  We consider a system of globally coupled rotors, described by a set of
Langevin equations, and examine stability of the incoherent phase. The
corresponding Fokker-Planck equation, providing a unified description of
microcanonical and canonical ensembles, bears a few solutions, depending upon
the ensemble. It is found that the stability of each solution varies
differently with the temperature, revealing the inequivalence between the two
ensembles. This also suggests a physical explanation of the quasi-stationarity
observed in recent numerical results.
