Electrical conductivity of YBa2Cu3O7-δ single crystals under conditions of anionic ordering in Cu(1)O1-δ layers

Abstract

The influence of thermocycling annealing processes on the oxygen ordering degree (order parameter) in theYBa₂Cu₃O_7-δ single crystals have been studied. It was determined that an increase in the critical temperature of the onset of the transition to the superconducting state during step annealing procedures is consistent with decrease of the σ_с/σ_аb parameter. This fact indicates the redistribution of the electronic density between the Cu(2)O₂ and Cu(1)O_1-d structurally-inhomogeneous planes, due to the formation of the oxygen long-range ordering in the O(4)—Cu(1)—O(4) linear groups along the (b) crystal structure axis of the unit cell, and removal of the oxygen defects in the square nets of the Cu(2)O₂ planes. The existence of a critical value of the conductivity anisotropy σс/σаb, below which its behavior does not correlate with the change of Т_с, has been proved. In this case, the increase of Тс and the orthorhombic distortion of the crystal structure at the isothermal annealing processes occur due to 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 in the electronstate density on the Fermi level increases. These layers could be the superconducting ones by means of the Cooper pairs tunneling from the Cu(2)О₂ planes, forming the induced superconductivity there.