Chemical Strategies to Induce Superconductivity in Rare-Earth Nickelate Thin Films

Since the discovery of high-temperature superconductivity in copper oxides, there have been continued efforts to better understand the origins of the superconducting phase and to identify related materials families. Rare-earth nickelates, particularly Ruddlesden-Popper phases of the type Ln<i>_n</i>₊₁Ni<i>_n</i>O_2<i>n</i>+2, are a promising class of materials owing to the many shared properties of isoelectronic Ni¹⁺ and Cu²⁺ cations. Recently, superconductivity was observed in thin films of the infinite layer (<i>n</i> = ∞) nickelate Nd_0.8Sr_0.2NiO₂ as well as in thin films of the five-layer (<i>n</i> = 5) phase Nd₆Ni₅O₁₂. Theoretical and experimental studies suggest that increased dimensional confinement of the nickelate layers will lead to more ‘cuprate-like’ superconductivity. The <i>n </i>= 1 phase Ln₂NiO₄ is therefore expected to display the highest degree of dimensional confinement with single layers of nickel oxide separated by blocking rare-earth oxide layers. Here, we describe efforts to synthesize thin films of the parent compound Ln₂NiO₄ through molecular beam epitaxy. We then explore different chemical strategies to tune the nickel valence of as-grown films to induce superconductivity.