[Paper Review] Paramagnetically driven superconducting re-entrance in Eu-doped infinite layer nickelates

Lucia Varbaro, Lukas Korosec+12 more|arXiv (Cornell University)|Jan 27, 2026

Magnetic and transport properties of perovskites and related materials0 citations

TL;DR

The paper reports field-induced re-entrant superconductivity in Eu-doped NdNiO2 thin films and shows it arises from competing polarization of Eu2+ and Nd3+ moments, supported by Hall effect modeling and Gorkov-based pair-breaking theory.

ABSTRACT

The breakthrough discovery of superconductivity in infinite-layer nickelates, and subsequently in several superconducting nickelates with more complex layered structures, capped a search spanning more than two decades and opened an entirely new field of research. Significant efforts aim to increase the critical temperature, to determine the electronic structure of the system, the underlying pairing mechanism, and the similarities between this system and cuprates - Ni1+ in infinite-layer nickelates being isoelectronic to Cu2+ in high-Tc cuprates. Here, we explore the unique role of magnetic rare earth ions in superconducting Eu-doped NdNiO2. We show that the field-induced re-entrant superconductivity which we evidence in this compound is the result of a delicate balance between the competing effects of the Eu2+ and Nd3+ ions. Our analyses of the extraordinary Hall effect and modeling of the superconducting critical fields demonstrate that the influence of these ions on magneto-transport is only felt when they are polarized by a magnetic field.

Motivation & Objective

• Investigate how magnetic rare-earth ions (Eu2+ and Nd3+) influence superconductivity in Nd1−xEuxNiO2.
• Determine whether a Jaccarino-Peter–like mechanism can explain field-induced re-entrant superconductivity in Eu-doped nickelates.
• Characterize magnetotransport (including Hall effect) and superconducting phase boundaries under high magnetic fields.
• Develop a quantitative model linking rare-earth spin polarization to the extraordinary Hall effect and superconducting suppression/recovery.

Proposed method

• Grow Nd1−xEuxNiO2 thin films (x=0.3) by RF magnetron sputtering and reduce in situ with metallic Al to realize infinite-layer phase.
• Measure resistivity under perpendicular and parallel magnetic fields up to 30 T to map Tc(B) and observe re-entrant superconductivity.
• Perform Hall effect measurements on non-superconducting and superconducting samples to extract R0(T) and the extraordinary Hall contribution.
• Model Rxy(B,T) as R0(T)B + c[xNd⟨Nd3+(B,T)⟩ + xEu⟨Eu2+(B,T)⟩] using Brillouin functions for Eu2+ (J=7/2) and Nd3+ (J=9/2).
• Use Gorkov’s pair-breaking theory to compute Bc2(T) with total field Btot including exchange contributions from Eu2+ and Nd3+.
• Fit model parameters (diffusion D, spin-orbit time τSO, effective mass m*, exchange coupling Γ) to experimental Bc2(T) data.

Figure 1: (a) High-resolution X-ray diffraction scan around the (002) reflection of an optimally doped Nd 0.7 Eu 0.3 NiO 2 film grown on LSAT. The scan reveals an intense film peak, and the pronounced finite-size oscillations, marked by arrows, indicate high crystalline quality and smooth interfaces

Experimental results

Research questions

• RQ1 What causes the anomalous field-induced re-entrant superconductivity in Eu-doped NdNiO2?
• RQ2 How do Eu2+ and Nd3+ magnetic moments individually and collectively influence magnetotransport and superconductivity?
• RQ3 Can a Jaccarino-Peter–like mechanism, extended to two magnetic ion species, explain the observed Hall response and Tc recovery under field?
• RQ4 How does the total effective field, including exchange fields from rare-earth ions, determine the Bc2(T) phase boundary?

Key findings

• Re-entrant superconductivity appears at intermediate magnetic fields (around 3 T) and persists up to at least 30 T in optimally doped Nd0.7Eu0.3NiO2 on LSAT.
• Hall effect data show nonlinear, temperature-dependent contributions attributed to Eu2+ and Nd3+ moments; a two-sublattice model fits Rxy(B,T).
• Brillouin-function-based polarization of Eu2+ (J=7/2) and Nd3+ (J=9/2) moments partially compensates under field, reducing the internal field felt by conduction electrons.
• Gorkov-based pair-breaking theory with a total field Btot that includes exchange terms reproduces the observed Bc2(T) nonlinearity for both field orientations.
• X-ray magnetic circular dichroism measurements corroborate the field-dependent spin polarizations of Eu and Nd, consistent with the model.
• Eu/Nd magnetic interplay is essential for the observed low-T, field-induced superconductivity; higher Tc samples do not exhibit re-entrance under the same conditions.

Figure 2: (a) Resistivity versus temperature curves for an optimized Nd 0.7 Eu 0.3 NiO 2 film on LSAT under perpendicular magnetic fields ranging from 0 T to 3.5 T in the top panel and 4 to 12 T in the bottom one. A conventional suppression of the superconducting transition temperature $T_{c}$ is ob