Direct observation of optical magnons in YBa2Cu3O6.2.

Abstract

We have used high-energy inelastic neutron scattering to detect optical magnons directly in antiferromagnetic Y${\mathrm{Ba}}_{2}$${\mathrm{Cu}}_{3}$${\mathrm{O}}_{6.2}$. The optical magnon gap is 67\ifmmode\pm\else\textpm\fi{}5 meV. This implies an intrabilayer superexchange constant perpendicular to the Cu${\mathrm{O}}_{2}$ layers of ${J}_{\ensuremath{\perp}}=0.08{J}_{\ensuremath{\parallel}}$ where ${J}_{\ensuremath{\parallel}}$ is the in-plane nearest-neighbor superexchange constant.