Using large single crystals (∼2 ${\mathrm{cm}}^3$), we have extended previous elastic and inelastic neutron scattering studies of the spin dynamics in the two-dimensional (2D) quantum antiferromagnet ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. We observe well-defined magnetic peaks for constant energy scans across the 2D magnetic rod for a wide range of temperatures and energies. The spectra are compared quantitatively with theoretical models. Simple spin-wave theory satisfactorily describes the experimental results well below $T_N$. The thermal excitations of the low-energy (ħω≤3 meV) spectra, however, appear to deviate from the theory as $T_N$ is approached. The overall temperature and energy dependence of the spectra above $T_N$ (245–520 K) can be explained by the formula of Chakravarty, Halperin, Nelson, and Tyc for the dynamical structure factor S(q,ω) with no adjustable parameters. We do, however, observe a 2D elastic component which grows rapidly as $T_N$ is approached; it is hypothesized that this arises primarily from magnetic defects and is analogous to the ‘‘central peak’’ observed for many structural phase transitions.

• Received 10 May 1989

DOI:https://doi.org/10.1103/PhysRevB.40.4557