A 20K nickelate superconductor exhibits a clear pairing pattern

A 20 K Nickelate Superconductor Shows Clean‑Limit d‑Wave Pairing

Researchers using time‑resolved terahertz spectroscopy have provided compelling evidence that an infinite‑layer samarium nickelate film, superconducting at 20 K, operates in the clean limit with d‑wave pairing symmetry. The result narrows the long‑standing debate over nickelate pairing mechanisms and strengthens the analogy between nickelates and the well‑studied cuprate superconductors.

Clean‑Limit Behavior Revealed by Terahertz Probes

The team measured the ratio of the electronic mean free path (ℓ) to the superconducting coherence length (ξ) and found ℓ/ξ ≈ 1.5. This value exceeds the threshold for the clean limit, meaning electrons form Cooper pairs with minimal impurity scattering. Unlike surface‑sensitive techniques such as ARPES, bulk‑sensitive terahertz spectroscopy can access the superfluid response directly, even when the material’s surface quality is poor.

By delivering an optical pump pulse and monitoring the ensuing terahertz probe, the researchers tracked the transient complex conductivity. The photo‑induced transfer of spectral weight from the superfluid component to a Drude‑like response signaled Cooper‑pair breaking, while the subsequent recovery followed a bimolecular recombination kinetics characteristic of quasiparticle recombination in a clean superconductor.

Linear Superfluid Density and d‑Wave Symmetry

A hallmark of clean‑limit d‑wave superconductivity is a linear temperature dependence of the normalized superfluid density, ρs(T)/ρs(0). The measurements displayed precisely this linear drop from near 18 K down to 5 K, contrasting with the quadratic behavior expected for dirty‑limit d‑wave or the exponential decay of conventional s‑wave pairing. This observation aligns the samarium nickelate’s behavior with that of cuprates, where nodal quasiparticles give rise to the same linear trend.

Gap Magnitude and Weak‑Coupling Regime

Analysis of the terahertz response yielded a superconducting gap Δ(0) = 2.5 ± 0.1 meV. The corresponding gap‑to‑critical‑temperature ratio, 2Δ(0)/kB Tc ≈ 3, places the material in the weak‑coupling regime, indicating relatively modest electron‑electron attraction compared with strong‑coupling superconductors. This quantitative benchmark offers a useful point of comparison with neodymium‑based nickelates, where reports of nodal versus nodeless gaps have been inconsistent.

Implications for Nickelate‑Cuprate Connections

Establishing clean‑limit d‑wave pairing in samarium nickelates addresses a central question: do nickelates share the same pairing mechanism as cuprates despite structural differences? The present findings suggest they do, at least for the infinite‑layer family, implying that the same underlying electronic correlations may drive high‑temperature superconductivity across both material classes.

The methodology—ultrafast terahertz spectroscopy—also demonstrates a practical route for probing superconductivity in systems where low superfluid density or strong normal‑state backgrounds hinder conventional measurements. By focusing on bulk response, researchers can bypass surface preparation challenges and obtain reliable symmetry information.

Looking Ahead

While the clean‑limit behavior clarifies intrinsic pairing properties, synthesizing high‑quality nickelate films remains a bottleneck. Future work will need to improve film uniformity and explore other rare‑earth elements to map out how the clean‑limit condition varies across the nickelate family. If the cuprate‑like d‑wave pairing proves robust, it could guide the design of new materials that push the superconducting transition temperature even higher.

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This article synthesizes recent experimental results on samarium nickelate superconductivity, highlighting the significance of clean‑limit d‑wave pairing and its broader relevance to high‑temperature superconductivity research.