Two-Dimensional Antiferromagnetic Excitations from a Large Single Crystal of ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.2}$

A large single crystal (7 × 7 × 3 ${\mathrm{mm}}^3$) of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6.2}$ has been grown and its magnetic excitations have been observed by neutron-scattering techniques. A two-dimensional magnetic ridge, similar to that observed for ${\mathrm{La}}_2\mathrm{Cu}{\mathrm{O}}_{4-y}$, has been clearly identified by triple-axis measurements. The slope of the dispersion curve for spin-wave excitations is very large (≳0.5 eV Å), consistent with the result of spinpair Raman scattering by Lyons et al.