Magnetic correlations and energy gap in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6.6}$ with ${\mathit{T}}_{\mathit{c}}$=53 K

The dynamic magnetic correlations have been characterized in a large, orthorhombic single crystal of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ with x=0.6, having a ${\mathit{T}}_{\mathit{c}}$ of 53 K. Inelastic-neutron-scattering measurements reveal a gap (zero magnetic cross section) of 5 meV in the spin-excitation spectrum at 10 K. The size of the gap fits well between those reported by Rossat-Mignod et al. for x=0.51 and 0.69. However, the rapid change in the gap size with a relatively small change in ${\mathit{T}}_{\mathit{c}}$ is not understood.