Spin Dynamics in the Square-Lattice Antiferromagnet

We apply the Schwinger boson mean-field theory to the square-lattice Heisenberg antiferromagnet at low temperatures. For spin ½ we confirm the renormalized classical behavior of the correlation length ${\kappa }^{-1\mathrm{}}\mathrm{}\left(T\right)\mathrm{}$ without appealing to spin-wave theory. We also present a detailed calculation of the dynamical structure factor. A quasielastic peak is featured near $\mathbf{q}=(\pi ,\pi )$, while for $|\mathbf{q}-(\pi ,\pi \left)\right|\gg \kappa$, spin-wave ridges appear. The uniform susceptibility interpolates between spin-wave results at $T=0\mathrm{}$ and the high-temperature series of Rushbrooke and Wood. Our theory is distinct from theories in which fermionic spinon excitations determine the low-temperature spin dynamics.