4f-levels in rare earth cuprates: Crystal field and exchange interaction

Measurements of the infrared transmission spectra up to similar to 10 000 c m(-1), ab-initio calculations based on the density functional theory and th e use of an anisotropic exchange Hamiltonian to describe the magnetic coupl ing of rare earth (RE)-Cu are new instruments shown in this paper to provid e an additional insight into the nature of the 4f splitting in rare earth ( RE) cuprates. (i) Sets of the parameters of the crystal field Hamiltonian f or trivalent Nd and Sm in regular as well as in Ba-sites in RE1+xBa2-xCu3O6 +y are determined; (ii) Nd2CuO4 is found to be an exceptionally suitable sy stem to study the anisotropy of the exchange interaction: Kramers doublet s plittings due to the superexchange Cu-Nd interaction, deduced from the inte rmultiplet transitions spectra, are interpreted in terms of an effective Ha miltonian for the Nd3+ ion expressed in terms of single electron spherical tensor operators up to the sixth order; (iii) it is shown that the defects in Nd2CuO4, Nd2-xGdxCuO4 and Nd2-xCexCuO4, that give rise to satellites in the spectra of 4f excitations, are universal and related to a local lattice deformation associated with a charge inhomogeneity. (C) 2001 Elsevier Scie nce B.V. All rights reserved.