A two-dimensional Fermi gas with an attractive s-wave interaction is shown to display a finite number of bound fermion pairs at all densities and temperatures. We study the chemical equilibrium between bound and ionized fermion pairs by including single-pair fluctuations in the free energy. While this turns out to be sufficient to drive the critical temperature for the onset of long-range order to zero, higher-order processes must be invoked to describe correctly the low-temperature regime.

• Received 1 February 1989

DOI:https://doi.org/10.1103/PhysRevLett.63.445