Researchers have now atomically resolved an intrinsic superconducting gap within (La,Pr)3Ni2O7 films, an important indicator of superconductivity previously difficult to observe in nickelates. The work, led by Xinxin Wang, Yaqi Chen, Cui Ding and Lizhi Xu at the State Key Laboratory of Quantum Functional Materials, Southern University of Science and Technology, demonstrates this gap within Ruddlesden-Popper bilayer nickelate films, a structural arrangement vital for revealing superconducting properties. By combining atomic-layer-controlled growth with scanning tunneling microscopy and spectroscopy, the team observed homogeneous U-shaped double-gap spectra, indicating minimized sample transfer time is key to understanding these materials.

Ruddlesden-Popper Nickelates Emerge as Superconductivity Platform

The precise oxygen content during growth and measurement is critical for observing superconductivity in (La,Pr)3Ni2O7 films, as revealed by recent atomic-scale investigations. Researchers found that maintaining oxygen-sufficient conditions during sample transfer and measurement consistently produced U-shaped tunnelling spectra, indicative of a nodeless, multigap superconducting state. Conversely, allowing even slight oxygen loss during transfer resulted in V-shaped spectra, a gap shape that can be misleadingly interpreted as evidence for unconventional superconductivity, but which coexists with structural reconstruction and a measurable transport transition.

This sensitivity to oxygen stoichiometry provides a significant constraint when interpreting local spectroscopic measurements of these materials. The observed link between oxygen content and the shape of the superconducting gap is not merely a surface effect; it reflects a fundamental interplay between oxygen stoichiometry, ligand-hole density, interlayer coupling and magnetic correlations.

Interstitial oxygen order is known to compete with superconductivity in La2PrNi2O7+δ, while oxygen deficiency can promote density-wave-related electronic reconstruction. The superconducting state appears within a delicate balance of these factors. The U-shaped spectra observed under oxygen-sufficient conditions demonstrate a nodeless multigap state, while V-shaped spectra emerge when this balance is disrupted, even if a transport transition persists at 40-50 K. This suggests that the superconducting gap itself is more sensitive to oxygen loss than the macroscopic superconducting properties.

The team’s work employed a specific growth technique, gigantic oxidative atomic-layer-by-layer epitaxy, to create high-quality (La,Pr)3Ni2O7 films on SrLaAlO4 substrates. This method allows for precise control over the film’s structure and composition, essential for studying the delicate interplay of factors influencing superconductivity. The resulting films were then characterized using scanning tunneling microscopy and spectroscopy (STM/STS), enabling atomic-resolution imaging of the electronic structure and the direct observation of the superconducting gap.

The researchers emphasize the importance of reporting the transfer protocol alongside the growth recipe for STM studies, given the sensitivity of the results to oxygen content. The distinction between U-shaped and V-shaped spectra extends beyond a simple difference in gap shape; it represents different surface conditions of the same material platform. The wide-energy dip present in the V-shaped spectra is a background feature and an integral part of the oxygen-history-dependent electronic response.

This observation challenges the interpretation of V-shaped gaps as definitive evidence of d-wave superconductivity, a non-conventional pairing mechanism. The team’s findings demonstrate that a V-shaped tunnelling gap should not be taken as proof of d-wave superconductivity in isolation. Magnetic-field-dependent spectra further support the robustness of the superconducting features observed under oxygen-sufficient conditions. Application of a perpendicular field of 2.2 Tesla induced only weak changes in the spectrum, with no significant suppression of the gap magnitude.

Gigantic-Oxidative Epitaxy Synthesizes Bilayer Nickelate Films

The synthesis of high-quality (La,Pr)3Ni2O7 films relies on a technique called gigantic-oxidative atomic-layer-by-layer epitaxy, enabling the creation of ultrathin films with remarkably sharp interfaces and minimal structural defects. This method, employed by the team, consistently produces 1.5-unit-cell films on SrLaAlO4 substrates, confirmed by reflection high-energy electron diffraction exhibiting persistent oscillations and negligible decay during growth. These oscillations indicate layer-by-layer deposition and high crystallinity, essential for subsequent spectroscopic analysis and the observation of subtle electronic properties.

A reproducible √2 × √2 surface reconstruction consistently forms on these films, serving as an internal marker of surface order and a critical indicator of the growth process’s success. This reconstruction is not merely a structural feature. It is maintained even when samples undergo rapid cryogenic transfer, demonstrating its robustness and providing a reliable reference point for interpreting spectroscopic data.

Atomic Reconstruction Confirms Surface Termination Order

X-ray diffraction confirmed the phase purity and coherent epitaxy of the films, establishing a foundational quality for subsequent atomic-level investigations. A significant element in preserving the delicate superconducting state proved to be the rapid cooling of the as-grown films below 150 Kelvin, immediately followed by transfer within an ultrahigh-vacuum cryogenic suitcase. Large-scale topographical imaging revealed atomically flat terraces, demarcated by steps corresponding to the unit cell scale of strontium lanthanum aluminate, indicating maintained morphological coherence throughout the transfer process.

Atomic-resolution imaging on these terraces consistently revealed a √2 × √2 reconstruction relative to the underlying nickel-oxide lattice, a reproducible feature serving as an internal marker of well-ordered surface termination. This reconstruction was observed on samples rapidly transferred, confirming its persistence even during handling.

The coexistence of a preserved √2 × √2 reconstruction alongside a V-shaped spectrum in certain samples revealed an important insight: the surface lattice order is more resilient to degradation than the superconducting gap itself. By contrast, prolonged transfer times through room-temperature stages consistently resulted in V-shaped spectra, even when the √2 × √2 reconstruction and a 40-50 Kelvin transport transition remained detectable. This observation emphasised the sensitivity of the local superconducting gap to oxygen loss, establishing a critical constraint on interpreting V-shaped spectra.

A V-shaped tunnelling gap, the researchers found, should not be automatically interpreted as evidence of d-wave superconductivity, a different pairing symmetry. The observed oxygen-stoichiometry dependence also offers a framework for reconciling local spectroscopic results with the broader understanding of the material’s phase diagram.

Rapid Transfer Preserves Intrinsic U-Shaped Gap Spectra

Transfer times under five minutes consistently produced U-shaped superconducting spectra, revealing a fully opened gap inconsistent with a dominant nodal structure, while longer durations induced a shift toward V-shaped gaps. This sensitivity highlights the delicate balance between maintaining oxygen stoichiometry and accurately characterizing the material’s electronic properties.

The observed spectral changes are not merely superficial; the broad dip within the V-shaped spectra is demonstrably linked to density-wave energy scales and cannot be dismissed as a simple background feature. Researchers found that the emergence of this dip correlates directly with conditions prone to oxygen loss during transfer, suggesting a fundamental connection between oxygen content and the superconducting gap.

This observation is important because it distinguishes the observed gap from artifacts potentially arising from surface contamination or other experimental factors. The team’s methodology involved atomic-layer-controlled growth of 1.5-unit-cell (La,Pr)3Ni2O7 films on SrLaAlO4 substrates, followed by rapid transfer to the STM for spectroscopic analysis. Defining transfer time as the period from exiting the cryogenic suitcase to loading into the STM chamber, they systematically varied this parameter to investigate its impact on the observed spectra.

Samples undergoing transfer in under five minutes exhibited U-shaped gaps with outer and inner gap energies of approximately 20 and 12-15 meV, respectively, while those transferred in over ten minutes displayed V-shaped gaps around 16 meV. Wide-energy-range spectra further revealed asymmetric suppression in the U-shaped samples, with edges at roughly +120 and -85 meV, contrasting with the broader dip observed in the V-shaped spectra, which exhibited edges near +80 and -90 meV.

This decoupling suggests that the degradation of superconductivity is a primary consequence of oxygen deficiency, rather than a general disruption of the surface structure. This finding emphasises the importance of carefully controlling the experimental environment to accurately probe the intrinsic properties of these materials. This level of detail is essential for ensuring reproducibility and accurately interpreting spectroscopic data, as the observed gap line shape is demonstrably sensitive to the sample’s oxygen content. In summary, atomic-resolution STM/STS on oxygen-sufficient 1.5-unit-cell (La,Pr)3Ni2O7 films reveals a homogeneous U-shaped double gap.

👉 More information
🗞 Atomically resolved superconducting gap in (La,Pr) ₃ Ni ₂ O ₇ films
✍️ Xinxin Wang et al.
🧠 DOI: http://link.aps.org/doi/10.1103/rkn7-f5rs

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