Spin waves in the flux-phase description of theS=1/2 Heisenberg antiferromagnet

Abstract

A so-called ‘‘flux-phase’’ projected-fermion mean-field theory has been shown to give a good account of the ground state and excitations of the S=1/2 Heisenberg antiferromagnet on a square lattice. In this paper it is shown that random-phase-approximation spin waves (or paramagnons) of the flux phase have an unusual spectrum, especially near the zone boundary. There is no singularity in the spin-wave density of states at maximum energy, and the small-momentum spin-wave-velocity renormalization is obtained. Noninteracting spin waves are shown to give reasonable description of Raman scattering. It is argued that the conventional picture of strongly interacting, damped Holstein-Primakoff spin waves may in fact be described in terms of less incoherent excitations in this new basis. The flux-phase basis arises through considering the antiferromagnet as the large-U limit of the half-filled Hubbard model. In that model it is suggested that the flux should turn on for U>${\mathit{U}}_{\mathit{c}}$∼3.1.