Superconductivity in Ruddlesden-Popper nickelates: a review of recent progress, focusing on thin films

This paper reviews recent experimental and theoretical progress in Ruddlesden-Popper nickelates, with a specific focus on thin films, to explore the mechanisms behind high-temperature superconductivity in these correlated electron systems following the discovery of superconductivity in La3Ni2O7 and La4Ni3O10 under high pressure and in ultra-thin films at ambient pressure.

The Gist

The Big Picture: A New Superconductor Star

Imagine the world of superconductors (materials that conduct electricity with zero resistance) as a sports league. For decades, the Cuprates (copper-based materials) were the undisputed champions, holding the record for the highest "winning temperature" (how hot they can be while still superconducting). Then, the Iron-based team joined the league.

Now, a new team has entered the arena: Nickelates (nickel-based materials). This paper is a review of their recent, explosive rise to stardom, specifically focusing on a new trick they learned: how to play the game without needing a giant pressure machine.

The Plot Twist: From "High Pressure" to "Room Temperature"

For a long time, these nickel superconductors were like deep-sea divers. They only worked when you squeezed them with massive pressure (like being at the bottom of the ocean). In 2023, scientists found that a specific nickel material called La₃Ni₂O₇ could superconduct at a scorching 80 Kelvin (about -310°F), but only if you crushed it with 14 billion Pascals of pressure.

The Problem: You can't put a giant pressure machine on a computer chip or a power line. It's too bulky and expensive.

The Breakthrough: In early 2025, scientists discovered a way to make this material superconduct at ambient pressure (normal room pressure) by growing it as an ultra-thin film on a special substrate.

• The Analogy: Imagine trying to fit a large, bulky suitcase into a tiny car trunk. You can't force it in (that's the high-pressure method). But, if you build a custom shelf inside the car that forces the suitcase to compress just the right way, it fits perfectly without needing a hydraulic press.
• The Trick: They grew the nickel material on a substrate called LSAO. This substrate acts like a "mold" that squeezes the nickel atoms from the sides (compressive strain). This squeezing mimics the effect of high pressure, unlocking the superconducting power without the heavy machinery.

The Players: The Nickel "Family"

The paper discusses different members of the Ruddlesden-Popper (RP) nickel family. Think of them as different floor plans in a building:

1. The Infinite Layer (NdNiO₂): The "studio apartment." It has one layer of nickel. It superconducts, but only at lower temperatures (around 15–40 K).
2. The Bilayer (La₃Ni₂O₇): The "duplex." It has two layers of nickel stacked on top of each other. This is the star of the show. When squeezed (either by pressure or thin-film strain), it hits the high 80 K mark.
3. The Trilayer (La₄Ni₃O₁₀): The "triplex." Three layers. It also superconducts under pressure, but the bilayer is currently the strongest.
4. The Hybrids (1212, 1313): These are like "mixed-use buildings" with different floor plans stacked together. They are a new, experimental frontier.

The Mystery: How Do They Work?

The scientists are still trying to figure out the "secret sauce" of how these electrons pair up to flow without resistance.

• The "Dimer" Concept: In the bilayer nickel, the two layers of nickel are so close they act like a single unit, or a "dimer" (a pair). It's like two dancers holding hands so tightly they move as one. The paper suggests that the electrons in these pairs are the key to the high temperature.
• The "Gamma" Pocket: There is a specific shape to the electron map (Fermi surface) called a "gamma pocket."
  • Analogy: Imagine a dance floor. For the superconducting dance to happen, the dancers need a specific empty space (the pocket) to spin.
  • The Debate: In the high-pressure bulk crystals, this pocket is clearly there. But in the thin films, some experiments say the pocket is there, while others say it's missing. This is a major mystery the paper highlights.
• The "Strange Metal" Behavior: Before they become superconductors, these materials act like "strange metals." Their resistance changes in a straight line as temperature changes, rather than the usual curve. This suggests they are "strongly correlated," meaning the electrons are constantly talking to and influencing each other, much like a crowded mosh pit where everyone's movement affects everyone else.

The Thin Film Advantage: Why It Matters

Why are we so excited about the thin films?

1. Accessibility: You can't do many experiments on a sample inside a diamond anvil cell (a tiny pressure machine). But you can put a thin film on a table and use powerful tools like ARPES (Angle-Resolved Photoemission Spectroscopy) to take "photos" of the electrons.
2. Tunability: By changing the substrate (the floor the film is built on), scientists can stretch or squeeze the material to see how it reacts. It's like tuning a guitar string to find the perfect note.
3. The Strain vs. Pressure Difference:
   • High Pressure: Squeezes the material from all sides (top, bottom, and sides).
   • Thin Film Strain: Squeezes from the sides but lets the top and bottom expand.
   • The Result: This difference changes the "dance floor" geometry. The paper notes that this might actually change the type of superconductivity (from one theoretical type called s-wave to another called d-wave), adding a new layer of complexity to the puzzle.

The Future: What's Next?

The paper concludes that while we have made huge leaps, the game isn't over.

• The Goal: We want to push the temperature higher, ideally to "liquid nitrogen temperature" (77 K), which is cheap and easy to cool with. The thin films are already hitting 63 K, getting very close!
• The Challenge: We need to understand exactly why the thin films work. Is it the strain? Is it the extra oxygen? Is it the specific way the atoms are stacked?
• New Frontiers: Scientists are now building "hybrid" structures (stacking different layers) and trying to grow even thicker films to see if the superconductivity survives.

Summary in a Nutshell

This paper celebrates a major victory in physics: Nickel superconductors are no longer just a "high-pressure curiosity." By growing them as ultra-thin films on special substrates, scientists have unlocked their power at normal pressure. This opens the door to using advanced tools to understand how they work, bringing us one step closer to the holy grail of room-temperature superconductivity—a technology that could revolutionize power grids, maglev trains, and quantum computers.

Technical Summary

1. Problem Statement

The discovery of high-temperature superconductivity ($T_c \sim 80$ K) in the bilayer Ruddlesden-Popper (RP) nickelate La$_3$Ni$_2$O$_7$ under high pressure (14–90 GPa) in 2023 opened a new frontier in condensed matter physics. However, the requirement for extreme hydrostatic pressure severely limited experimental characterization, particularly preventing the use of surface-sensitive techniques like Angle-Resolved Photoemission Spectroscopy (ARPES) to map the Fermi surface and pairing symmetry. Furthermore, the theoretical understanding of the pairing mechanism (e.g., $s_{\pm}$ vs. $d$-wave) and the role of electronic correlations in these nickelates remains debated, especially regarding the necessity of specific Fermi surface features (the $\gamma$ pocket) and the influence of lattice distortions.

The central problem addressed by this review is the transition from high-pressure bulk studies to ambient-pressure superconductivity in ultra-thin films. The authors aim to synthesize recent experimental and theoretical progress to understand how compressive strain in thin films can mimic high-pressure effects, stabilize the superconducting state, and provide a platform to resolve the mechanisms of unconventional superconductivity in correlated electron systems.

2. Methodology

This paper is a comprehensive review that synthesizes data from recent experimental and theoretical literature (primarily 2023–2026). The methodology involves:

• Experimental Synthesis: Analyzing results from thin-film growth techniques, specifically Pulsed Laser Deposition (PLD) and Molecular Beam Epitaxy (MBE), often utilizing "Gigantic-oxidative atomic-layer-by-layer epitaxy" (GAE) to control oxygen stoichiometry.
• Comparative Analysis: Contrasting bulk high-pressure data with thin-film ambient-pressure data, focusing on lattice parameters ($a, b, c$), strain types (compressive vs. tensile), and electronic structures.
• Theoretical Review: Summarizing various computational approaches used to model these systems, including:
  • Density Functional Theory (DFT) and DFT+U for electronic band structures.
  • Random Phase Approximation (RPA) and Functional Renormalization Group (FRG) for pairing instabilities.
  • Quantum Monte Carlo (QMC) and Dynamical Mean Field Theory (DMFT) for strong correlation effects.
  • Slab Geometry Models: Specifically addressing the difference between 0.5 unit cell (UC) models (single bilayer) and 1 UC models (two bilayers with symmetry breaking) to accurately simulate thin films.

3. Key Contributions and Results

A. Ambient-Pressure Superconductivity in Thin Films

• Discovery: The review highlights the landmark 2025 discovery of superconductivity in La$_3$Ni$_2$O$_7$ ultra-thin films (1–3 unit cells) grown on LaSrAlO$_4$ (LSAO) substrates at ambient pressure.
• Strain Engineering: The LSAO substrate provides a compressive strain of $\sim -2\%$. This strain transforms the low-pressure orthorhombic ($Amam$) structure into the high-symmetry tetragonal ($Fmmm$) structure, which is identical to the high-pressure bulk phase.
• Critical Temperature: Films exhibit an onset $T_c$ of 42–63 K (with a record of 63 K reported for (La,Pr)$_3$Ni$_2$O$_7$ films), significantly lower than the bulk high-pressure $T_c$ ($\sim 80$ K) but sufficient for extensive study.
• Strain vs. Pressure: A crucial finding is that while hydrostatic pressure reduces all lattice constants ($a, b, c$), compressive strain reduces in-plane constants ($a, b$) but expands the out-of-plane constant ($c$) due to Poisson effects. This difference in $c/a$ ratio is a key variable distinguishing thin-film physics from bulk high-pressure physics.

B. Electronic Structure and Fermi Surface

• The $\gamma$ Pocket Controversy: The presence of a $\gamma$ pocket (derived from Ni $d_{3z^2-r^2}$ orbitals) at the Fermi level is theoretically linked to $s_{\pm}$ pairing.
  • Bulk High-Pressure: The $\gamma$ pocket is present.
  • Thin Films: Experimental ARPES results are conflicting. Some studies report the $\gamma$ pocket is absent (lying $\sim 70$ meV below $E_F$), while others (using specific photon energies or buffer layers) observe it crossing $E_F$. The review suggests this may depend on accidental hole doping (Sr diffusion from the substrate) or oxygen stoichiometry.
• Normal State Behavior: The resistivity $\rho(T)$ of optimal films shows a linear dependence ($\rho \propto T$), indicative of "strange metal" behavior similar to cuprates, suggesting strong electronic correlations.

C. Pairing Symmetry

• Theoretical Debate:
  • $s_{\pm}$ Wave: Supported by RPA and FRG studies on bulk high-pressure systems and some thin-film models, driven by interlayer coupling and Fermi surface nesting involving the $\gamma$ pocket.
  • $d$-Wave: Recent theoretical work on 1 UC thin films (accounting for interface symmetry breaking) suggests that the expansion of the $c$-axis and specific hole doping levels can stabilize a $d$-wave pairing state, particularly when the $\gamma$ pocket is absent.
• Experimental Evidence: STM and ARPES studies on thin films generally report nodeless gaps (favoring $s_{\pm}$), though some data show very small gaps that could be consistent with nodes. The consensus is shifting toward $s_{\pm}$ as the dominant channel, but the possibility of $d$-wave in specific strain/doping regimes remains open.

D. Hybrid Stacking Nickelates

• The review extends to hybrid structures (e.g., 1212 and 1313 phases) where single-layer and bilayer/trilayer blocks alternate.
• 1212 Nickelate: Shows ambient-pressure superconductivity ($T_c \sim 50$ K) in thin films, with the $\gamma$ pocket present in the bilayer blocks.
• 1313 Nickelate: Shows superconductivity only under high pressure in bulk; thin films do not show ambient superconductivity, and the $\gamma$ pocket is absent in the trilayer blocks.

4. Significance

• Experimental Accessibility: The ability to achieve superconductivity at ambient pressure in thin films is the most significant breakthrough. It enables the use of ARPES, STM, and RIXS to probe the electronic and magnetic structure of the superconducting state, which was impossible under high-pressure conditions.
• Mechanism Insight: The comparison between bulk (hydrostatic) and film (compressive strain) systems provides a unique "knob" to tune the $c/a$ ratio and orbital splitting. This helps disentangle the roles of interlayer coupling, crystal field splitting, and orbital occupancy in driving high-$T_c$ superconductivity.
• Theoretical Challenge: The review highlights that thin films are not merely scaled-down bulk systems; the symmetry breaking at the interface and the specific strain tensor require distinct theoretical models (e.g., 1 UC slab models). The potential for $d$-wave pairing in films, contrasting with $s_{\pm}$ in bulk, challenges existing paradigms and suggests a rich phase diagram dependent on dimensionality and strain.
• Future Directions: The paper outlines the need for:
  1. Resolving the $\gamma$ pocket controversy via improved ARPES and doping control.
  2. Determining the intrinsic magnetic structure (double-stripe vs. spin-charge stripe) via inelastic neutron scattering on high-quality single crystals.
  3. Exploring higher-order RP nickelates ($m=4, 5$) and hybrid stacking structures to further tune the pairing mechanism.

In conclusion, this review establishes that Ruddlesden-Popper nickelate thin films have emerged as a viable and powerful platform for studying high-$T_c$ superconductivity, offering a pathway to resolve the long-standing questions regarding the pairing mechanism in strongly correlated electron systems.