Neutron crystal-field spectroscopy in underdoped and overdoped copper oxide superconductors

In many cuprate superconductors rare-earth (R) ions can be placed close to the superconducting copper oxide planes; thus, the crystal-field interaction at the R site constitutes an ideal probe of the local symmetry as well as the local charge distribution and thereby monitors directly changes of the carrier concentration induced by doping. For several compounds the crystal-field spectra observed by inelastic neutron scattering separate into different local components whose spectral weights distinctly depend on the doping level, i.e., there is clear experimental evidence for the formation of clusters which make the systems inhomogeneous. Moreover, it is found that the intrinsic linewidths of crystal-field transitions vary with temperature, which is essentially a reflection of the density of states associated with the charge carriers at the Fermi energy. Linewidth studies can therefore reveal information about the energy gap. For underdoped systems there is evidence for the formation of a pseudogap at T* > T-c.