Electrical conductivity of YBa ₂Cu ₃O _7-δ single crystals under conditions of anionic ordering in Cu(1)O _1-δ layers

The influence of thermocycling annealing processes on the oxygen ordering degree (order parameter) in YBa₂Cu₃O_7-δ single crystals has been studied. It has been shown that an increase in the critical onset temperature of the transition to the superconducting state during thermocycling annealing procedures is consistent with the decrease of the σ_с/σ_аb parameter. This fact indicates a redistribution of the electronic density between the structurally inhomogeneous Cu(2)O₂ and Cu(1)O_1-δ planes, due to the formation of oxygen long-range order in the O(4)–Cu(1)–O(4) linear groups along the (b) crystal structure axis of the unit cell, and elimination of oxygen defects in the square nets of the Cu(2)O₂ planes. The existence of the critical value of the conductivity anisotropy σ_с/σ_аb, below which its behavior does not correlate with the change of Т_с, has been confirmed. In this case an increase in Т_с and orthorhombic distortion of the crystal structure during isothermal annealing are caused by the amplification of the “interlayer” interaction between the Cu(2)О₂ and Cu(1)О_1-δ planes. As a result, the contribution of the Cu(1)О_1-δ chain layers to the electron state density at the Fermi level increases. These layers can acquire superconducting properties due to tunneling of Cooper pairs from the Cu(2)О₂ planes resulting in the formation of the induced superconductivity in these planes.