  After decades of research, superconductivity was finally found in
nickel-based analogs of superconducting cuprates, with infinite-layer (IL)
structure. These results are so far restricted to thin films in the case of
IL-nickelates. Therefore, the nature of the interface with the substrate, and
how it couples with the thin film properties is still an open question. Here,
using scanning transmission electron microscopy (STEM)- electron energy loss
spectroscopy (EELS) and four-dimensional (4D)-STEM, a novel chemically sharp
p-type interface is observed in a series of superconducting IL-praseodymium
nickelate samples, and a comparative study is carried out with the previously
reported n-type interface obtained in other samples. Both interfaces have
strong differences, with the p-type interface being highly polar. In
combination with ab-initio calculations, we find that the influence of the
interface on the electronic structure is local, and does not extend beyond 2-3
unit cells into the thin film. This decouples the direct influence of the
interface in driving the superconductivity, and indicates that the IL-nickelate
thin films do not have a universal interface model. Insights into the spatial
hole-distribution in SC samples, provided by monochromated EELS and total
reflection-hard x-ray photoemission spectroscopy, suggest that this particular
distribution might be directly influencing superconductivity.
