Anharmonic vibrations of apex oxygen atom in YBa2Cu3O7-δ and doping effects on the out-of-plane electrical resistivity

The effects of the quantum tunneling of the apex oxygen atom O(4) on the electrical resistivity ρ{variant}c in YBa2Cu3O7-δ have been investigated at different oxygen depletions. Path techniques have been applied to derive the temperature-dependent coupling constants between the double-well potential (DWP) provided by the O(4) atom and the normal electrons. It is found that: (a) in a strong-coupling regime, the scattering mechanism gives rise to ρ{variant}c∼Tα with 1.5<α<2; (2) in a weak-coupling regime, ρ{variant}c∼T0.4 at temperatures higher than 2Tc whereas dρ{variant}c dT is slightly negative in the region of critical fluctuations. ρ{variant}c is below the Mott-Ioffe-Regel limit in YBa2Cu3O7 in agreement withthe experiments. Therefore the O(4) atom should be weakly coupled to the electrons and the tunneling energy between the minima of the DWP is ∼ 10 meV at T=100K, consistent with the estimation given in Ref. [8]. By reducing the oxygen content, the tunneling is slowed down and the charge transfer along the c-axis is smaller; then, the ρ{variant}c values increase and a non-metallic temperature dependence is obtained. © 1994.