USE OF THE 7.6 MEV O-16(ALPHA,ALPHA) RESONANCE IN STUDYING THE ANOMALOUS CHANNELING BEHAVIOR OF YBA2CU3O7-X NEAR T-C

By using the intense O-16 (He-4, He-4) elastic scattering resonance at 7.3-7.6 MeV, we have been able to extend the earlier investigations [ 1-4] of the channeling behaviour in YBa2Cu3O7-x around T-c to include the backscattering signal from the oxygen sublattice. We have also dev eloped a computer code enabling us to simulate c-axis angular scans fo r each atomic sublattice as a function of depth. These simulations rep roduce very well the observed oxygen angular scans at 70 K (below T-c) and at 100 K (above T-c), with magnitudes of the thermal vibrational amplitudes which are in reasonable agreement with neutron-scattering d ata. Concerning small changes at T-c, as observed in earlier channelin g investigations, we have concentrated on the analysis of oxygen scans . A direct fit of the simulations to our measurements indicates an inc rease above T-c of about 0.5 pm in the amplitude of oxygen vibrations perpendicular to the c-axis, or a smaller increase combined with a sta tic displacement. Within the estimated uncertainties, this result is c onsistent with the neutron data. The oxygen (O4) atoms on the CuO rows along the c-axis contribute a wider channeling dip than the other oxy gen atoms (O1, O2 acid O3), which form pure oxygen rows along the c-ax is. Hence, the magnitude of the O4 vibrational amplitude is important only for the yield in the shoulders of the combined oxygen dip and it can be determined independently from the fit to measurements. Our resu lts do not confirm the large change of this amplitude at T-c suggested by Remmel et al. [8].