Neutron Spectroscopy in RBa₂Cu₃O _x (R=Rare Earth, 6≤x≤7) Compounds: Charge Transfer, Phase Separation, Spin Fluctuations

Summary

Inelastic neutron scattering has been employed to study the perovskitetype high-T _c superconducting compounds RBa₂Cu₃O _x (R = rare earth;6 = x = 7). The variation of the energies and intensities of the observed crystalline-electric-field (CEF) transitions versus the oxygen content x is shown to be predominantly related to a charge transfer process between the chains and the planes. The observed energy spectra are the result of a superposition of two different metallic components and a semiconducting one, i.e., there is clear experimental evidence for phase separation. A two-dimensional bond percolation model explains the appearance of superconductivity as well as the critical oxygen concentrations associated with the two-plateau structure of T _c . The line shape of some low-energy R³⁺ excitations turns out to be highly asymmetric which we interpret in terms of an exchange interaction between the R³⁺ spins and fluctuating Cu²⁺ spins. The latter are most likely associated with low-energy spin excitations of spin-polarized polynuclear clusters of Cu²⁺ ions. From a line-width analysis of the CEF transitions we derive the evolution of the fractal sizes of the clusters versus the oxygen concentration x. Magnetic field-dependent neutron spectroscopic experiments give further evidence for the existence of spin-polarized clusters which are suggested to be the elementary building blocks giving rise to high-temperature superconductivity above the percolation threshold.