Key New Cuprates Tailored by MBE to Comprehend High-T_c Superconductivity

While a wide variety of high-T_c superconducting cuprates have been discovered, the search for new materials remains at the forefront of research, and we have been striving to create novel superconductors by means of molecular beam epitaxy (MBE) [1]. Here, we introduce two families of cuprates newly synthesized by MBE, in which the playground of superconductivity is exclusively within CuO₂ planes with square-planar-coordinated Cu. Our results may challenge the widely accepted notion that the high-T_c superconductivity is induced by carrier doping into Mott-insulating parent compounds.
We start with the Nd₂CuO₄ (T') structure. T'-Ln₂CuO₄ (Ln = La, Pr, Nd) are known as the parent compounds of electron-doped cuprates and heterovalent substitution (Ce⁴⁺ for Ln³⁺) is required to induce superconductivity in bulk specimens. However, our T'-Ln₂CuO₄ thin films show superconductivity without the heterovalent substitution [1]. The stark contradiction between our results and commonly achieved data likely originates from the complicated oxygen chemistry. Unlike that hole doping into CuO₂ planes with octahedral- and/or pyramidal-coordinated Cu induces superconductivity, electron doping into CuO₂ planes with square-planar-coordinated Cu alone is insufficient for the induction of superconductivity. Instead, treatment of the as-grown specimens of T’-Ln_2-xCe_xCuO₄ under reducing environments is necessary, irrespective of the dopant (Ce) concentration x. Such a reduction treatment is vital for elimination of defects, for example, removal of excess oxygen at apical sites while simultaneously regular oxygen sites remain occupied. As the annealing process is a diffusion process, thin-film samples are advantageous for achieving an ideal and uniform oxygen configuration. Nonetheless, the two regular oxygen sites in T’-cuprates—O(1) in the CuO₂ plane and O(2) in the Ln₂O₂ layer—may favor oxygen off-stoichiometries, and therefore unintentional doping.
This can be tested more rigorously by infinite-layer (IL) cuprates as the oxygen sites are confined within the CuO₂ planes. Single-crystalline specimens of such cuprates can only be prepared by MBE. We focus on IL-CaCuO₂, the common ingredient of cuprates with T_c > 100 K. While the bare IL-CaCuO₂ thin films are insulating, our in-depth crystallographic analysis revealed the cause. Particularly, in-plane scanning transmission electron microscopy (STEM) shows that cationic stripes are formed in the CuO₂ planes and their density is sufficient to quench metallic conduction [2]. This stripe formation is considered to buffer charge imbalances, which are inevitably introduced during the growth triggered by point defect formation. To eliminate those stripes, we inserted Ca₂Fe₂O₅ layers after every 13 or 17 layers of CaCuO₂ by MBE. These superstructures drive the system metallic and superconducting [3]. As superconductivity is induced when CaCuO₂/Ca₂Fe₂O₅ superlattices are cooled from growth temperatures under strong oxidizing conditions, we also explored the possibility of hole-doping by excess oxygen introduced into the Ca₂Fe₂O₅ layers. We find this to be unlikely for the following reasons: (1) excess oxygen is not observed in STEM measurements; (2) Ca₂Fe₂O_5+δ only accepts small amount of excess oxygen (δ ≤ 0.08) as far as the brownmillerite structure is preserved [4]; (3) a large Meissner signal excludes the possibility of interface superconductivity and the carrier concentration would be too small if the carriers were injected into the 13-17 layers of CuO₂. After all, the square-planar CuO₂ planes have a metallic and superconducting ground state per se and nearly disorder-free CuO₂ planes are the only prerequisite for high-T_c superconductivity.

[1] H. Yamamoto et al., in “Epitaxial growth of complex metal oxides” (Elsevier, 2022).
[2] Y. Krockenberger et al., ACS Omega 6 (2021) 21884.
[3] A. Ikeda et al., ACS Appl. Electron. Mater. 4 (2022) 2672.
[4] A. Shaula et al., DOI: 10.1109/OMEE.2012.6464764.