2001, Volume 4, Number 1, pp.13-17

We analyze radiative interaction of two-level molecules in a thermal gas and find that their cooperative dynamics can result in the self-organization of molecular dipole moments, i.e., the spontaneous formation of a static polarization structure. Within quasiclassical consideration of translational motion of molecules in the local field of the corresponding transverse polarization wave, it is shown that actual energy levels of a single molecule coincide with its quasienergies, which are well known in quantum optics. This allows us to formulate a generalized Gibbs distribution over quasienergy states of molecules and to apply it for investigation of the statistical properties and specific features of the antiferroelectric phase in the mean-field approach. We find that the latter may appear, in principle, via not only the second-order, but also the first-order phase transition. However, the first-order one is weak and exists in a rather narrow range of parameters because it is caused by a small group of molecules taking part in the resonance interaction with the polarization wave. The effect of the second-order antiferroelectric phase transition is owing to the collective interaction of the greater part of molecules and therefore takes place under less exotic conditions, which seem to be realizable experimentally.
Key words: cooperative phenomena, mean field approach, quasienergy states, Gibbs distribution, antiferroelectric phase transition

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