Crystal field, phase separation, and percolative superconductivity in ErBa2Cu3Ox (6<x<7) (abstract)

Abstract

Inelastic neutron scattering has been employed to study the crystalline-electric-field (CEF) level scheme of Er3+ in ErBa2Cu3Ox (6<x<7). We have been able to resolve all the seven ground-state CEF transitions for a series of samples covering the superconducting as well as the semiconducting phases. All the CEF transitions are characterized by a monotonic behavior of their energies and intensities versus the oxygen content x, with the exception of the lowest-lying CEF state which exhibits a most unusual behavior: when going from x=7 to x=6, we observe a major shift to lower energies and a significant decrease of the intensity: moreover, there is evidence for real substructures associated with this CEF transition as shown in Fig. 1. More specifically, we find the observed energy spectra to be the result of a superposition of three stable states, whose spectral weights distinctly depend on the oxygen content x. We identify the three stable states A1, A2, and A3 by two local regions of metallic (Tc≊90 K, Tc≊60 K) and a local region of semiconducting character, respectively, i.e., there is clear experimental evidence for phase separation. The superconducting phases are shown to result from the formation of a two-dimensional percolative network. A bond percolation model correctly predicts the critical oxygen concentrations associated with the two-plateau structure of Tc. A preliminary account of the present work has recently been published.1