Strong dependence on doping of a low-activation-energy relaxation process in YBa2Cu3O6+x: Possible polaron relaxation

The elastic energy loss of YBa2Cu3O6+x is measured below room temperature with x close to the maximum stoichiometry, where three low-activation-energy peaks are present. It is found that the intensity of the process with the lowest activation energy, E~0.076 eV decreases very steeply when x is lowered just below its maximum value; it becomes undetectable when the sample is still in the Tc=90°K plateau and all the other peaks are nearly unaffected. The possible mechanisms for this process are discussed, mainly in terms of hopping of polarons or off-center atoms. The characteristics that these defects should possess in order to produce anelastic relaxation are specified. The most natural way to interpret the peak is the assumption of the formation and reorientation of bipolarons among orbitals which are occupied by holes only at the highest O stoichiometries; according to Tolentino et al. [ Physica C 192 115 (1992)], such holes could reside in the pz orbitals of the apical O atoms.