Chapter 194 Electronic 4f state splittings in cuprates

This chapter describes the studies undertaken to characterize the electronic splitting of the rare-earth 4f states in high-TC superconductors and introduces the crystal-electric-field (CEF) formalism used in the analysis, together with an overview of the method by which the potentials were determined. Systematic sets of CEF parameters are presented in tables, together with the energies, wavefunctions, and transition-matrix elements for each R in the series. These representations of the R-ion energetics form the basis from which to build an understanding of the magnetic properties of high-TC superconductors. The use of the rare-earth f states as a local probe to monitor changes of the local charge distribution, and resulting models of clustering and percolative superconductivity, are discussed. Also discussed are s-f, f-f, and f-phonon interactions. Whereas optical spectroscopy is the technique most often used to provide data about the splitting of the 4f configuration in a solid material, the conductivity of the high-temperature superconductors renders them opaque to optical photons. Most of the data available for analysis are obtained from inelastic neutron scattering (INS). INS is usually limited to energies less than about 150 meV.