Electron microscopy study of interface structure in infinite-layer nickelate-based superconducting thin films

<sec>The discovery of superconductivity in infinite-layer nickelate Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub>2</sub> has established another type of unconventional superconductors, whose structure and electron pairing mechanism are similar to those of cuprate superconductors. Unlike in cuprate superconductors, superconductivity in infinite-layer nickelates has only been observed in thin film samples, where heterointerface structures, elemental doping, and the infinite-layer configuration are critical for epitaxial systems. Therefore, the film-substrate interfacial effects require exploration for understanding superconductivity. However, comparative studies on the interfacial structures between superconducting and non-superconducting Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub><i>x</i></sub> nickelate thin films have not been reported in the literature so far.</sec><sec>This work focuses on Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub>3</sub>/SrTiO<sub>3</sub> and Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub>2</sub>/SrTiO<sub>3</sub>, and the phase distribution and interfacial structural changes in superconducting and non-superconducting nickelate thin films are characterized in detail by using scanning transmission electron microscopy (STEM). Further analysis of the corresponding atomic high-angle annular dark filed (HAADF), integrated differential phase contrast (iDPC) and energy dispersive X-ray spectroscopy (EDS) maps reveals the phenomena such as elements mixing, atomic steps, and changes in lattice parameters at the interfaces. These results also show that in the Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub>2</sub> film, the first 1−2 unit cells near the interface are not fully reduced to the superconducting infinite-layer structure. Such findings contribute to alleviating the strong polarity discontinuity at the sharp interface.</sec><sec>This study also emphasizes the atomic reconstruction and the modulation effect at the interface between the substrate and the film, thus enriching the understanding of the structural properties of the Nd<sub>0.8</sub>Sr<sub>0.2</sub>NiO<sub><i>x</i></sub> films, and providing crucial experimental evidence for understanding the interfacial structure of infinite-layer nickelates.</sec>