Fermi-liquid transport beyond the upper critical field in superconducting La$_2$PrNi$_2$O$_7$ thin films

Unconventional superconductivity typically emerges out of a strongly correlated normal state, manifesting as a highly renormalized Fermi liquid or a strange metal with $T$-linear resistivity. In Ruddlesden-Popper bilayer nickelates, superconductivity with a critical temperature $T_{\rm c}$ exceeding 80 and 40K has been respectively realised in pressurized bulk crystals and epitaxially strained thin films. These advancements call for the characterisation of fundamental normal-state and superconducting parameters in these new materials platforms of high-$T_{\rm c}$ superconductivity. Here we report detailed magnetotransport experiments on superconducting La$_2$PrNi$_2$O$_7$ (LPNO) thin films under pulsed magnetic fields up to 64T and access the normal-state behaviour over a wide temperature range between 1.5 and 300~K. We find that the normal state of thin-film LPNO exhibits the hallmarks of Fermi-liquid transport, including $T^2$ temperature dependence of resistivity and Hall angle, and $H^2$ magnetoresistance obeying Kohler scaling. Using the empirical Kadowaki-Woods ratio, we estimate a quasiparticle effective mass $m^*/m_e \simeq 10$, thereby revealing the highly renormalized Fermi liquid state therein. Our results demonstrate that thin-film LPNO follows the same $T_{\rm c}/T_{\rm F}$ scaling observed across a myriad of strongly correlated superconductors and establish key normal-state characteristics of strained bilayer superconducting nickelates.

💡 Research Summary

In this work the authors investigate the normal‑state transport properties of compressively strained La₂PrNi₂O₇ (LPNO) thin films, a member of the Ruddlesden‑Popper bilayer nickelate family that exhibits superconductivity with a critical temperature above 40 K under epitaxial strain. By employing pulsed magnetic fields up to 64 T, they suppress superconductivity and access the underlying normal state over a broad temperature range (1.5–300 K). The resistivity in the field‑induced normal state follows a clear T² dependence, ρ(T)=ρ₀+A₂T², with ρ₀≈102 µΩ cm and A₂≈8.1 nΩ cm K⁻², indicating dominant electron‑electron scattering characteristic of a Fermi liquid. Hall measurements reveal a monotonic increase of the Hall coefficient as temperature decreases, while the Hall angle obeys cot θ_H∝T², confirming that both longitudinal and transverse transport are governed by a single quasiparticle scattering rate. Magnetoresistance is small (≤4 % at 53 T) and scales as H²; when plotted as Δρ/ρ₀ versus μ₀H/ρ₀, data from all temperatures collapse onto a single curve, demonstrating Kohler scaling and further supporting a single‑lifetime Fermi‑liquid picture.

The upper critical field Hc₂ is extracted for fields parallel (H∥c) and perpendicular (H∥ab) to the film plane. Using a 90 % ρ_n criterion, the authors find μ₀Hc₂^⊥(0)≈42 T and μ₀Hc₂^∥(0)≈106 T, giving an anisotropy factor γ≈2.5, comparable to that of optimally doped YBa₂Cu₃O₇₋δ and larger than in infinite‑layer nickelates. Ginzburg‑Landau analysis yields an in‑plane coherence length ξ_ab≈2.76 nm and an effective superconducting thickness d_sc≈3.88 nm, consistent with the 5 nm film thickness and indicating bulk‑like superconductivity. From ξ_ab and an estimated Fermi velocity the superconducting gap is inferred to be Δ₀≈6 meV (≈2.3 k_BTc), in line with recent tunneling studies.

Because direct specific‑heat measurements are not feasible under the required strain conditions, the authors employ the empirical Kadowaki‑Woods relation A₂/γ₀²≈10 µΩ cm mol² K² J⁻² to estimate the electronic specific‑heat coefficient γ₀ and thus the quasiparticle effective mass. The analysis yields m*≈10 m_e, signifying a heavily renormalized Fermi liquid. Finally, the ratio Tc/TF≈0.01 falls on the universal scaling line observed across a wide variety of strongly correlated superconductors, suggesting that the same underlying principle governs the superconducting transition temperature in strained bilayer nickelates as in cuprates, iron‑based, and heavy‑fermion systems.

Overall, the paper provides a comprehensive transport study that establishes the normal state of strained LPNO thin films as a conventional Fermi liquid with a large effective mass, demonstrates robust anisotropic superconductivity with high upper critical fields, and places nickelate superconductivity within the broader context of correlated‑electron superconductors. This work therefore offers crucial benchmarks for theoretical models of pairing in nickelates and guides future experimental efforts aimed at tuning their electronic correlations via strain, pressure, or chemical substitution.