Abstract
High temperature superconductivity is typically associated with strong coupling and a large superconducting gap, yet these characteristics have not been demonstrated in the nickelates. Here, we provide experimental evidence that Eu substitution in the spacer layer of Nd1-xEuxNiO2 (NENO) thin films enhances the superconducting gap, driving the system toward a strong-coupling regime. This is accompanied by a magnetic-exchange-driven magnetic-field-enhanced superconductivity. We investigate the upper critical magnetic field, Hc2, and the superconducting gap of superconducting NENO thin films with x = 0.2 to 0.35. Magnetoresistance measurements reveal magnetic-field-enhanced superconductivity in NENO films. We interpret this phenomenon as a result of an interaction between magnetic Eu ions and superconducting states in the Ni dx2-y2 orbital. The upper critical magnetic field strongly violates the weak-coupling Pauli limit. Infrared spectroscopy confirms a large gap-to-Tc ratio \(2\Delta /{k}_{B}{T}_{{\rm{c}}}\simeq 5-6\), indicating a stronger coupling pairing mechanism in NENO relative to the Sr-doped NdNiO2. The substitution of Eu in the rare-earth layer causes pronounced modifications of the superconducting gap and magnetic interactions in Nd-based nickelates, opening new pathways to engineer high-Tc superconductivity in infinite-layer nickelates.
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Acknowledgements
This work was supported by U.S. Department of Energy, Office of Science, Office of Basic Energy Sciences, under award number DE-SC0019211. This work involves the use of resources from the Yale Materials Characterization Core. A portion of this work was performed at the National High Magnetic Field Laboratory, which is supported by National Science Foundation Cooperative Agreement No. DMR-2128556 and the State of Florida and the U.S. Department of Energy. D.V.C. acknowledges financial support from the National High Magnetic Field Laboratory through a Dirac Fellowship. C.L. was supported by start-up funds from Florida State University and the National High Magnetic Field Laboratory. W. L. was partly supported by the James Kouvel Fellowship. Y. H. acknowledges support from the U.S. Air Force Office of Scientific Research under Award No. FA9550-24−1-0048. Computational studies in this work were supported by Grant No. NSF DMR 2237469, by NSF ACCESS supercomputing resources via allocation TG- MCA08X007, and by the guidance and use of research computing infrastructure at the Yale Center for Research Computing. Work in Hamburg was supported by the Deutsche Forschungsgemeinschaft (DFG) by the Cluster of Excellence CUI: Advancing Imaging of Matter (EXC 2056, project ID 390715994). We thank Dr. Danilo Ratkovski for supporting our magnet time at the National High Magnetic Field Laboratory.
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D.V., C.H.A, A.C., and F.J.W. developed the concept of the work. D.V., H.L., D.N., and Y.H. designed the experiments. W.W. synthesized the thin films. D.V., A.S., C.A.M., T.Q., and B.M. performed the high-magnetic-field transport measurements. D.V. performed the X-ray diffraction measurements and Hall measurements. H.L., D.N., M.B. performed the optical measurements. W.L., X.Y., R.W. performed the mutual inductance measurement. Z.J. and S.I-B. performed the ab-initio calculations. D.V.C., C.L. and D.V. performed theoretical modelling and fitting of transport data. H.L., D.N., M.B., performed theoretical modelling and fitting of optical data. D.V., H.L., D.N., Z.J. wrote the manuscript. All the authors contributed to the data analysis and editing of the manuscript.
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Vu, D., Lee, H., Nicoletti, D. et al. Re-entrant unconventional superconductivity induced by rare-earth substitution in Nd1-xEuxNiO2 thin films. Nat Commun (2026). https://doi.org/10.1038/s41467-026-70254-0
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DOI: https://doi.org/10.1038/s41467-026-70254-0
