We have explored the magnetic excitation spectrum of the $\mathrm{S}=1∕2$ square lattice Heisenberg antiferromagnet, ${\mathrm{K}}_2{\mathrm{V}}_3{\mathrm{O}}_8$, using both triple-axis and time-of-flight inelastic neutron scattering. The long-wavelength spin waves are consistent with the previously determined Hamiltonian for this material. A small energy gap of $72\pm 9\mu \mathrm{eV}$ is observed at the antiferromagnetic zone center and the near-neighbor exchange constant is determined to be $1.08\pm 0.03\mathrm{meV}$. A finite ferromagnetic interplanar coupling is observed along the crystallographic $c$ axis with a magnitude of $J_c=-0.0036\pm 0.0006\mathrm{meV}$. However, upon approaching the zone boundary, the observed excitation spectrum deviates significantly from the expectation of linear spin wave theory resulting in split modes at the $(\pi ∕2,\pi ∕2)$ zone boundary point. The effects of magnon-phonon interaction, orbital degrees of freedom, multimagnon scattering, and dilution/site randomness are considered in the context of the mode splitting. Unfortunately, no fully satisfactory explanation of this phenomenon is found and further theoretical and experimental work is needed.

• Received 28 September 2006

DOI:https://doi.org/10.1103/PhysRevB.74.214424