PRESSURE-INDUCED STRUCTURAL AND ELECTRONIC-PROPERTIES OF HIGH-TC SUPERCONDUCTING MATERIALS STUDIED BY NEUTRON-SCATTERING

We describe the Inelastic-neutron-scattering technique to observe the crystal field in high-Tc superconductors as a direct probe of the local symmetry and the charge distribution at the rare-earth site. This is exemplified for the compounds Nd2 - xCexCuO4, ErBa2Cu3Ox and ErBa2Cu4O8. For ErBa2Cu3Ox we succeeded to directly prove the oxygen-vacancy Induced charge redistribution in the CuO2 planes. An empirical relation between Tc and the observed charge transfer Δo was derived which is highly nonlinear close to Tc = 90 K. Crystal-field studies performed for ErBa2Cu3O7 and ErBa2Cu4O8 under external pressure up to 10 kbar can be consistently described within this picture. © 1990.