image: The diagram shows the position of nickel in the group of 3d elements, its electron configuration, as well as the structures of several typical nickelates and the valence states of nickel atoms within them.
Credit: ©Science China Press
In recent years, research on nickel-based superconductors has remained a hotspot in condensed matter physics and materials science. As a new high-temperature superconductor system following cuprate and iron-based superconductors, nickel oxides have attracted extensive attention due to their unique electronic structure and multilayer crystal configurations. Recently, a joint team from the Institute of Physics, Chinese Academy of Sciences, and the University of Science and Technology of China systematically summarized the research progress in this field in recent years, providing an important reference for the scientific community.
As early as the theoretical prediction stage, scientists noted the similarity between the electronic configuration of LaNiO₂ and that of high-temperature cuprate superconductors: the 3d⁹ state of Ni¹⁺ resembles the electronic distribution of Cu²⁺ in layered structures, suggesting potential superconductivity. However, experimental verification remained elusive for a long time. It was not until 2019 that researchers achieved the first superconductivity in Nd₀.₈Sr₀.₂NiO₂ thin films, with a critical temperature of about 15 K, marking the beginning of nickel-based superconductor research. Subsequently, studies quickly expanded to bilayer, trilayer, and higher-layer nickel oxide systems.
A breakthrough study in 2023 revealed that bilayer La₃Ni₂O₇ under high pressure exhibits high-temperature superconductivity with a transition temperature up to 80 K, setting a new record for nickel-based superconductors. Soon after, in 2024, strain engineering via substrates enabled the realization of superconductivity in La₃Ni₂O₇ thin films at ambient pressure, further expanding the experimental platform for this system. At the same time, trilayer La₄Ni₃O₁₀ and other multilayer nickel oxides were also confirmed to exhibit superconducting behavior, enriching the material family of nickel-based superconductors.
These nickel oxide materials commonly exhibit coexistence of density waves, magnetic order, and superconductivity. Fine-tuning of their electronic structures and the issue of pairing symmetry have gradually become key research focuses. In addition, the field still faces several core challenges: first, the difficulty in preparing high-quality single crystals and thin films; second, the incomplete understanding of the impact of defects and lattice disorder on electronic properties; and third, ongoing academic debates regarding the superconducting mechanism and electron pairing. In the future, scientists will need to combine advanced spectroscopic techniques, innovative experimental methods, and theoretical modeling to further reveal the nature of superconductivity in nickel oxides and provide new perspectives for understanding high-temperature superconductivity.