SPIN-WAVE EXCITATIONS IN THE SQUARE-LATTICE HEISENBERG-ANTIFERROMAGNET

From the second time derivative of the z-component spin Sz we have derived two types of spin wave excitations in the square lattice Hesenberg antiferromagnet within the effective-field approximation: In the long-wavelength limit, one has a vanishing energy and the other a finite energy, i.e., a gap. On the assumption that the fluctuations of Sz strongly correlate each other between the two sublattices, we have calculated the correlation function and the temperature-dependent correlation length from the excitation with a gap. In the zero-temperature limit, the correlation length diverges and the correlation function behaves as 1/x, where x is the distance between two spins. This result seems to support the fact that the ground state does not have the long-range order with a broken symmetry. © 1990, THE PHYSICAL SOCIETY OF JAPAN. All rights reserved.