Abstract
The two-magnon spectrum of the nearest-neighbour Heisenberg ferromagnet on a square lattice is studied. An exact Green function formalism is used to obtain the spectral functions at zero temperature. The lattice Green functions of this system are written in terms of elliptic integrals. The behaviour of the spectral functions at the critical points is studied analytically. Plots of the spectral functions against energy are given for some values of the wave-vector and their various features are discussed. Comparisons with the analogous 3D and 1D systems are made.