Calcium-free double-layered cuprate superconductors with critical temperature above 100\u2009K

Hiroki Ninomiya,Hiroshi Fujihisa,Hiraku Ogino,Hiroshi Eisaki,Akira Iyo,Yoshiyuki Yoshida,Shigeyuki Ishida,Kenji Kawashima,Yoshito Gotoh

doi:10.1038/s43246-020-00116-9

Abstract

Calcium is a vital constituent in multilayered cuprate superconductors with critical temperatures (Tc) 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)2SrCu2Oy(M = Hg/Re, Tl, and B/C), where strontium exists between the CuO2 planes. Oxyfluoride and mercury-based materials show a Tc of 107 K and 110 K, respectively, which are high compared to existing calcium-free cuprates. These findings indicate Tc 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)Sr2SrCu2Oy 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.

Highlights

Calcium is a vital constituent in multilayered cuprate superconductors with critical temperatures (Tc) above 100 K, because it plays a key role in separating CuO2 planes
The difficulty with introducing the hole carriers arising from the size mismatch of ionic radii between Cl− and O2− has been reported for the Ae0 1⁄4 Ca case[14]
The main X-ray diffraction (XRD) peaks of each pattern were found to be indexed by a tetragonal structure with double CuO2 layers

Summary

Introduction

Calcium is a vital constituent in multilayered cuprate superconductors with critical temperatures (Tc) above 100 K, because it plays a key role in separating CuO2 planes. We demonstrate the synthesis of calcium-free double-layered cuprates: Sr2SrCu2O4(X,O)2(X = F, Cl, and Br) and M(Sr,Ba)2SrCu2Oy(M = Hg/Re, Tl, and B/C), where strontium exists between the CuO2 planes. The Ae0 metal separates the CuO2 planes, forming infinite-layered Ae0CuO2 units. With increasing n, the structure of the single-layered phase (0201-X or M-1201) pulls in infinite-layered Ae0CuO2 units one by one between the blocking layers. In most such multilayered cuprates, either Ba, Sr, or Ca are employed as the Ae metal; the choice of Ae0 tends to be limited to metals with relatively small ionic radii, such as divalent Ca and/or trivalent rare-earth cations[9]. To the best of our knowledge, La2−xSr1+xCu2O6+y (0212 phase)[12] is the only double-layered cuprate in which Sr separates the CuO2 planes; it is known to exhibit no superconductivity due to the lattice disorder associated with the random occupation at the Ae0 site by the mixing of Sr and La13

Objectives

Methods

Results

Conclusion