Researchers have found evidence of clean-limit d-wave superconductivity in an infinite-layer samarium nickelate film exhibiting superconductivity at 20 K. The work, led by Qingming Huang of Xiamen University and Liang Qiao of the University of Electronic Science and Technology of China, used ultrafast terahertz spectroscopy to determine a ratio of mean free path to coherence length of approximately 1.5. This finding confirms the material operates in a “clean limit” where electrons pair without significant scattering, establishing parallels between nickelate and cuprate superconductors and offering insight into the origins of high-temperature superconductivity.

Ultrafast Terahertz Spectroscopy Probes 20K Samarium Nickelate

This observation aligns with theoretical predictions for clean-limit d-wave pairing, a phenomenon previously established in high-temperature cuprate superconductors. The study employed time-resolved optical pump-THz probe spectroscopy to analyze photoinduced conductivity and quasiparticle dynamics within the superconducting material, providing a robust method to characterize its superfluid response across a broad temperature range. Current limitations in synthesizing these materials present challenges for surface-sensitive techniques like angle-resolved photoemission spectroscopy when probing the superconducting state of infinite-layer nickelates.

Unlike these methods, bulk-sensitive terahertz spectroscopy circumvents these issues, offering precise characterization of the superfluid response. Analysis of the transient complex conductivity uncovered a photoinduced spectral weight transfer from superfluid to Drude response following optical excitation, a clear indication of Cooper pair breaking and the subsequent formation of quasiparticles. The recovery kinetics of the terahertz response were determined to be governed by the bimolecular recombination of these photogenerated quasiparticles.

Extraction of data from the observed linear relationship yielded a superconducting gap of Δ(0) = 2.5 ± 0.1 meV and a gap-to-𝑇� ratio of 2∆(0) /𝑘�𝑇� ≈3. These values suggest the samarium nickelate sample exists within the weak-coupling regime of superconductivity, where the interactions between electrons are relatively weak.

The findings offer a useful comparison point to neodymium-based nickelates, where reports on pairing symmetry have been inconsistent. While some studies using scanning tunneling spectroscopy and penetration depth measurements have suggested both nodal d-wave and nodeless s-wave behavior, previous terahertz work by Cheng et al. provided evidence for d-wave pairing in the dirty limit.

Linear Superfluid Density Confirms Clean-Limit d-wave Pairing

The temperature-dependent superfluid density in an infinite-layer samarium nickelate film exhibited a linear decrease as temperature increased, a finding consistent with the theoretical expectations for clean-limit d-wave pairing. This observation provides a strong connection between the behavior of this nickelate superconductor and that of well-studied cuprates, materials known to exhibit high-temperature superconductivity through a similar mechanism.

The observed linear relationship between the photo-destroyed superfluid density and the equilibrium superfluid density, maintained across temperatures from near 18 K down to 5 K, further reinforces this conclusion and distinguishes it from the expected quadratic temperature dependence of dirty-limit d-wave pairing or the exponential dependence of s-wave pairing.

The ability to selectively probe the superfluid response using time-resolved terahertz spectroscopy proved broadly applicable, especially in systems where equilibrium measurements are complicated by low superfluid density or strong normal-state contributions. The confirmation of clean-limit superconductivity in samarium nickelates strengthens the parallels with cuprates, where d-wave pairing and the presence of nodal quasiparticles are already well established. The observed linearity in the temperature dependence of the normalized superfluid density, 𝜌�(𝑇)/𝜌�(0), is a key indicator of clean-limit d-wave pairing, a phenomenon comprehensively validated through decades of research on cuprate superconductors.

This approach, they argue, is broadly applicable for investigating superconductivity in systems where equilibrium measurements are difficult. The findings emphasize the importance of quasiparticle coherence and scattering in determining superconducting properties in strongly correlated materials, offering a deeper understanding of the mechanisms driving superconductivity in these complex systems.

Superconducting Gap and Weak-Coupling Ratio in Nickelates

A superconducting gap of 2.5 ± 0.1 meV has been measured in an infinite-layer samarium nickelate film, providing key insight into the material’s pairing mechanism and its relationship to high-temperature cuprate superconductors. The ratio of 𝑙 𝜉 / = 𝜋∆(0) ℏ𝜏�� /, where 𝜏�� is the scattering rate for the equilibrium quasiparticles, was independently estimated to be around 1.5, further validating the clean-limit condition. The findings establish strong parallels between the pairing mechanisms in nickelate and cuprate superconductors.

Mean Free Path to Coherence Length Ratio Establishes Clean Limit

Establishing a ratio of 1.5 between the mean free path and coherence length within an infinite-layer samarium nickelate film confirms superconductivity occurring in the clean limit, a condition where electrons pair with minimal scattering. This finding is significant because it parallels observations in high-temperature cuprate superconductors, suggesting shared mechanisms driving superconductivity in these materials.

Previous terahertz spectroscopy on strontium-doped neodymium nickelate indicated superconductivity in the “dirty limit,” where impurity scattering obscures intrinsic pairing features; demonstrating clean-limit behavior is therefore key for isolating the fundamental properties of nickelate superconductivity. From this linear relation, researchers extracted a superconducting gap Δ(0) = 2.5 ± 0.1 meV and a gap-to-𝑇� ratio, 2∆(0) /𝑘�𝑇� ≈3, suggesting that this sample lies in the weak-coupling regime.

Analysis of the time-resolved terahertz signal revealed a clean recovery process, indicating minimal formation of transient species that could complicate the interpretation of the superconducting gap. The team’s methodology involved extracting the frequency-dependent transient complex conductivity, ∆𝜎�, from the ratio of the change in the terahertz electric field, ∆𝐸�(𝜔; ∆𝑡), to the original electric field, 𝐸�(𝜔).

At 3 K, this analysis revealed a positive, Drude-like real part and a negative imaginary part following a 1/𝜔 dependence, further supporting the clean-limit scenario. This technique circumvents challenges faced by surface-sensitive methods, providing a more direct assessment of the superconducting properties.

Nickelate Superconductivity Parallels Cuprate Pairing Mechanisms

This finding directly addresses a key question in the field: whether nickelates exhibit pairing mechanisms analogous to those driving superconductivity in cuprates, despite differences in their crystal and electronic structures. Establishing clean-limit behavior is essential for isolating intrinsic pairing properties from the influence of material imperfections, a challenge that has previously obscured interpretations of nickelate superconductivity. This ratio, combined with the clean-limit confirmation, suggests a shared underlying mechanism governing electron pairing in both material families.

The team’s work offers an important step toward clarifying these symmetries and assessing the extent to which nickelate superconductivity mirrors that of cuprates, potentially unlocking new avenues for materials discovery and technological advancement. The study’s findings build on previous research, including work by Li et al. in Nature (2019) and Sakakibara et al. in Physical Review Letters (2020), which explored the potential for cuprate-like pairing in nickelates. The research emphasizes the importance of clean-limit measurements for disentangling intrinsic pairing properties from extrinsic influences.

👉 More information
🗞 Evidence for clean d-wave superconductivity in the samarium nickelates
✍️ Qingming Huang, Xiaofang Fu, Junlong Wu, Laifeng Li, Liang Qiao and Ye Yang
🧠 DOI: http://link.aps.org/doi/10.1103/fz8w-w61f

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