Observation of flat-bottom U-shaped energy gap in high-Tc nickelate (La,Pr)3Ni2O7 thin films

The Gist

The Big Picture: Finding a Superhighway in a New Material

Imagine you are trying to build a super-fast train (electricity) that can travel without any friction or energy loss. This is called superconductivity. Scientists have known about this for a long time, but usually, these "super-trains" only work when things are frozen to temperatures near absolute zero (colder than outer space) or when you squeeze the material with massive pressure, like a hydraulic press.

Recently, scientists discovered a new family of materials called nickelates (specifically a bilayer nickelate) that can become superconducting at much higher temperatures. However, to get them to work, they usually needed to be squeezed under high pressure.

The Breakthrough:
This paper reports a major step forward. The researchers took a thin film of this nickelate material and placed it on a specific crystal "floor" (a substrate). The floor was slightly smaller than the film, so it squeezed the film gently from the sides (compressive strain). This allowed the material to become a superconductor at room pressure (no heavy squeezing needed) and at temperatures above 40 Kelvin (about -230°C). While still very cold, this is a huge jump from the near-absolute-zero temperatures usually required.

The Main Discovery: The "Flat-Bottom U"

To understand how this material works, the scientists used a super-powerful microscope called a Scanning Tunneling Microscope (STM). Think of this microscope as a blind person's cane that can feel the energy of individual electrons.

When they looked at the energy of the electrons, they found something very special:

1. The Shape: Instead of a sharp "V" shape or a messy curve, the energy gap looked like a flat-bottomed "U".
2. The Meaning: In physics, a "gap" is like a moat around a castle. Electrons need energy to jump across it. A "flat-bottom U" with zero energy at the very bottom means the moat is completely empty. There are no "leaks" or weak spots where electrons can sneak through.
3. The Analogy: Imagine a swimming pool.
   • A normal metal is like a pool with water everywhere (electrons moving freely).
   • A superconductor usually has a "hole" in the middle where no water exists (the energy gap).
   • This new material has a perfectly flat, dry bottom in the middle of the pool. This suggests the superconductivity is very strong and uniform (what scientists call "nodeless").

The Mystery: How It Changes with Heat

The most surprising part of the paper is how this "U" shape changes as the material warms up.

• At Ultra-Cold Temperatures (60 mK): The "U" is deep and flat. The pool is perfectly dry at the bottom.
• As it Warms Up (to 10 K): The bottom of the "U" starts to fill up with water. It turns into a "V" shape.
• The Weird Part: Usually, when a superconductor gets warm, the gap just gets smaller and smaller until it disappears. But here, the gap fills up with "water" (electrons) very quickly, changing its shape entirely.

The Scientists' Theory:
They suggest the material might be made of tiny "islands" of superconductivity.

• At very low temps: The islands are connected by strong bridges, acting like one giant, solid continent (the flat U-shape).
• As it warms: The bridges get weak. The islands break apart. Now, instead of one solid continent, you see the individual islands, which have a different shape (the V-shape).

The "Liquid Nitrogen" Dream

The researchers did some math based on the size of this energy gap. They found that the gap is massive (about 41.6 meV).

In the world of superconductors, the size of the gap is linked to how hot the material can get before it stops working.

• The Calculation: If this huge gap is real, it suggests the material could theoretically stay superconducting at temperatures around 107 Kelvin.
• Why this matters: Liquid nitrogen (the stuff used to freeze things in labs) boils at 77 Kelvin. If the material works at 107 K, it means we could use cheap, common liquid nitrogen to power these superconductors, rather than expensive, rare liquid helium.

What They Did (The Process)

1. Growth: They grew a very thin film of the nickelate on a special crystal.
2. Cleaning: The surface was a bit rough (like a dirty window). They used the tip of their microscope to gently scrape off a tiny layer of the surface to get a fresh, clean view.
3. Measurement: They measured the electricity flow (transport) and then used the microscope to look at the electron energy (STM).
4. Verification: They checked the material again after the microscope work, and it was still a superconductor, proving the microscope didn't break it. They also tested it with strong magnets, and the "U" shape shrank, which is exactly what a superconductor should do.

Summary

The paper claims to have found a new, clean view of a superconducting material that works without high pressure. They saw a unique, flat-bottomed energy gap that suggests the material is a very strong, uniform superconductor. While the material currently works at about -230°C, the size of the energy gap hints that it might be possible to make it work at temperatures as high as -166°C (above the boiling point of liquid nitrogen), which would be a massive leap for future technology.

Note: The paper stops at these observations and theoretical hints. It does not claim to have built a working device or a commercial product yet; it is purely a discovery of the material's fundamental properties.

Technical Summary

Technical Summary: Observation of Flat-Bottom U-Shaped Energy Gap in High-Tc Nickelate (La,Pr)3Ni2O7 Thin Films

Problem and Context
The discovery of high-transition temperature (high-$T_c$) superconductivity in Ruddlesden-Popper (R-P) bilayer nickelates, specifically $La_3Ni_2O_7$, under high pressure has stimulated significant research into the underlying mechanisms of unconventional superconductivity. However, the requirement for high-pressure conditions has historically limited the application of conventional experimental tools. While recent advances have enabled ambient-pressure superconductivity in R-P bilayer nickelate thin films (with onset $T_c$ above 40 K) via compressive strain from substrates, a comprehensive microscopic understanding of the superconducting gap structure in these films remains an open challenge. Specifically, direct spectroscopic evidence of the gap symmetry and its evolution in these strained thin films is required to distinguish between various theoretical models.

Methodology
The authors employed ultra-low temperature scanning tunneling microscopy/spectroscopy (STM/S) and electrical transport measurements on (La,Pr)$_3$Ni$_2$O$_7$ thin films grown on SrLaAlO$_4$ substrates.

• Sample Preparation: Three-unit-cell thick (La,Pr)$_3$Ni$_2$O$_7$ films were grown using gigantic-oxidative atomic layer-by-layer epitaxy (GAE). Due to surface roughness induced by air exposure during transfer, a local surface refreshment technique was utilized, involving pushing the STM tip into the sample (~1–8 nm) and retracting it to expose a fresh surface.
• Transport Measurements: Standard four-probe electrical transport measurements were conducted before and after STM/S experiments to verify the stability of the superconducting state.
• STM/S Measurements: Spectroscopic measurements were performed at base temperatures as low as 50–60 mK. The study included:
  • Tunneling conductance ($dI/dV$) mapping to identify the energy gap structure.
  • Magnetic field dependence studies (up to 14 T perpendicular to the sample surface).
  • Temperature evolution studies (from 60 mK up to 45 K).
  • Thermal broadening analysis, where low-temperature spectra were convoluted with thermal broadening functions to distinguish intrinsic gap evolution from thermal effects.

Key Contributions and Results

1. Observation of a Flat-Bottom U-Shaped Gap: The primary finding is the first successful observation of an energy-symmetric, flat-bottom U-shaped energy gap with coherence peaks at the gap edges in (La,Pr)$_3$Ni$_2$O$_7$ thin films. The gap size ($\Delta$) was determined to be approximately 41.6 meV, with zero residual density of states at the Fermi level at ultra-low temperatures, suggesting a nodeless gap function.
2. Magnetic Field Response: The U-shaped gap structure persists under a magnetic field of 14 T but is clearly reduced in size. The spatially averaged tunneling spectra show a suppression of coherence peaks and a reduction in the gap width, consistent with the behavior of a superconducting gap with a high critical field.
3. Unconventional Temperature Evolution: The tunneling spectra exhibit a highly unconventional temperature dependence. As temperature increases from 60 mK to 7 K, the U-shaped gap rapidly fills, and coherence peaks are suppressed. Above 10 K, the gap evolves into a V-shaped structure with a non-zero density of states at the Fermi level. This transition occurs at temperatures significantly lower than the transport-measured $T_c$ (onset > 40 K, zero resistance > 20 K).
4. Exclusion of Alternative Mechanisms: The authors argue against the gap originating from Coulomb blockade or trivial band gaps. The gap's symmetry, its reduction under magnetic fields, and its rapid filling with temperature (despite $k_BT$ being much smaller than the gap width at 10 K) support a superconducting origin. The rapid filling is attributed to a potential transition from a global nodeless $s$-wave-like pairing (at low $T$) to nodal $d$-wave-like grains as inter-grain Josephson coupling weakens with increasing temperature.
5. Reproducibility: These observations were replicated in multiple regions of the same film and in a second sample (Sample 2) with similar stoichiometry and transport properties, confirming the reliability of the results.

Significance and Claims
The paper claims that the observed energy-symmetric, flat-bottom U-shaped gap, combined with its magnetic field and temperature dependence, is consistent with a superconducting gap, indicating a nodeless gap function at ultra-low temperatures.

The authors highlight the unprecedented magnitude of the gap size ($\Delta \approx 41.6$ meV). By comparing the ratio $2\Delta/k_B T_c$ with known superconductors, they note that if this gap corresponds to the transport-measured onset $T_c$, the ratio would be approximately 20, placing the system deep in the strong-coupling regime. Alternatively, if the ratio is assumed to be similar to high-$T_c$ cuprates (around 9), the observed gap size would imply a local superconducting transition temperature ($T_c$) of approximately 107 K, well above the liquid nitrogen boiling temperature (77 K).

The authors suggest that the tip treatment process may locally alter the out-of-plane lattice constant, potentially inducing a higher "local $T_c$" in the specific region probed by the STM compared to the macroscopic transport measurements. They conclude that these findings provide new insights into the pairing mechanism of high-$T_c$ nickelate superconductors and offer an encouraging hint that ambient-pressure superconductivity above liquid nitrogen temperatures may be achievable in nickelate thin films. The work underscores the potential of R-P bilayer nickelates as a platform for studying unconventional superconductivity and hints at the possibility of local superconductivity with $T_c$ exceeding 77 K at ambient pressure.