High-energy spin waves in ${\mathrm{La}}_2$${\mathrm{CuO}}_4$

Time-of-flight spectrocopy using neutrons produced by a spallation source is used to measure the one-magnon scattering throughout the Brillouin zone for ${\mathrm{La}}_2$${\mathrm{CuO}}_4$. The zone-boundary magnons have an energy ħ${\mathrm{\omega }}_{\mathrm{ZB}}$=0.312±0.005 eV and are good eigenstates of the quantum Heisenberg Hamiltonian in that they possess lifetimes >10/ω. A multiplicative renormalization of the overall frequency scale of classical spin-wave theory accounts for the quantum effects in the one-magnon spectrum.