Abstract Body

Cuprate superconductors are the materials that still have the highest T_c at ambient pressure. Most high-T_c cuprates are composed of CuO₂ planes, and block layers of other compounds often referred to as charge reservoir layers (CRLs). The infinite-layer (IL) cuprates are exceptional, where only simple cations, e.g., Ca²⁺ and Sr²⁺, separate adjacent CuO₂ planes. Moreover, IL-CaCuO₂ is the common ingredient of cuprates with T_c > 100 K, and therefore vital to comprehend the mechanism of the high-T_c superconductivity. However, synthesis of the IL-cuprates requires high-pressures, and hence, bulk single crystals are unavailable. In contrast, single-crystalline and single-phase IL-cuprates can be prepared by thin-film synthesis, notably by molecular beam epitaxy (MBE). MBE is privileged to synthesize superlattice structures and this verity includes complex transition metal oxides that include the IL-cuprates. Early efforts of our research team on the development of novel superconducting materials1-2) resulted in our recent reports on superconductivity in IL-CaCuO₂/Ca₂Fe₂O₅ superlattices.³) Our custom-designed MBE apparatus, which combines a high-precision flux rate control and an elemental source sequencing, incepted the successful development of artificial superlattice-superconductors. Our results of IL-CaCuO₂/Ca₂Fe₂O₅ superlattices and IL-CaCuO₂/SrTiO₃ superconducting superlattices synthesized by Di Castro et al.⁴) assure that superlattice formation is a promising approach to customize novel cuprate superconductors, hopefully with higher T_c’s.

The growth was performed by MBE on cubic (001) (LaAlO₃)_0.3(SrAl_0.5Ta_0.5O₃)_0.7 (LSAT) substrates (a = 3.868 Å), as it is almost lattice-matched to IL-CaCuO₂ (3.853 Å). IL-CaCuO₂ can be only stabilized on lattice-matched substrates at low temperatures (< 600 °C) while maintaining strong oxidizing conditions, i.e., atomic oxygen. These synthesis constraints are also imposed to the synthesis conditions of the combining materials to avoid decomposition and/or structural transformation of the IL-CaCuO₂ layers. In addition to Ca₂Fe₂O₅, we have examined three more material candidates for the synthesis of superconducting superlattices: (a) BaBiO₃, (b) LaCrO₃, and (c) EuFeO₃. In case (a), BaBiO₃ (4.355 Å) was epitaxially grown on the LSAT substrate despite the large lattice mismatch. However, the large lattice mismatch of BaBiO₃ with IL-CaCuO₂ prevents the growth of the IL-CaCuO₂ layer, and the superlattices do not form. In case (b), formation of superlattices seems promising, considering the lattice constant of LaCrO₃ (3.885 Å) being only ~0.8 % larger than that of IL-CaCuO₂. However, incompatible growth conditions between IL-CaCuO₂ and LaCrO₃ impede the synthesis of a superlattice. Specifically, IL-CaCuO₂ layers do require supply of atomic oxygen though its presence enhances the formation of volatile CrO₃ and its desorption from the growth front. Accordingly, the LaCrO₃ layers are not formed on the IL-CaCuO₂ layer. In case (c), the sequential stacking of nominal IL-CaCuO₂/EuFeO₃ was realized thanks to lattice matching (3.865 Å for EuFeO₃) and growth condition compatibility. However, Ca/Eu interdiffusion likely occurs and the quality of the resultant superlattice is subpar and shows insulating behavior. In summary, our case studies indicate that (1) lattice-matching, (2) growth condition compatibility, and (3) interdiffusion-free, are the criteria for achieving the IL-CaCuO2-based superlattices as designed.

While the superconducting IL-CaCuO₂/Ca₂Fe₂O₅ superlattices satisfy the above criteria, it turned out that more factors are involved in the emergence of superconductivity. We recall the fact that bare IL-CaCuO₂ is insulating though it is the common ingredient of cuprates with T_c > 100 K. Our in-depth crystallographic analysis, particularly by in-plane scanning transmission electron microscopy, clarified the secret; cationic stripes (anti-phase boundaries) are formed in the CuO₂ planes of bare IL-CaCuO₂ and their density is sufficient to quench superconductivity and cause electronic localization.⁵⁾ The cationic stripe formation is considered to buffer charge imbalances, which are inevitably introduced during the growth triggered by point defect formation. The reason why superconductivity is achieved when IL-CuCuO₂ is embedded in a superlattice structure is that the so-called CRLs (here, the Ca₂Fe₂O₅ layer) can buffer the charge imbalances without distorting the CuO₂ planes;³) there are more functionalities associated to the CRLs than previously acknowledged. Actually, the crystal quality (degree of cationic ordering) of the IL-CaCuO₂ layers was significantly improved and the cationic stripes disappeared by inserting the Ca₂Fe₂O₅ layers. A high degree of anion (oxygen) ordering appears to be also crucial for the induction of superconductivity as strong oxidation during the cooling process after the fabrication of the superlattices was required to reduce oxygen vacancies in the CuO₂ planes. After all, nearly disorder-free CuO₂ planes achieved by superlattice formation is the criteria for superconductivity.

References

1) Y. Krockenberger, A. Ikeda, K. Kumakura, H. Yamamoto, J. Appl. Phys. 124, 073905 (2018).
2) A. Ikeda, Y. Krockenberger, H. Yamamoto, Phys. Rev. Materials 3, 064803 (2019).
3) A. Ikeda, Y. Krockenberger, Y. Taniyasu, H. Yamamoto, ACS Appl. Electron. Mater. 4, 2672 (2022).
4) D. Di Castro, M. Salvato, A. Tebano, D. Innocenti, C. Aruta, W. Prellier, O. I. Lebedev, I. Ottaviani, N. B. Brookes, M. Minola, M. Moretti Sala, C. Mazzoli, P. G. Medaglia, G. Ghiringhelli, L. Braicovich, M. Cirillo, G. Balestrino, Phys. Rev. B 86, 134524 (2012).
5) Y. Krockenberger, A. Ikeda, H. Yamamoto, ACS Omega 6, 34, 21884 (2021).

Acknowledgment

The authors are tremendously grateful to Takayuki Ikeda for his sedulous support in STEM, EELS, and EDS measurements. The authors are particularly grateful to Hiroshi Irie for his enduring support regarding the fabrication of Hall bars and measurement.