QUARTET STATE OF THE 2-DIMENSIONAL HEISENBERG-MODEL WITH THE SPIN-1 2ON A SQUARE LATTICE/

The low-energy properties of the two-dimensional Heisenberg model with spin-1/2 on a square lattice are investigated on the basis of the loc al dimer order. The lattice is divided into square blocks consisting o f the quarters of spins. The spin variables and the Heisenberg Hamilto nian are expressed in terms of the low-energy quartet variables. On th e basis of the Dyson-Maleev representation, the spin-wave theory of th e quartet state is developed. The spectrum of the lower magnon excitat ions consists of three degenerate modes with the energy gap DELTA = 0. 17J. The ground state energy per spin E/N = -0.6J. Calculations of the basic corrections make the work complete.