Synthesis of Chemically Sharp Interface in NdNiO₃/SrTiO₃ Heterostructures

The nickel-based superconductivity provides a fascinating new platform to explore high-T_c superconductivity. As the infinite-layer nickelates are obtained by removing the apical oxygens from the precursor perovskite phase, the crystalline quality of the perovskite phase is crucial in synthesizing high quality superconducting nickelates. Especially, cation-related defects, such as the Ruddlesden–Popper-type (RP-type) faults, are unlikely to disappear after the topotactic reduction process and should be avoided during the growth of the perovskite phase. Herein, using reactive molecular beam epitaxy, we report the atomic-scale engineering of the interface structure and demonstrate its impact in reducing crystalline defects in Nd-based nickelate/SrTiO₃ heterostructures. A simultaneous deposition of stoichiometric Nd and Ni directly on SrTiO₃ substrates results in prominent Nd vacancies and Ti diffusion at the interface and RP-type defects in nickelate films. In contrast, inserting an extra NdO monolayer before the simultaneous deposition of Nd and Ni forms a sharp interface and greatly eliminates RP-type defects in nickelate films. A possible explanation related to the polar discontinuity is also discussed. Our results provide an effective method to synthesize high-quality precursor perovskite phase for the investigation of the novel superconductivity in nickelates.