Chinese scientists have made significant progress in high-temperature (T_C) nickelate superconductors. For the first time, they observed a nodeless superconducting gap and discovered electron-boson coupling by measuring the electronic structures of Ruddlesden-Popper bilayer nickelate superconducting thin films. These findings provide crucial evidence for two fundamental issues in the mechanism of high-T_C nickelates: "superconducting gap symmetry" and "superconducting pairing mechanism."

The study was published in Science on May 21. It was conducted by a research team led by Prof. HE Junfeng from the University of Science and Technology of China (USTC) of the Chinese Academy of Sciences, along with a research team led by Prof. XUE Qikun and Prof. CHEN Zhuoyu from the Southern University of Science and Technology (SUSTech).

Since its discovery in 1911, superconductivity, with extreme electromagnetic properties, has become an important research direction. In the past century, copper-based and iron-based high-T_C superconductors have been discovered. However, the high-T_C superconducting mechanism remains unresolved. The recent emergence of nickel-based high-T_C superconductors (nickelates) has provided a new opportunity for understanding this mechanism.

In this study, the SUSTech team led the efforts of thin film growth, and the USTC team led the measurements of the electronic structures of the thin films. To address the issue of oxygen loss during sample transfer, the scientists developed a new technology based on liquid-nitrogen-cooled ultra-high vacuum low-temperature sample quenching and transfer. This technology enabled the transfer of samples from Shenzhen to Hefei, and ensured the success of the experiments.

In high-T_C superconductors, exploring the "superconducting gap symmetry" in nickelates is extremely important. Specifically, whether the superconducting gap has "nodes" (points where the superconducting gap is zero) in momentum space is key for revealing the superconducting gap symmetry. The scientists carried out measurements on Ruddlesden-Popper bilayer nickelate superconducting thin films using angle-resolved photoemission spectroscopy. No gap node was observed in the entire momentum space, consistent with s-wave (s±) superconducting gap symmetry.

Understanding how "electron pairs" are formed is another key issue for the mechanism of high-T_C superconductivity. In principle, electrons may pair through "electron-boson coupling." The scientists found a dispersion kink at ~70 meV below the Fermi level, which is a "finger print" of electron-boson coupling. This observation provides crucial evidence for understanding the electron pairing mechanism.