More fundamentally, it has been recently pointed out in a series of theoretical work that owing to its DD character, polariton condensation actually belongs to a universality class which is different from that of equilibrium systems [21], and which is common to phenomena as diverse as selection in predator-prey dynamics or the build up of traffic jam [22,23].In this work, we use a warm thermal bath of phonons (T = 150 â 250 K) as a strong heat source interacting with polaritons, to realize and characterize a condensation regime in which, as illustrated in Fig.1.b, thermal equilibration and the DD dynamics play an equally significant role in the phenomenon, and as a result lies
FIG. 1. (Color online)Schematic representation of two different regimes of polariton condensation. P is the nonresonant pump rate, ÎłT(T ) is the polariton-thermal phonons inelastic scattering rate, and Îł is the overall polariton loss rate. In a) ÎłT Îł such tha...