A two-level model of a weakly nonideal Bose gas for a metastable (superfluid) state with pairwise interaction of the following type is considered: any pair of bosons (that happen to be) at the upper level give off a quantum of energy of the order of $1/N$, where $N$ is the number of particles. It is demonstrated that at a certain temperature a second-order phase transition with critical index $1/2$ and with fountain effect as a thermo-mechanical effect takes place.
Department of Quantum Statistics and Field Theory, Faculty of Physics, Moscow State University, Leninskie Gory, Moscow, 119992, Russia
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