Why is Superconductivity absent in bulk Infinite-layer Nickelates?

For decades, physicists have been chasing a specific kind of electrical magic: the ability for electricity to flow through a material with absolutely zero resistance. This phenomenon, known as superconductivity, usually requires cooling materials to temperatures so cold they are barely above absolute zero. However, a family of materials called cuprates, which contain copper, broke this rule by becoming superconductors at much higher, more manageable temperatures. This discovery sparked a decades-long hunt to find other families of materials that could do the same, hoping that understanding them would reveal the secret mechanism behind high-temperature superconductivity. One promising candidate emerged in the form of nickelates, materials containing nickel that look structurally similar to the copper-based cuprates. When scientists created thin films of these nickelates and doped them with a specific element, they successfully observed superconductivity. Yet, a puzzling contradiction arose: when other teams tried to create the exact same material in large, bulk chunks rather than thin films, the superconductivity vanished completely.

This mystery led a team of researchers to investigate why the bulk samples failed while the thin films succeeded. They proposed that the difference was not due to the substrate or the interface where the thin film sits, as some had speculated, but rather due to tiny imperfections inherent in the bulk synthesis process. Through a combination of magnetic modeling and computer simulations, the team discovered that the bulk samples contain microscopic islands of pure nickel metal that form alongside the main material. Because pure nickel is magnetic, these tiny islands act like miniature magnets scattered throughout the sample. The researchers calculated that the magnetic field generated by these clusters is strong enough to disrupt the delicate state required for superconductivity, effectively killing it before it can begin.

The investigation began by addressing a long-standing debate about the role of the surface on which thin films are grown. Some scientists had suggested that the superconductivity in thin films was an interface phenomenon, meaning it only happened where the material touched the substrate. However, the authors noted that experiments measuring the expulsion of magnetic fields confirmed the substrate does not directly participate in the superconducting mechanism. Instead, the focus shifted to the internal quality of the material itself. It had been previously reported that bulk samples of these nickelates suffer from a lack of nickel atoms in certain areas, and that the missing nickel atoms do not simply disappear. Instead, they clump together to form distinct clusters, some as large as a few micrometers, which remain trapped between the grains of the main material.

To understand the impact of these clusters, the researchers treated them as tiny ferromagnets, since nickel is a magnetic metal. They estimated the size of these clusters to be roughly one micrometer in radius and calculated the magnetic field they would produce. Using two different physical models to describe how these magnetic fields behave, they found that even at a distance of ten micrometers from a cluster, the magnetic field remains strong, reaching values in the range of milliteslas. This is a critical finding because the threshold at which superconductivity in these materials breaks down is extremely low, reported at just 0.79 milliteslas. The magnetic fields generated by the nickel clusters are orders of magnitude stronger than this limit, meaning that if these clusters are present, they create a magnetic environment that is hostile to superconductivity, preventing the material from ever entering that state.

In a parallel line of inquiry, the team used computer simulations to examine how the absence of nickel atoms affects the material's electronic structure. They modeled the material with varying degrees of nickel deficiency and found that the missing atoms reduce the importance of a specific electron orbital, known as the Ni 3dx2−y2 orbital, at the energy level where superconductivity occurs. This orbital is considered essential for the superconducting behavior in these materials. When its influence is diminished by the presence of nickel-deficient regions, the material loses a key ingredient needed for the phenomenon to occur. This suggests that even if the magnetic clusters were removed, the chemical imperfections in the bulk samples would still hinder the formation of a superconducting state.

The authors conclude that the absence of superconductivity in bulk infinite-layer nickelates is likely due to a combination of these two factors. The nickel-deficient regions alter the electronic landscape in a way that is detrimental to superconductivity, while the nickel atoms that have gathered into clusters generate magnetic fields strong enough to destroy the superconducting state entirely. These imperfections appear to be a specific consequence of the current chemical methods used to synthesize bulk samples, which differ from the techniques used to create the successful thin films. The researchers suggest that if future synthesis methods can eliminate these nickel clusters and correct the nickel deficiency, it may be possible to observe superconductivity in bulk nickelate samples, finally resolving the mystery of why the thick samples have remained silent while the thin films sing.