T_c increased to 90 K in YBa₂Cu₄0₈ by Ca doping

Abstract

THE two superconducting phases YBa₂Cu₄O₈ (124), containing double Cu–O chains, and Y₂Ba₄Cu₇O₁₅ (247), containing both single and double Cu–O chains, were originally discovered as lattice defects in the 90-K superconductor YBa₂Cu₃O₇ (123)¹. The synthesis of bulk superconducting 124 may be effected by a high-oxygen-pressure annealing technique^2,3 and by reaction with ambient oxygen in the presence of alkali carbonates⁴. The oxygen stoichiometry of 124 is much more stable against high temperatures than that of 123 (ref. 2), a feature that is important for practical applications. But the lower T_c (80 K) of 124 presents a major obstacle to practical uses of this phase at liquid-nitrogen temperature (77 K). Here we report that partially substituting Ca for Y in 124 increases T_c. A superconducting transition of ∼90 K was achieved for the composition Y_0.9 Ca_0.1, Ba₂Cu₄O₈.