Imagine a world where electricity flows without resistance, where power lines never waste energy as heat, and where computers operate at speeds we can barely fathom today. This is the promise of superconductivity, a phenomenon where certain materials conduct electricity perfectly when cooled to extremely low temperatures. For decades, scientists have been hunting for new superconducting materials that might one day transform our technological landscape. Now, a breakthrough discovery is rewriting what we thought we knew about an exotic class of materials called nickelates.

In a stunning development that has caught the attention of physicists worldwide, researchers have demonstrated that thin films of praseodymium nickel oxide, a compound with the chemical formula PrNiO₂, can become superconducting without any chemical additives. This finding challenges a fundamental assumption that has guided nickelate research since these materials first showed superconducting behavior in 2019.

The Quest for Better Superconductors

To appreciate why this discovery matters, we need to understand what makes superconductors special and why scientists are so obsessed with finding new ones.

When electricity flows through ordinary wires, some energy is always lost as heat due to electrical resistance. This is why your phone charger gets warm and why power companies lose billions of dollars worth of electricity during transmission. Superconductors eliminate this problem entirely by allowing electricity to flow without any resistance whatsoever.

The catch? Most superconductors only work at temperatures far below zero, often colder than outer space. The quest for materials that superconduct at higher, more practical temperatures has been one of the holy grails of physics for nearly a century.

Enter nickelates. These materials belong to a special category called infinite layer compounds, named for their unique atomic structure where nickel and oxygen atoms stack in flat, repeating layers. When scientists first discovered superconductivity in nickel-based compounds a few years ago, it sparked enormous excitement because these materials share intriguing similarities with cuprates, copper-based superconductors that have been studied intensively since the 1980s.

But there was a widely accepted rule: nickelates only became superconducting when researchers added extra elements like strontium or calcium, a process called chemical doping. Pure, undoped nickelates were thought to be non-superconducting "bad metals" with unusual electrical properties but no superconductivity.

The new research shatters this assumption.

A Delicate Dance of Atoms

Creating infinite layer nickelates is extraordinarily difficult. Think of it as atomic scale architecture requiring precision that pushes the limits of modern materials science.

The process begins with growing thin films of a precursor material, praseodymium nickel oxide in its perovskite form, on top of a strontium titanate substrate. This step alone requires careful control, as the atoms must arrange themselves in perfect crystalline order, layer by atomic layer.

Then comes the truly tricky part: converting the perovskite into the infinite layer structure. This transformation involves removing specific oxygen atoms from the crystal lattice through a process called topotactic reduction. Researchers heat the samples with calcium hydride powder at precisely controlled temperatures, typically around 260 degrees Celsius, creating conditions where oxygen atoms are extracted while the overall crystal structure is preserved.

It's a bit like removing specific bricks from a building without causing it to collapse, except you're working with structures just a few nanometers thick, where a nanometer is about 100,000 times smaller than the width of a human hair.

The research team discovered that the quality of this reduction process is absolutely critical. They found that applying a thin protective cap of strontium titanate over the nickelate film before reduction dramatically improved the results. Samples with a cap at least six atomic layers thick consistently showed a complete transition to superconductivity, with electrical resistance dropping to zero below about 4 Kelvin, which is roughly minus 269 degrees Celsius.

This protective capping layer does more than just shield the material during chemical processing. It appears to help maintain the compressive strain imposed by the substrate underneath, which is essential for stabilizing the infinite layer phase in its pristine form.

Seeing is Believing

One of the most compelling aspects of this research is the extraordinary level of evidence supporting the findings. The team employed multiple sophisticated techniques to verify that their samples were truly pure and undoped.

Using scanning transmission electron microscopy, a technique that can image individual columns of atoms, the researchers examined their samples at unprecedented resolution. The images revealed a beautifully ordered infinite layer structure with remarkably few defects. Unlike previous nickelate samples that often contained stacking faults and imperfections, these new films displayed exceptional crystalline quality throughout.

But atomic scale imaging was just the beginning. The team also used a specialized variant called divergence of center of mass imaging, which is particularly sensitive to the positions of lighter atoms like oxygen. This analysis confirmed the complete absence of the apical oxygen atoms that should be removed during the infinite layer conversion. The nickel atoms sat in perfect square planar arrangements, exactly as required for the infinite layer structure, with no signs of residual oxygen lurking between the layers.

To rule out unintentional chemical doping, which might occur if strontium from the substrate diffused into the film or if the praseodymium to nickel ratio was off, the researchers performed elemental mapping with electron energy loss spectroscopy. Within the sensitivity limits of the technique, they found a perfect one to one ratio of praseodymium to nickel throughout the films, with no detectable strontium contamination.

X-ray absorption measurements provided further confirmation. These experiments probe the electronic structure of the material by measuring how it absorbs X-rays at different energies. The results showed extremely sharp features characteristic of nickel in the +1 oxidation state, exactly what's expected for a fully reduced infinite layer compound. The absence of broader peaks that would signal nickel in the +2 state ruled out incomplete reduction or excess oxygen.

The Magnetic Connection

Perhaps the most intriguing aspect of the research involves understanding the magnetic properties of these superconducting films.

Superconductivity and magnetism have a complicated relationship. In many superconductors, particularly the cuprates that nickelates resemble, the superconducting state emerges from a parent material that exhibits strong antiferromagnetic behavior. In an antiferromagnet, adjacent magnetic moments point in opposite directions, creating an alternating pattern like a checkerboard.

The research team used a technique called resonant inelastic X-ray scattering to probe the magnetic excitations in their nickelate films. This sophisticated method allows scientists to observe magnons, collective oscillations of the magnetic moments, similar to how sound waves are collective oscillations of atoms in a solid.

The measurements revealed remarkably sharp magnon excitations with energies around 200 millielectron volts, which is quite substantial on the energy scale relevant for electronic properties in solids. These magnons appeared most prominently at the edge of the magnetic Brillouin zone, a technical term that describes specific directions in the crystal lattice where magnetic correlations are strongest.

What makes these observations particularly striking is the sharpness of the magnetic excitations. In materials with significant disorder or chemical doping, magnetic peaks tend to be broad and washed out. The crisp, well defined magnons observed here indicate long range magnetic correlations persisting throughout the material, suggesting that superconductivity is emerging in a system with well preserved antiferromagnetic fluctuations, not one disrupted by chemical dopants.

The Self-Doping Puzzle

If these nickelate films aren't chemically doped, how do they become superconducting? The answer may lie in a phenomenon called self doping.

In most superconductors, charge carriers, the electrons or holes that carry electrical current, need to be introduced through chemical substitution. It's like adding a pinch of salt to change the flavor of a dish. But theoretical calculations have suggested that nickelates might be different.

The praseodymium atoms in these compounds have partially filled 5d electron orbitals that can hybridize, or mix, with the nickel 3d orbitals that dominate the material's electronic properties. This hybridization naturally creates charge carriers without requiring any chemical additives. The material dopes itself through its intrinsic electronic structure.

Think of it like self rising flour that already contains leavening agents. You don't need to add anything extra because the ingredients for the desired outcome are built in from the start.

The experimental evidence supports this self doping picture. Measurements of the Hall coefficient, a property that reveals information about the charge carriers, showed behavior consistent with both electron-like and hole-like carriers, as predicted by calculations that include the praseodymium 5d states. The X-ray absorption data revealed strong linear dichroism, meaning the material absorbs light differently depending on its polarization direction, which directly demonstrates that holes preferentially occupy specific nickel orbitals.

Why This Changes Everything

This discovery has profound implications for how we understand superconductivity in nickelates and potentially for the broader search for new superconducting materials.

First, it means the phase diagram of nickelate superconductors, the map that shows under what conditions these materials superconduct, needs to be redrawn. Scientists had assumed you needed a dome shaped region of chemical doping, similar to cuprates, with superconductivity appearing only within a specific doping range. But if the parent compound itself is superconducting, this simple picture no longer holds.

Second, it highlights the critical importance of materials quality. Previous studies that reported non-superconducting undoped nickelates may have been limited by subtle imperfections in their samples: incomplete reduction, excess defects, or other issues that prevented the superconducting state from fully developing. The researchers in this study achieved their breakthrough through painstaking optimization of every step in the synthesis process, from growing defect-free perovskite precursors to carefully controlling the reduction conditions.

The quality of the initial perovskite phase turned out to be especially crucial. The team demonstrated that their precursor films exhibited sharp metal to insulator transitions, a hallmark of stoichiometric praseodymium nickel oxide. When they grew the same material on different substrates with better lattice matching, the transitions became even sharper, approaching the behavior of bulk single crystals. This pristine starting point appears essential for achieving a clean infinite layer phase capable of superconductivity.

Third, the results suggest that the pairing mechanism responsible for superconductivity in nickelates might be more robust than previously thought. If superconductivity can emerge without chemical doping, the underlying physics, whatever causes electrons to pair up and condense into the superconducting state, must be intrinsic to the infinite layer structure itself.

The Bigger Picture

While this research won't immediately lead to room temperature superconductors or levitating trains, it represents a significant step forward in our fundamental understanding of quantum materials.

Nickelates are part of a broader family of strongly correlated electron systems, materials where the interactions between electrons are so strong that they cannot be understood by treating electrons as independent particles. These systems display a bewildering array of emergent phenomena, from superconductivity to magnetism to exotic metallic states that defy conventional classification.

Understanding how superconductivity emerges in nickelates could provide insights applicable to other material families. The multi-orbital nature of nickelates, with both nickel 3d and rare earth 5d electrons playing important roles, creates a richer and more complex electronic structure than found in cuprates. Unraveling this complexity might reveal new knobs to turn in the search for superconductors with more favorable properties.

There's also a practical lesson about materials synthesis. The extraordinary difficulty of making high quality infinite layer nickelates has been a major impediment to progress in this field. Different research groups using different synthesis methods have reported widely varying properties, making it hard to establish what represents the intrinsic behavior of these materials versus artifacts of sample quality.

This study demonstrates that with sufficient care and optimization, it is possible to reproducibly synthesize nickelate films that exhibit the same superconducting properties across multiple samples. The detailed protocols developed here, from substrate preparation to film growth to reduction procedures, will serve as a roadmap for other laboratories working in this area.

Questions Still to Answer

As often happens in science, answering one question raises many others.

One immediate puzzle concerns why superconductivity in these undoped films appears below 4 Kelvin, while chemically doped nickelates have shown superconducting transitions up to 15 Kelvin or higher in some reports. If self doping is present, why does adding chemical dopants increase the transition temperature? Does chemical doping simply add more charge carriers on top of the self doped background, or does it modify the material in more fundamental ways?

The spatial uniformity of superconductivity also deserves further investigation. The measurements in this study examined bulk transport properties averaged over the entire film. Local probe techniques that can map superconductivity at the nanoscale might reveal whether the superconducting state is perfectly homogeneous or contains spatial variations.

Another open question involves the interplay between superconductivity and magnetism. The sharp magnon excitations observed in these films indicate robust antiferromagnetic correlations, but the exact relationship between these magnetic fluctuations and the superconducting pairing mechanism remains unclear. Are the antiferromagnetic fluctuations responsible for mediating superconductivity, as in many cuprates, or is the relationship more subtle?

The role of the substrate and interface also warrants deeper study. The research showed that a capping layer is critical for achieving robust superconductivity, but the exact mechanism remains uncertain. Is it purely a matter of maintaining compressive strain, or does the interface between the nickelate and the strontium titanate cap play a more active role in the superconducting properties?

A New Chapter Begins

The discovery of superconductivity in undoped praseodymium nickel oxide marks a turning point in nickelate research. It demonstrates that these materials are even more intriguing than previously recognized and that there is still much to learn about their fundamental properties.

For the broader condensed matter physics community, this work serves as a reminder that subtle details in materials synthesis can have profound consequences for the properties that emerge. The difference between a non-superconducting bad metal and a superconductor may come down to achieving the right level of structural perfection and eliminating unintentional sources of disorder.

As research groups around the world work to reproduce and extend these findings, we can expect rapid progress in understanding the phase diagram of nickelate superconductors. Each new piece of the puzzle brings us closer to a complete picture of how superconductivity emerges in these fascinating materials.

Whether nickelates will ultimately lead to practical applications remains to be seen. The transition temperatures are still far too low for most real world uses, and the synthesis requirements are demanding. But in the quest to understand quantum materials and potentially discover new superconductors, every advance in fundamental knowledge opens new possibilities.

The story of nickelate superconductors is far from over. If anything, this latest chapter suggests that the most interesting discoveries may still lie ahead, waiting to be uncovered by researchers willing to push the boundaries of materials synthesis and characterization.

In science, as in life, sometimes the most important discoveries come from questioning assumptions we thought were settled. The assumption that nickelates require chemical doping to superconduct appeared well established based on multiple studies over several years. Yet with careful optimization and comprehensive characterization, researchers have now shown that this assumption was premature.

It's a valuable reminder that in the exploration of nature's possibilities, we should always be prepared for surprises. The materials we think we understand may still have secrets left to reveal, if only we look closely enough.

Publication Details

Published: 2025
Journal: Advanced Materials
Publisher: Wiley-VCH GmbH
DOI: https://doi.org/10.1002/adma.202416187

Credit and Disclaimer

This article is based on original research published in Advanced Materials by an international collaboration involving researchers from institutions in France (Université de Strasbourg, Université Paris-Saclay), Italy (Politecnico di Milano, CNR-SPIN), and other European countries. The content has been adapted for a general audience while preserving scientific accuracy. For complete methodological details, comprehensive structural analyses, full spectroscopic datasets, and in-depth theoretical interpretations, readers are strongly encouraged to consult the original peer-reviewed research article through the DOI link provided above. All scientific findings, data interpretations, and conclusions presented here are derived directly from the original publication, and full credit belongs to the research team and their institutions.