Ion-channeling study of anomalous atomic displacements at the superconducting transition in high-(Tc) materials.

摘要

Ion channeling along the (001) direction in high-quality single crystals of (Y/Er)Ba(sub 2)Cu(sub 3)O(sub 7-x) revealed an abrupt change in displacements in the a-b plane of the Cu and O atoms at the superconducting transition, (Tc); normal ''Debye-like'' vibrations were found for the Y/Er and Ba atoms. The anomalous change in Cu-O displacements was found to shift directly with stoichiometry-induced changes in (Tc), implying a direct link between the observed phonon anomaly and the superconducting transition. Recent measurements of ion-channeling along the (001) axis in (Bi(sub 1.7)Pb(sub 0.3))Sr(sub 2)Ca(sub 1)Cu(sub 2)O(sub x) single-crystals revealed a similar change at (Tc), suggesting that this phonon anomaly is a general feature of high-(Tc) superconductivity. In order to identify more specifically the crystallographic directions and displacement amplitudes associated with the anomalous phonon behavior, axial channeling scans using RBS, as well as characteristic x-ray production, were taken at several temperatures between 30 and 300K along the (301) and (331) directions of YBa(sub 2)Cu(sub 3)O(sub 7-x) single crystals. Twins present in the specimens, and the existing static atomic displacements present along these directions, caused the channeling to be poorer along these axes compared to the (001) direction. Also, a much stronger dependence of the minimum yield on depth was observed. However, since only one twin variant generally dominated over sufficiently wide areas of the specimens, reasonably good ((approximately)10%) minimum yields could be obtained along the appropriate (331) axis, and detwinned crystals produced good results along (301). 22 refs., 5 figs.