Atomically resolved intrinsic superconducting gap in (La,Pr)3Ni2O7 films

Using atomic-resolution scanning tunneling microscopy and spectroscopy on cryogenically transferred (La,Pr)₃Ni₂O₇ films, this study reveals an intrinsic nodeless superconducting gap with two distinct energy scales, distinguishing it from V-shaped spectra caused by oxygen loss and providing key insights into the pairing symmetry of bilayer nickelates.

The Gist

Imagine you are trying to listen to a very quiet, delicate song being played by a tiny, invisible orchestra inside a special material called a nickelate. Scientists have been arguing for a while about what kind of music this orchestra is playing: is it a smooth, continuous melody (a "nodeless" gap), or does it have a sharp, jagged edge (a "nodal" gap)?

This paper is like a detective story where the researchers finally figured out that the "music" they hear depends entirely on how carefully they handle the instrument.

Here is the breakdown of their discovery in simple terms:

1. The Material: A Tiny, Special Sandwich

The scientists created a super-thin film of a material called (La,Pr)3Ni2O7. Think of this material as a microscopic sandwich with layers of atoms. It's famous because, under the right conditions, it can conduct electricity with zero resistance (superconductivity) at relatively high temperatures.

They grew these films atom-by-atom, like stacking Lego bricks perfectly, on a special base. The result was a smooth, flat surface that looked like a perfectly tiled floor when viewed under a super-powerful microscope.

2. The Problem: The "Oxygen Thief"

The biggest mystery was that when scientists looked at these films with a Scanning Tunneling Microscope (STM)—which acts like a super-sensitive ear listening to electrons—they got two different results:

• Result A: A smooth, "U-shaped" curve with a deep, flat bottom. This looked like a perfect, gapless song (nodeless superconductivity).
• Result B: A jagged, "V-shaped" curve that looked like the song had a hole in the middle (nodal superconductivity).

For a long time, scientists didn't know which result was the "real" one. Was the material naturally V-shaped, or was something wrong with the experiment?

3. The Solution: The "Cold Suitcase" Race

The researchers realized the culprit was oxygen. The material is like a sponge that desperately wants to hold onto oxygen atoms. If it loses even a tiny bit of oxygen, its electronic "song" changes completely.

They set up a race against time using a cryogenic (super-cold) vacuum suitcase:

• The Fast Run (Good): They took the film out of the growth machine, put it in the cold suitcase, and rushed it to the microscope in less than 5 minutes. Because the film stayed cold and sealed, it kept all its oxygen. The microscope heard the U-shaped, smooth song.
• The Slow Run (Bad): They let the film sit in the suitcase or transfer it slowly (taking more than 10 minutes). During this time, the film warmed up slightly and lost some oxygen to the air. Even though the film still looked perfect under the microscope and still conducted electricity in a big test, the local "song" heard by the microscope had changed to a V-shaped, jagged sound.

4. The Discovery: Two Different "Personalities"

The paper concludes that the material has two "personalities" depending on its oxygen levels:

• The Healthy Version (Oxygen-rich): When the film is fresh and full of oxygen, it shows a U-shaped gap. This is the "intrinsic" or true nature of the superconductivity in this material. It means the electrons are pairing up in a very specific, smooth way without any holes.
• The Sick Version (Oxygen-poor): When the film loses oxygen, it starts showing a V-shaped gap. This isn't the true superconducting state; it's a mix of the superconductivity and some "noise" caused by the missing oxygen (which creates something called a "density wave").

5. The Takeaway

The most important lesson from this paper is a warning to other scientists: Don't trust the shape of the gap just because the material looks good.

Even if the film has a perfect atomic pattern (the "tiles" are still aligned) and conducts electricity well, if you took too long to move it to the microscope, you might be listening to the "sick" version. To hear the true, smooth song of superconductivity, you must move the sample quickly and keep it cold to protect its oxygen.

In short: The material is a nodeless superconductor (smooth U-shape), but it is so sensitive to losing oxygen that if you aren't careful, it looks like it has a hole in it (V-shape). The "U" is the real deal; the "V" is a side effect of oxygen loss.

Technical Summary

Technical Summary: Atomically Resolved Intrinsic Superconducting Gap in (La,Pr)₃Ni₂O₇ Films

Problem Statement
Ruddlesden–Popper (RP) bilayer nickelates have emerged as a critical platform for investigating high-temperature superconductivity, yet the pairing symmetry of the superconducting state remains a subject of intense debate. While momentum-resolved techniques like ARPES and RIXS have provided insights into orbital correlations and Fermi surface reconstruction, the intrinsic nature of the superconducting gap at the atomic scale is complicated by oxygen stoichiometry. Previous studies suggest that oxygen loss can induce density-wave orders and reorganize unoccupied orbital characters, potentially obscuring the true superconducting gap. A central challenge is distinguishing between intrinsic nodeless superconductivity and spectral features arising from oxygen-deficient conditions or density-wave states, particularly when surface reconstructions and macroscopic transport transitions appear robust despite local spectral degradation.

Methodology
The authors employed a combination of atomic-layer-controlled growth, cryogenic ultrahigh-vacuum (UHV) sample transfer, and scanning tunneling microscopy/spectroscopy (STM/STS) to investigate 1.5-unit-cell (equivalent to three bilayers) (La,Pr)₃Ni₂O₇ films grown on SrLaAlO₄ substrates.

• Growth: Films were synthesized using the gigantic-oxidative atomic-layer-by-layer epitaxy (GAE) method, resulting in a Ni–O terminated surface with a nominal La:Pr ratio of 1.35:1.65.
• Transfer Protocol: A critical variable was the sample transfer time. Samples were rapidly cooled below 150 K and transferred in a cryogenic UHV suitcase. The authors defined "transfer time" as the duration from removing the sample from the suitcase to loading it into the STM chamber.
• Spectroscopy: Atomic-resolution STS was performed at 4.2 K. The study specifically compared spectra from samples transferred in less than 5 minutes (oxygen-sufficient condition) against those transferred in more than 10 minutes (oxygen-loss-prone condition).
• Analysis: Spectra were analyzed using nodeless two-gap Dynes models and compared against BCS-like temperature dependencies. Wide-energy-range spectra were also examined to identify density-wave-related features.

Key Results

1. Structural and Transport Baseline: All samples exhibited atomically flat terraces and a reproducible $\sqrt{2} \times \sqrt{2}$ surface reconstruction, regardless of transfer time. Transport measurements on the growth batch showed a superconducting onset near 53 K. Even samples with longer transfer times retained the surface reconstruction and a transport transition onset of 40–50 K.
2. U-Shaped Spectra (Rapid Transfer): Samples transferred in under 5 minutes displayed homogeneous, U-shaped tunneling spectra with strongly suppressed zero-bias conductance. These spectra revealed a nodeless double-gap structure with characteristic energy scales of approximately 14 meV (inner gap) and 20 meV (outer gap). The spectra were well-fitted by a nodeless two-gap Dynes model, indicating a fully opened gap with no residual density of states at the Fermi level.
3. V-Shaped Spectra (Slow Transfer): Samples transferred for longer than 10 minutes exhibited V-shaped spectra with a broad dip and a gap scale of approximately 16 meV. While the zero-bias conductance was not fully suppressed, wide-energy-range measurements revealed a broad asymmetric dip with edges near $\pm$80–90 meV. This energy scale corresponds to density-wave features observed in oxygen-deficient nickelate systems.
4. Oxygen Control as a Determinant: The study establishes a direct correlation between transfer time, oxygen loss, and spectral line shape. The preservation of the $\sqrt{2} \times \sqrt{2}$ reconstruction and macroscopic superconductivity in the slow-transfer samples did not guarantee the observation of the intrinsic superconducting gap. Instead, the V-shaped spectra in these samples were attributed to a mixture of degraded superconductivity and density-wave-related spectral weight induced by oxygen loss.

Key Contributions

• Identification of Intrinsic Gap: The paper provides the first atomic-scale observation of a homogeneous, nodeless double-gap superconducting state in bilayer nickelate ultrathin films under oxygen-sufficient conditions.
• Decoupling Reconstruction from Superconductivity: The work demonstrates that surface lattice order ($\sqrt{2} \times \sqrt{2}$ reconstruction) and macroscopic transport transitions are more robust against oxygen loss than the local superconducting gap. This finding challenges the assumption that structural reconstruction alone confirms the intrinsic superconducting state.
• Methodological Criterion: The authors establish rapid cryogenic UHV transfer as a prerequisite for resolving the true pairing symmetry in RP nickelates. They provide a practical criterion for interpreting local spectra: U-shaped spectra indicate intrinsic superconductivity, while V-shaped spectra in nominally superconducting films likely reflect oxygen-deficient conditions mixed with density-wave orders.
• Gap Symmetry Constraints: By demonstrating that V-shaped spectra can arise from oxygen loss rather than intrinsic nodal pairing, the results place constraints on interpretations of pairing symmetry, suggesting that the intrinsic state in these films is likely nodeless rather than d-wave.

Significance
The paper claims to resolve a critical ambiguity in the study of nickelate superconductors by isolating the effects of oxygen stoichiometry on the local electronic structure. It argues that previous observations of V-shaped gaps or nodal behavior in similar systems may have been artifacts of oxygen loss and density-wave mixing rather than intrinsic pairing symmetry. By defining the oxygen-controlled transfer protocol as essential, the study provides a standardized framework for future investigations into the pairing mechanism of RP nickelates, ensuring that spectroscopic data reflects the intrinsic superconducting state rather than extrinsic surface degradation. The findings support a model where superconductivity in bilayer nickelates exists within a specific window of oxygen stoichiometry, balancing ligand-hole density and interlayer coupling, and that outside this window, density-wave orders can dominate the spectral response.