Energy gap, dynamic correlations, and correlation length in two-dimensional antiferromagnets

We present a phenomenological theory of the low-temperature behavior of two-dimensional, square-lattice antiferromagnets. At finite temperatures the instantaneous correlations behave as in the ordered (T=0) state up to a distance of the order of the thermal wavelength ${\lambda }_T$=ħc/$k_B$T (c is the spin-wave velocity) and then decay with a characteristic length ${\xi }_c$. Antiferromagnetic excitations have a minimum energy ħ${\omega }_g$=ħc/${\xi }_c$. We argue that neutron scattering experiments in ${\mathrm{La}}_2$ ${\mathrm{CuO}}_4$ provide a sensitive measurement of the effect of quantum fluctuations on the ground-state sublattice magnetization.