Superconductivity-Induced Anomalies in the Spin Excitation Spectra of Underdoped YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ $O_{6+\mathit{x}}$

Polarized and unpolarized neutron scattering have been used to determine the effect of superconductivity on the magnetic excitation spectra of YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_{6.5}$ $(T_c=52\mathrm{K})$ and YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_{6.7}$ $(T_c=67\mathrm{K})$. Pronounced enhancements of the spectral weight centered around 25 and 33 meV, respectively, are observed below $T_c$ in both crystals, compensated predominantly by a loss of spectral weight at higher energies. The data provide important clues to the origin of the 40 meV magnetic resonance peak in YB${\mathrm{a}}_2$C${\mathrm{u}}_3$ ${\mathrm{O}}_7$.