Spectral characteristics of the antiferromagnetic spin-1/2 Heisenberg model on the square lattice in a magnetic field

We predict that spin-waves in an ordered square quantum antiferromagnet in a magnetic field (h) perpendicular to antiferromagnetic plane may demonstrate three modes of spin excitations. Starting from the self-consistent rotation-invariant Green's function method, a new mean-field theory is constructed for h not equal 0. The method preserves the translational and the axial symmetries, and provides exact fulfillment of the single-site constraint for each of the three modes. We examine the dynamical structure factors S-alpha alpha(k, omega),alpha(beta) = x, y, z. It is shown, that the introduction of h leads to the hybridization of two degenerate spin modes due to the appearance of a non-diagonal on alpha, beta spin-spin Green's functions. The comparison of the theory with the exact diagonalization study and with results on inelastic neutron scattering experiments is discussed at T = 0. We discuss also the correspondence of the present theory to the existing theories, which allow only two spin excitations modes for the total S(k, omega).

Automated summary

In this study, we investigate the spin excitations in an ordered square quantum antiferromagnet under a magnetic field, predicting the emergence of three modes and a hybridization effect due to the field. The theoretical predictions are compared with exact diagonalization and neutron scattering experiments to confirm the validity of the model.