Electronic structure and ultrafast dynamics of nickel-based high-temperature superconductors

Nickel-based superconductors have attracted widespread attention due to their electronic configuration similar to that of copper-based high-temperature superconductors. Recently, the discovery of superconductivity with a transition temperature as high as 80 K in the bilayer nickelate La<sub>3</sub>Ni<sub>2</sub>O<sub>7</sub> under pressure has not only reignited research interest in nickel-based superconductors but also opened new avenues for the study of unconventional superconductivity. Layered nickel-based superconductors are similar to copper- and iron-based superconductors in crystal structure, superconducting properties, and electronic structure, but they also show significant differences. A deeper investigation into the electronic structure of nickel-based superconductors is expected to reveal the mechanisms behind these similarities and differences, which will further offer critical insights into developing a unified theoretical model and deepen the understanding of unconventional superconductivity. Moreover, the study of nonequilibrium ultrafast dynamics offers new perspectives and regulations for unconventional superconductivity, which has become a vital tool. This paper focuses on the electronic structure and ultrafast dynamics of Ruddlesden-Popper phase layered nickel-based superconductors, systematically reviewing the successful applications of angle-resolved photoemission spectroscopy (ARPES) and ultrafast optical spectroscopy in nickel-based superconductivity research. Specifically, the new properties of different nickelates are compared, including strong electron correlation, Hund coupling, non-Fermi liquid behavior, energy gap formation, and ultrafast electron dynamics. These advances offer important experimental insights into elucidating the mechanisms of unconventional superconductivity and characterizing the properties of their normal states in these materials.