Green's-function theory of the ferroelectric phase transition in potassium dihydrogen phosphate (KDP)

The method of the two-time Green's function is applied to the investigation of the ferroelectric phase transition in K${\mathrm{H}}_2$P${\mathrm{O}}_4$. The zero-frequency difficulties are circumvented by means of the fermion Green's function. It is shown that the softening of the mixed mode and the onset of static long-range order take place at the same temperature.