Use of the 7.6 MeV 16O(α, α) resonance in studying the anomalous channeling behaviour of YBa 2Cu 3O 7 − x near Tc

Abstract

By using the intense 16O( 4He, 4He) 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 YBa 2Cu 3O 7 − x around T c to include the backscattering signal from the oxygen sublattice. We have also developed a computer code enabling us to simulate c-axis angular scans for each atomic sublattice as a function of depth. These simulations reproduce 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 data. Concerning small changes at T c, as observed in earlier channeling investigations, we have concentrated on the analysis of oxygen scans. A direct fit of the simulations to our measurements indicates an increase 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 static displacement. Within the estimated uncertainties, this result is consistent with the neutron data. The oxygen (O4) atoms on the CuO rows along the c-axis contribute a wider channeling dip than the other oxygen atoms (O1, O2 and O3), which form pure oxygen rows along the c-axis. 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 results do not confirm the large change of this amplitude at T c suggested by Remmel et al. [8].