Possible observation of polaron pairs in highly doped YBa2Cu3O6+x by elastic energy loss

Anelastic relaxation measurements were made on polycrystalline YBa2Cu3O6+x with x close to 1. Among the three low-activation-energy processes which are present, that with the lowest activation enthalpy (H ~ 0.08 eV) is the most sensible to small variations of the O stoichiometry and becomes undetectable when x < 0.85. The relaxation process, which occurs in the superconducting state, is interpreted in terms of pairs of pz holes in the apical oxygens, which have been previously observed by X-ray absorption spectroscopy in highly doped YBa2Cu3O6+x, and which are supposed to be far less mobile than the conducting holes.