  Nickel-based complex oxides have served as a playground for decades in the
quest for a copper-oxide analog of the high-temperature (high-Tc)
superconductivity. They may provide key points towards understanding the
mechanism of the high-Tc and an alternative route for a room-temperature
superconductor. The recent discovery of superconductivity in the infinite-layer
nickelate thin films has put this pursuit to an end. Having complete control in
material preparation and a full understanding of the properties and electronic
structures becomes the center of gravity of current research in nickelates.
Thus far, material synthesis remains challenging. The demonstration of perfect
diamagnetism is still missing, and understanding the role of the interface and
bulk to the superconducting properties is still lacking. Here, we synthesized
high-quality Nd0.8Sr0.2NiO2 thin films with different thicknesses and
investigated the interface and strain effects on the electrical, magnetic and
optical properties. The perfect diamagnetism is demonstrated, confirming the
occurrence of superconductivity in the thin films. Unlike the thick films in
which the normal state Hall coefficient (RH) changes signs from negative to
positive as the temperature decreases, the RH of the films thinner than 6.1-nm
remains negative at the whole temperature range below 300 K, suggesting a
thickness-driven band structure modification. The X-ray spectroscopy reveals
the Ni-O hybridization nature in doped finite-layer nickelates, and the
hybridization is enhanced as the thickness decreases. Consistent with band
structure calculations on nickelate/SrTiO3 interfaces, the interface and strain
effect induce the dominating electron-like band in the ultrathin film, thus
causing the sign-change of the RH.
