Calcium-free double-layered cuprate superconductors with critical temperature above 100 K

Abstract

Calcium is a vital constituent in multilayered cuprate superconductors with critical temperatures (T_c) above 100 K, because it plays a key role in separating CuO₂ planes. Here, we demonstrate the synthesis of calcium-free double-layered cuprates: Sr₂SrCu₂O₄(X,O)₂(X = F, Cl, and Br) and M(Sr,Ba)₂SrCu₂O_y(M = Hg/Re, Tl, and B/C), where strontium exists between the CuO₂ planes. Oxyfluoride and mercury-based materials show a T_c of 107 K and 110 K, respectively, which are high compared to existing calcium-free cuprates. These findings indicate T_c greater than 100 K can be realized by replacing both barium and calcium, which have been indispensable in conventional multilayered cuprates, with strontium. Furthermore, the non-toxicity of Sr₂SrCu₂O₄F₂ and (B,C)Sr₂SrCu₂O_y simplifies the synthesis process and ensures their safety in potential applications. We also perform a comparison of the characteristic structural parameters between the calcium-free and calcium-containing cuprates considering the number of CuO₂ planes.