In multilayered cuprate superconductors, calcium is a vital constituent, because it plays a key role in separating CuO₂ planes and enhancement of critical temperatures (T_c) above 100 K^1,2). We demonstrated the synthesis of calcium-free double-layered cuprates and developed multilayered cupurate, such as Sr₂SrCu₂O₄(X,O)₂ (X = F, Cl, and Br) and M(Sr,Ba)₂SrCu₂O_y (M = Hg/Re, Tl, and B/C). We revealed that the Sr metal could be employed to partition two CuO₂ planes and the oxyfluoride and mercury-based materials show a T_c of 107 K and 110 K^3,4). Moerover, we found that the length of the a-axis of developed cuprates closed to that of the infinite-layered SrCuO₂ by increasing a number of CuO₂ planes, n. It is comparable to known cuprate with Ca element. Furthermore, the superconducting properties, including critical current density was investigated through magnetic properties measutements⁵⁾. In Hg-based cuprate superconductors, a comparison of the irreversibility lines between Ca-free and representative Ca-containing Hg-based compounds highlighted that the replacement of Ca by Sr in the CuO₂ planes does not significantly impact the temperature dependence of B_irr. The development of Ca-free cuprate superconductors and their critical current properties will be reported in detail.

[1] E. Takayama-Muromachi, Chem. Mater. 10, 2686–2698 (1998).
[2] A. Iyo, et al., J. Phys. Soc. Jpn. 76, 094711 (2007).
[3] H. Ninomiya, et al., Commun Mater 2, 13 (2021).
[4] H. Ninomiya et al., Chem. Mater. 33, 24, 9690–9697 (2021).
[5] H. Ninomiya et al., Supercond. Sci. Technol. 36 115014 (2023).

Keywords: Ca-free cuprate, cuprate superconductor