A major breakthrough in understanding the mechanism of novel nickel-based high-temperature superconductivity has been achieved. A collaborative research effort between Professor He Junfeng's group at the University of Science and Technology of China (USTC) and the research groups led by Academician Xue Qikun and Associate Professor Chen Zhuoyu at the Southern University of Science and Technology (SUSTech) has provided crucial experimental evidence for two core issues: the symmetry of the superconducting gap and the pairing mechanism. The findings were published in the journal *Science* on May 21.
Exploring high-temperature superconducting materials and understanding their mechanisms is a central scientific challenge in global superconductivity research. While copper-based and iron-based high-temperature superconductors were discovered in the century following the initial discovery of superconductivity, their underlying mechanisms remain unresolved. The recent emergence of nickel-based high-temperature superconductors presents a new opportunity to unravel this mystery, prompting a global scientific race to obtain key experimental evidence.
In prior work, the SUSTech team achieved ambient-pressure high-temperature superconductivity in nickel oxide films, creating a vital material platform for probing electronic structures. In this latest study, the SUSTech team focused on optimizing film growth to produce high-quality samples. To overcome the technical hurdle of oxygen loss in films, which destroys superconductivity, the USTC team led the joint development of a novel liquid-nitrogen-based ultra-high-vacuum cryogenic quenching and sample transfer technique. This enabled the successful "ultra-high-vacuum cold-chain" transport of samples from Shenzhen to Hefei.
The researchers then utilized a high-resolution laser-based angle-resolved photoemission spectroscopy system developed by the USTC team to perform critical electronic structure measurements on the superconducting thin films, supplemented by measurements at the Shanghai Synchrotron Radiation Facility.
The symmetry of the superconducting gap is a milestone for understanding high-temperature superconductivity. A key indicator is the presence or absence of "nodes"—points in momentum space where the superconducting gap goes to zero. The team conducted electronic structure measurements on Ruddlesden-Popper phase bilayer nickel oxide superconducting films. They observed a superconducting quasiparticle coherence peak and further revealed the magnitude and momentum dependence of the superconducting gap, finding no gap nodes across the entire momentum space (Brillouin zone). This result differs from a d-wave nodal gap and is more consistent with an s-wave (s±) superconducting gap symmetry.
In high-temperature superconductivity, "electron pairing" is the crucial step for forming the superconducting state. Understanding how electrons pair is a central question. Theory suggests that two electrons, which would normally repel each other, may pair via an intermediary boson through "electron-boson coupling." The research team discovered a kink in the electronic band dispersion at approximately ~70 meV below the Fermi level, a characteristic spectroscopic signature of electron-boson coupling. Quantitative analysis confirmed its existence. A similar electron-boson coupling is also observed in copper-based high-temperature superconductors. Therefore, its discovery in nickel-based superconductors demonstrates important universality and provides key experimental evidence for understanding the electron pairing mechanism in high-temperature superconductivity.