Superconducting Properties of Oxygen Deficient YBa₂Cu₃O_7-δ

Abstract

An important and interesting feature of the high temperature superconductor YBa₂Cu₃O_7-δ is the dramatic effect of oxygen vacancies on its superconducting properties. 1–10 As oxygen is systematically removed from the structure, the orthorhombic distortion decreases and the system undergoes a structural transition to a tetragonal phase. Simultaneously, T_C falls monotonically, with superconductivity disappearing at approximately the stoichiometry where the orthorhombic to tetragonal structural transition occurs. The curve of T_C versus δ shows two regions of stable superconductivity where T_C is not a strong function of δ. For δ near zero, there is a nearly flat region where T_C is close to 90 K and for.35 < δ <. 45 there is a plateau where T_C remains constant at approximately 60 K. The width of the inductive transition in these two regions is narrow suggesting that the superconductivity is due to a homogeneous well defined phase. In order to investigate the mechanisms and properties of superconductivity in YBa₂Cu3O_7-δ it is necessary to characterize and compare the superconducting behavior in these two phases. In this paper we present resistivity and magnetization data from which we determine the upper critical field H_c2 and compare the relative pinning strengths of polycrystalline samples with δ = 0.04 and 0.4.