Chinese Scientists Identify Nodeless Gap in Nickelate Superconductors

Chinese scientists have made a significant breakthrough in the study of high-temperature superconductivity by identifying a nodeless superconducting gap and detecting electron-boson coupling in nickelate superconductors.

The research focused on Ruddlesden-Popper bilayer nickelate superconducting thin films, marking the first time these electronic structures have been examined in such detail. These findings address two pivotal questions in the field: the symmetry of the superconducting gap and the mechanism behind superconducting pairing.

Investigation of Superconducting Gap Symmetry

Superconductivity, discovered in 1911, is characterized by unique electromagnetic properties and remains a central topic in condensed matter physics. Despite the discovery of copper-based and iron-based high-temperature superconductors, the underlying mechanism of high-temperature superconductivity is still not fully understood. Nickelate superconductors provide a new avenue for exploration.

One critical aspect is whether the superconducting gap exhibits nodes—points where the gap is zero—in momentum space. Using angle-resolved photoemission spectroscopy (ARPES), the research team examined bilayer nickelate thin films and found no nodes in the superconducting gap across momentum space. This observation aligns with s-wave (s±) superconducting gap symmetry.

Evidence Supporting Electron-Boson Coupling

The formation of electron pairs in high-temperature superconductors is another central question. Theoretically, electron pairing can occur via electron-boson coupling. The researchers detected a dispersion kink approximately 70 meV below the Fermi level, which is recognized as a signature of electron-boson coupling. This discovery provides valuable insight into the possible pairing mechanism in nickelate superconductors.

The collaborative effort involved the Southern University of Science and Technology (SUSTech) team, led by Qikun Xue and Zhuoyu Chen, responsible for thin film growth, while the University of Science and Technology of China (USTC) team, led by Junfeng He, conducted electronic structure measurements. To avoid oxygen loss during sample transfer, the teams developed a technique based on liquid-nitrogen-cooled ultra-high vacuum low-temperature sample quenching and transfer, enabling successful movement of samples from Shenzhen to Hefei and facilitating the experiments.