Effects of Li doping on superconducting properties of citrate-gel prepared Y_1–xLi_xBa₂Cu₃O_7–δ Compound

Mahshid Amiri–Moghadam, Hassan Gashmard, Seyed Sajjad Hosseini, Hamideh Shakeripour, Hadi Salamati

The Y_1–xLi_xBa₂Cu₃O_7–δ polycrystalline bulk superconductors doped with Li substituting at the Y site at different concentrations (x = 0, 0.01, 0.02, 0.1) were prepared using the citrate-gel method to study the effects of doping on the superconducting temperature and critical current density. The question was whether Li addition characterized by a high Debye frequency would have any positive effects on T_c. The optimum citrate-gel and heat treatment conditions were identified as those yielding samples with a maximum grain size on the order of 50 μm (up to the optimum Li doping level, x = 0.01). Li substitution at the Y site was verified by structural, electrical, and magnetic measurements of the produced samples, whereas X-ray diffraction (XRD) analysis revealed the formation of a pure phase with no visible impurity phases. Moreover, AC magnetic susceptibility measurements showed no increase in the superconducting transition temperature T_c, consistent with the predicted results obtained by machine learning method, although it was theoretically expected to increase owing to the high Debye frequency of Li. This observation is consistent with magnetic coupling models for pairing mechanism in cuprates. Finally, because of the optimum conditions of the preparation procedure, nearly identical values of the critical current density (J_c) were recorded for samples with different Li doping levels (up to the optimum Li doping level). It was found that improved compound preparation conditions would have a critical and extensive effect on J_c enhancement, with nearly no T_c suppression.