Two Simultaneous Papers Report Field Re-Entrant Superconductivity in Europium-Doped Nickelates, Defying the Pauli Limit
Independent teams publishing in Nature and Nature Communications show that doping infinite-layer nickelates with europium revives superconductivity at magnetic fields that should destroy it.
Overview
Two independent research groups published convergent findings in late April 2026 showing that europium substitution in infinite-layer nickelates can revive superconductivity at magnetic field strengths that ordinarily destroy electron pairing. A Nature paper published online on 23 April 2026 by Yang, Tang, Wu and colleagues and a Nature Communications paper from a Yale-led collaboration that Yale Engineering announced both report a phenomenon known as field re-entrant superconductivity, in which the superconducting state vanishes under moderate magnetic fields and then reappears at much higher ones.
The two reports point to the same underlying mechanism: localized magnetic moments from europium ions interacting with the conducting electrons in the nickel-oxygen layers. Together they suggest that rare-earth substitution can be used as a deliberate design tool to harden unconventional superconductors against magnetic fields, addressing one of the long-standing fragilities of the nickelate family, which has been studied as a potential high-temperature superconductor host since 2019.
What We Know
The Yale-led paper, titled “Re-entrant unconventional superconductivity induced by rare-earth substitution in Nd1-xEuxNiO2 thin films,” was authored by Dung Vu and 19 collaborators including principal investigator Charles Ahn, according to the arXiv preprint. The same preprint reports that the team studied europium doping levels in the range x = 0.2 to 0.35 in neodymium nickelate thin films, and observed a superconducting gap-to-temperature ratio of roughly 2Δ/k_BTc ≈ 5 to 6 — substantially larger than the BCS weak-coupling value of 3.5, which the authors take as evidence that the system has been pushed into a strong-coupling regime. The arXiv abstract also reports that the upper critical field strongly violates the weak-coupling Pauli limit, meaning the superconducting state survives magnetic fields well beyond what conventional theory predicts.
The Nature paper, “Field re-entrant superconductivity in Eu-doped infinite-layer nickelates,” lists Mingwei Yang, Jiayin Tang, Xianfeng Wu and 19 additional authors in its arXiv preprint. That preprint describes a europium-doped samarium-calcium nickelate system (Sm0.95-xCa0.05EuxNiO2) in which superconductivity is initially suppressed at low fields and then re-emerges, accompanied by zero-resistance transport, high-field diamagnetic screening, nonlinear Hall transport, and hysteretic magnetoresistance. The preprint attributes the effect to a compensation between the europium-derived exchange field and the externally applied field — a mechanism in which the magnetic moments of europium effectively cancel part of the field that would otherwise break apart the superconducting Cooper pairs.
The Yale Engineering announcement quotes lead author Dung Vu describing the difficulty of producing the films at all: “Reaching that point took us months to years of process optimization, because nickelate films are highly unstable.” The same announcement explains that europium ions “partially shield the superconducting electron pairs from the applied field, helping the superconducting state survive to much higher magnetic fields,” as Yale Engineering reports. A Phys.org summary published on 27 April 2026 confirms the Nature Communications publication, identifies the experimental work as having been carried out “at some of the world’s strongest magnets,” and notes that the team’s central observation is that doping changed both how electrons conduct electricity and how the superconducting state responds to magnetic fields.
What We Don’t Know
Neither preprint nor the public announcements specify the absolute transition temperature of the optimally doped europium nickelate films, leaving open the question of whether the rare-earth substitution improves Tc as well as field tolerance. The full text of the Nature paper is paywalled, and the public summaries do not reproduce the abstract’s quantitative parameters in detail.
The two papers also use different host materials — neodymium-based films at Yale and a samarium-calcium system in the Nature paper — so it remains to be established whether the field re-entrant behavior is a general property of europium substitution or specific to certain rare-earth host lattices. The exchange-coupling mechanism proposed in both papers, in which europium 4f magnetic moments compensate the external field acting on the nickel 3dx2-y2 conducting electrons, will need to be tested directly through neutron scattering or other probes of the local magnetic structure.
Whether the effect can be pushed to higher temperatures, and whether it can be reproduced in bulk samples rather than thin films, are also open. Nickelate superconductors have been a frontier topic since their discovery in 2019, but as the Phys.org report notes, the materials remain difficult to synthesize reproducibly.
Context
The Pauli paramagnetic limit is a textbook bound on how strong a magnetic field a conventional superconductor can tolerate before the field aligns the electron spins inside Cooper pairs and breaks them apart. Reports of superconductivity that violates this limit are uncommon and usually point to either unconventional pairing symmetries or to internal magnetic structures that effectively shield the pairs. The two new papers add nickelates to the short list of materials where this kind of field-enhanced or field-revived superconductivity has been observed.
The Machine Herald has previously reported on related developments in the same family of materials, including the Penn State team’s reidentification of iron telluride as a superconductor and the Rice University magneto-ARPES results on a kagome superconductor, both of which involved interactions between magnetism and unconventional superconductivity. The simultaneous appearance of two field re-entrant nickelate papers in the same week extends that line of work and reinforces the picture that magnetism, rather than being purely antagonistic to superconductivity, can in carefully chosen systems stabilize it.