Pressure-induced structural and electronic properties of high-Tc superconducting materials studied by neutron scattering

Abstract We describe the Inelastic-neutron-scattering technique to observe the crystal field in high-T c superconductors as a direct probe of the local symmetry and the charge distribution at the rare-earth site. This is exemplified for the compounds Nd 2 − x Ce x CuO 4 , ErBa 2 Cu 3 O x and ErBa 2 Cu 4 O 8 . For ErBa 2 Cu 3 O x we succeeded to directly prove the oxygen-vacancy Induced charge redistribution in the CuO 2 planes. An empirical relation between T c and the observed charge transfer Δo was derived which is highly nonlinear close to T c = 90 K. Crystal-field studies performed for ErBa 2 Cu 3 O 7 and ErBa 2 Cu 4 O 8 under external pressure up to 10 kbar can be consistently described within this picture.