Eu-doped nickelates: re-entrance without a settled pairing mechanism

Yang and colleagues study Sm0.95−xCa0.05EuxNiO2 (SCEₓ), an infinite-layer nickelate film series with fixed nominal Ca substitution. It is not Eu-substituted bilayer La3Ni2O7. The measured Eu valence is mixed and varies with composition, so nominal x is not a direct count of identical Eu²⁺ moments.

This note reads the revised author manuscript dated 25 April 2026. The journal article was published online on 23 April 2026. The publisher abstract supports the broad re-entrance report, but access to that abstract does not establish equivalence of all final and author-version figures. The separate Nd/Eu nickelate study discussed in G3 and G5 concerns another material series; its magnetic interpretation cannot simply be transferred here.

The selected author text and Figure 2 distinguish the following observations. Fields are perpendicular to the film plane (H∥c) for these comparisons.

These temperatures describe measurement conditions, not interchangeable transition temperatures. Figure 3 instead marks open-circle phase boundaries using 0.5Rn, with Rn the high-field normal-state resistance. Those contours must not be relabeled zero-resistance boundaries. No material-Tc record is created from this table.

The angle θ is measured from the film normal. The angular maps use different temperatures: 4.2 K for x=0.33 and 0.36, 1.5 K for x=0.38, and 0.3 K for x=0.40. Their changing angular windows cannot support a universal assertion that in-plane re-entrance is absent. The manuscript also distinguishes intermediate-doping compensation-like behavior from stronger deviations at x=0.40.

The inherited notes attribute persistence to 45 T at 0.3 K to SCE0.36. That exact sample–temperature–field tuple was not verified in the selected v3 pages. Extended Data Figure 9 labels SCE0.36 at 4.2 K and SCE0.40 at 0.3 K; it also has a caption saying “up to 45 T” although its upper panels extend to 60 T. These version and panel details require resolution before the inherited tuple is used as a current verified result. A measurement ceiling would in any case provide a bound, not a measured high-field endpoint.

Nonlinear Hall response after linear-background subtraction and hysteretic magnetoresistance motivate magnetic explanations. They do not by themselves identify uniform ferromagnetism, its volume fraction, or a particular pairing state. The v3 discussion explicitly leaves long-range magnetic order and its structure undetermined.

The authors fit a WHH-based model including orbital and Pauli pair breaking, spin-orbit scattering and an exchange field. Compensation partially describes intermediate compositions; the fitted exchange fields and scattering parameters remain phenomenological. An empirical factor of two in a Brillouin-function argument improves some fits but is not universal and does not adequately describe x=0.40. Neither this adjustment nor analogy with other re-entrant systems proves spin-triplet pairing.

D5 supplies the material-family context; D15 separates substitution, scattering and magnetism; G3 and G5 distinguish re-entrance from a demonstrated compensation mechanism. For H7 and H9, superconducting recovery fields must not become Hall-sign-reversal or anomalous-Hall threshold fields. This note reports no extracted ordinary Hall coefficient.

A stronger scientific assessment still needs version-resolved final figures, criterion and noise-floor checks, sample calibration and local magnetic probes. The present review neither fits the deposited source-data files nor certifies an intrinsic bulk phase or unique microscopic mechanism.