The behaviour and properties of in-situ formed YBa2Cu 3Ox thin films during reversible oxidation/deoxidation

The authors have reversibly oxygenated and deoxygenated in situ-formed thin films of YBa₂Cu₃O_x by means of low-temperature anneals in oxygen and argon. The transition shape for slightly deoxygenated material is consistent with the existence of two superconducting phases, which may be distinguished by different oxygen ordering. The authors investigated J_c(B) as a function of oxygen content in samples whose oxygen content has been controlled by low-temperature anneals. The steep rise in J_c with T_c. has been demonstrated and the effect on J_c of change in c investigated. It is confirmed that in situ-formed films which have low T_c as-deposited have anomalously high c values, but it is demonstrated that good films (that is, films which as deposited have c~1.168 nm and T_c~90 K) follow the bulk c vs. T_c relationship when deoxygenated and reoxygenated in low-temperature anneals. The cause of the c expansion is attributed to a defect, possibly the substitution of Ba for Y, introduced at the time of film deposition under conditions of low mobility. Some data have been obtained which suggest that films deposited on substrates at higher temperatures have c vs. T_c more consistent with bulk data