Coexistence of Antiferromagnetic Spin Fluctuations and Superconductivity in La2SmNi2O7 Thin Films

The interplay between magnetic fluctuations and superconductivity is fundamental for understanding unconventional high-temperature superconductors. In the recently discovered Ruddlesden-Popper phase nickelates-which achieve superconducting transition temperatures up to ~100 K-this connection has been theoretically predicted but experimentally unverified. Using compressively strained La2SmNi2O7 thin films, we report direct evidence for this interplay. We observe a characteristic ‘Mexican hat’-shaped magnetoresistance, a signature of superconductivity coexisting with emergent antiferromagnetic (AFM) fluctuations. This distinctive feature arises from a competition between the magnetic field’s suppression of AFM fluctuation-driven scattering and its suppression of superconducting fluctuations. We quantify these AFM fluctuations by a crossover field, B*, whose magnitude decreases with increasing temperature and vanishes near the superconducting onset-behavior contrasting sharply with that in high-Tc cuprates. Our results establish not merely coexistence, but also a direct and novel correlation between AFM instability and superconductivity in nickelates, offering crucial insight into their pairing mechanism.

💡 Research Summary

The authors investigate compressively strained La₂SmNi₂O₇ (Ruddlesden‑Popper 3‑2‑7) thin films grown on (001) SrLaAlO₄ substrates, aiming to uncover the relationship between antiferromagnetic (AFM) spin fluctuations and superconductivity (SC). High‑quality 10 nm films are prepared by pulsed laser deposition, capped with a single‑unit‑cell SrTiO₃ layer, and annealed in dilute ozone to reduce oxygen vacancies. X‑ray diffraction, reciprocal‑space mapping, and half‑order diffraction confirm full epitaxial coherence (≈ 1.7 % compressive strain) and an orthorhombic octahedral rotation pattern described by the Glazer notation a⁻a⁻c⁺. Electrical transport shows three regimes: metallic behavior above ~68 K, an upturn (insulating tendency) between 21 K and 68 K, and a sharp drop below 21 K, defining the superconducting onset temperature (T_c^onset ≈ 21 K). The transition is broadened, likely due to residual oxygen vacancies, and zero resistance is not reached down to 0.3 K.

Magnetotransport is measured with magnetic fields both perpendicular and parallel to the film plane. The upper critical fields extracted from 2D Ginzburg‑Landau fits are H_c2⊥(0) ≈ 12 T and H_c2∥(0) ≈ 38 T, yielding a coherence length ξ ≈ 5.3 nm and an effective superconducting thickness d ≈ 5.4 nm, indicating a quasi‑2D superconducting state.

The central discovery is a “Mexican‑hat” magnetoresistance (MR) when an in‑plane field is applied. Near zero field (|B| < B*), the MR is negative, while at higher fields (|B| > B*) it becomes positive and follows a B² dependence. The crossover field B* marks the balance between two competing mechanisms: suppression of AFM‑fluctuation‑driven scattering (producing negative MR) and Zeeman pair‑breaking of superconducting fluctuations (producing positive MR). B* decreases with increasing temperature and vanishes as the temperature approaches T_c^onset, a behavior opposite to that observed in underdoped cuprates where AFM‑related negative MR disappears above T_c. This demonstrates that AFM spin fluctuations persist and even strengthen within the superconducting phase of the nickelate films.

Control experiments rule out alternative explanations: the negative MR is independent of the relative orientation of current and field, excluding vortex‑flow contributions; hysteresis loops are unchanged over a wide sweep‑rate range, dismissing eddy‑current heating; and the normal‑state upturn regime shows no negative MR, eliminating Kondo scattering. The zero‑field resistivity peak, together with the negative MR, is characteristic of AFM fluctuations rather than ferromagnetic effects.

Two samples are compared. Sample 1, with a modest resistivity upturn, exhibits B* ≈ 2.7 T at 2 K. Sample 2, annealed to reduce oxygen vacancies, shows a higher T_c^onset (≈ 24 K), no upturn, and a much larger B* that reaches ≈ 8 T at 2 K and grows monotonically as temperature decreases. This systematic dependence on oxygen stoichiometry indicates that oxygen vacancies suppress AFM correlations, likely by disrupting apical oxygens that mediate interlayer coupling.

Angular‑dependent MR measurements reveal that the negative MR appears only when the magnetic field lies within ~1.9° of the film plane. As the field tilts out of plane, the perpendicular component generates vortices, leading to a dominant positive MR and a rapid reduction of B*. Minor hysteresis loops show two characteristic bounding fields (B_HF ≈ 5 T and B_LF ≈ ‑2 T), suggesting that beyond B_HF superconducting pairs are largely broken, producing normal electrons that may interact with spin‑glass‑like correlations.

The authors discuss the implications for pairing mechanisms. Unlike cuprates, which involve a single Cu d_{x²‑y²} orbital, the 3‑2‑7 nickelates host both e_g (d_{x²‑y²}) and t_{2g} (d_{xz}, d_{yz}) orbitals near the Fermi level, allowing richer spin‑orbital interactions. Theoretical work predicts that compressive strain enhances interlayer AFM coupling, and the experimental observation of strong AFM fluctuations coexisting with SC supports a scenario where spin fluctuations act as the pairing glue, possibly leading to sign‑changing s± or d‑wave gaps. The sensitivity of B* to oxygen vacancies underscores the importance of apical oxygen in stabilizing the proposed interlayer coupling.

In summary, this work provides the first direct transport evidence of antiferromagnetic spin fluctuations coexisting with superconductivity in ambient‑pressure Ruddlesden‑Popper nickelate thin films. The distinctive “Mexican‑hat” MR, the temperature‑dependent crossover field B*, and the systematic influence of oxygen stoichiometry collectively reveal a robust coupling between magnetism and superconductivity, offering crucial insight into the unconventional pairing mechanism of the 3‑2‑7 nickelates.