Neutron spectroscopy in rba2cu3Ox (R  Ho, Er; 6 <x⩽7) compounds

Abstract

Abstract Inelastic neutron scattering has been employed to study the perovskite-type high c superconducting compounds RBa 2 Cu 3 O ϵ (R  Er, Ho; 6 ϵ ⩽ 7). For ErBa 2 Cu 3 O ϵ , we have determined the crystalline electric field (CEF) level scheme as a function of the oxygen content ϵ. The variation of the energies and intensities of the CEF transitions versus ϵ 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 . For HoBa 2 Cu 3 O 7 , we have measured the low-lying magnetic excitations of Ho 3+ ions in a grain-aligned sample. From the dispersive behavior of the spin-wave excitations, we derive the nearest-neighbor magnetic coupling parameters in the Heisenberg approximation. From complimentary experiments on a magnetically diluted Ho 0.1 Y 0.9 Ba 2 Cu 3 O 7 compound, we have been able to observe Ho 3+ dimer excitations which allow a direct determination of the magnetic coupling strength in the ( a , b ) plane. The line shapes of the Ho 3+ excitations turn out to be highly asymmetric which we interpret in terms of an exchange interaction between the Ho 3+ spins and the fluctuating Cu 2+ spins.