Crystal field interaction for the superconducting ceramics TmBa2Cu3O7 and TmBa2Cu4O8

For most high temperature ceramic superconductors, the rare earth ions are situated close to the Cu-O planes in which the superconducting carriers are believed to be located. The crystal electric field acting at these rare earth ions is sensitive to the structural arrangement of the oxygen nearest neighbours and their associated charge distribution. The charge distribution is dependent on the degree of charge transfer which, in turn, is closely linked with the existence (or non-existence) of superconductivity. Because of this, considerable experimental effort has been directed at characterising the rare earth crystal field for 1:2:3-type (RBa2Cu3O7; R=rare earth) superconductors, and at investigating how the crystal field is influenced by oxygen deficiency. The most reliable crystal field parameter determinations in the literature appear to be those based on inelastic neutron scattering measurements, although other techniques have been employed. This work compares the crystal field parameters available for both types of superconductors and tests them against existing 169Tm Moessbauer measurements for TmBa2Cu3O7 and TmBa2Cu3O6, and new measurements (this work) for TmBa2Cu4O8