  Nickel-based complex oxides have served as a playground for decades in the
quest for a copper-oxide analog of the high-temperature superconductivity. They
may provide clues towards understanding the mechanism and an alternative route
for high-temperature superconductors. The recent discovery of superconductivity
in the infinite-layer nickelate thin films has fulfilled this pursuit. However,
material synthesis remains challenging, direct demonstration of perfect
diamagnetism is still missing, and understanding of the role of the interface
and bulk to the superconducting properties is still lacking. Here, we show
high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and
demonstrate the interface and strain effects on the electrical, magnetic and
optical properties. Perfect diamagnetism is achieved, confirming the occurrence
of superconductivity in the films. Unlike the thick films in which the
normal-state Hall-coefficient changes signs as the temperature decreases, the
Hall-coefficient of films thinner than 5.5 nm remains negative, suggesting a
thickness-driven band structure modification. Moreover, X-ray absorption
spectroscopy reveals the Ni-O hybridization nature in doped infinite-layer
nickelates, and the hybridization is enhanced as the thickness decreases.
Consistent with band structure calculations on the nickelate/SrTiO3
heterostructure, the interface and strain effect induce a dominating
electron-like band in the ultrathin film, thus causing the sign-change of the
Hall-coefficient.
