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This paper reviews a burst of recent work on a family of nickel oxide materials called Ruddlesden–Popper nickelates. The headline result is that superconductivity — the loss of electrical resistance below a critical temperature (Tc) — was first found in layered La3Ni2O7 crystals under high pressure at around 80 kelvin. More recently, superconductivity was also seen in ultra-thin films of the same bilayer compound at ordinary pressure when the films are grown on substrates that squeeze them in-plane (compressive strain). The review focuses on experiments and theory for these thin films and on how they relate to other nickelate superconductors.
The authors summarize several important experimental routes. In one older thread, infinite-layer nickelates such as NdNiO2 are made by growing a perovskite oxide (NdNiO3) film and chemically removing some oxygen. Doping with calcium or strontium (replacing some Nd3+ with Sr2+) adds holes and can produce superconductivity; for example Nd0.8Sr0.2NiO2 shows Tc near 15 K. In a separate but related development, bulk La3Ni2O7 shows superconductivity above 77 K under hydrostatic pressure (pressure applied equally along all lattice directions). The superconducting pressure range reported extends roughly from 14 to 90 gigapascals, with the strongest response near 20 GPa, and later studies pushed Tc even higher after partial rare-earth substitution.
What makes the thin-film result notable is that compressive strain from a substrate changes only the in-plane lattice constants and can stabilize superconductivity at ambient pressure. That allows experiments that are hard or impossible under extreme pressure, such as angle-resolved photoemission spectroscopy (ARPES), to probe the superconducting state. The review also contrasts the effects of hydrostatic pressure and compressive strain. Hydrostatic pressure shrinks all three lattice directions. Compressive strain shrinks the in-plane lattice but can expand the out-of-plane spacing, and these different lattice changes can alter the electronic structure and magnetic tendencies of the nickel layers.
The paper highlights links between structure and electronic behavior. In La3Ni2O7 the nickel average valence is +2.5, meaning the material is intrinsically hole-doped. At low pressure the crystal is orthorhombic with tilted Ni–O–Ni bonds (bond angle ~168°). At high pressure it becomes nearly tetragonal with untilted octahedra and Ni–O–Ni bond angles close to 180°. The authors note that the low- and high-pressure regimes are separated by a first-order structural transition. It is not yet clear whether the superconducting state emerges smoothly from the low-pressure magnetic or charge-ordered states, as happens in some other superconductors.
The review is careful about uncertainties. Many experiments still require extreme pressures, and the ambient-pressure thin-film superconductivity has so far been reported only in very thin films grown under specific strain conditions. The microscopic mechanism that pairs electrons in these nickelates is not settled. The authors call for more experiments and for further theoretical work that compares the bilayer and trilayer nickelates, thin films, and the better-known infinite-layer compounds. In short, the Ruddlesden–Popper nickelates are a growing platform for unconventional superconductivity, but important questions about how and why they superconduct remain open.