Superconductivity and electronic structures of nickelate thin film superstructures.

Abstract

Ruddlesden-Popper nickelates have emerged as a crucial platform for exploring the mechanisms of high-temperature superconductivity^1-7. However, the Fermi surface topology required for superconductivity remains unknown. Here, beyond the superconducting pure bilayer (2222) phase, we report the thin film growth and ambient-pressure superconductivity of monolayer-bilayer (1212) and bilayer-trilayer (2323) superstructures, together with the absence of superconductivity in monolayer-trilayer (1313) superstructure, under identical compressive epitaxial strain. The onset superconducting transition temperatures range from 46 K to 50 K, exceeding the McMillan limit. Angle-resolved photoemission spectroscopy shows key Fermi surface differences in these atomically engineered structures. In superconducting 1212 and 2222 films, a dispersive hole-like band (γ^ΙΙ) forms an underlying Fermi pocket, surrounding the Brillouin zone corner. By contrast, the top of the flat band (γ^ΙΙΙ) is observed at about 70 meV below E_F in the non-superconducting 1313 films. Particularly, the superconducting 2323 films host both γ^ΙΙ and γ^ΙΙΙ bands. The polarization dependence of the γ bands reveals their Ni <math xmlns="http://www.w3.org/1998/Math/MathML"><msub><mi>d</mi> <msup><mi>z</mi> <mn>2</mn></msup> </msub> </math> origin. Our findings expand the family of ambient-pressure nickelate superconductors and establish a connection between structural configuration, electronic structure and the emergence of superconductivity in nickelates.