Spectral shift of the magnetic cross section in superconducting ${\mathrm{YBa}}_2$${\mathrm{Cu}}_3$${\mathrm{O}}_{6+\mathit{x}}$

A triple-axis neutron-scattering study has been done of the magnetic correlations in three large single crystals of ${\mathrm{YBa}}_2$ ${\mathrm{Cu}}_3$ ${\mathrm{O}}_{6+\mathit{x}}$ with x=0.5, 0.8, and 0.9 having ${\mathit{T}}_{\mathit{c}}$’s of 50, 60, and 80 K, respectively. The frequency dependence of the cross section at Q=(1/2,1/2,l) was found to peak near 5 meV for x=0.45 in an earlier study. Our data confirm that this peak shifts to ∼30 meV for x=0.5. Moreover, the low-frequency (<40 meV) spectra for x=0.8 and 0.9 become progressively suppressed with respect to those of the less oxygenated samples. These data are consistent with a picture in which the spectral weight of the magnetic scattering shifts to higher energy as the oxygen concentration x and carrier density increase.