SPECTRAL SHIFT OF THE MAGNETIC CROSS-SECTION IN SUPERCONDUCTING YBA2CU3O6+X

A triple-axis neutron-scattering study has been done of the magnetic correlations in three large single crystals of YBa2Cu3O6+x with x = 0.5, 0.8, and 0.9 having T(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 approximately 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.