Structural Basis for Achieving Superconductivity in a Ultrathin Multilayer-Nickelate

The discovery of infinite-layer and multilayer superconducting nickelates heralds a new chapter of superconductivity after a long time extensive pursuit in nickelate compounds for achieving cuprate-like unconventional superconductors. The ‘infinite-layer’ nickelate, e.g. Nd_0.8Sr_0.2NiO₂, mimicks the same 3d⁹ electronic configuration as Cu²⁺ in the cuprates via chemical substitution and stablized low Ni⁺¹ valence state. In contrast, the Ruddlesden-Popper type multilayer nickelate may exhibit altered electronic behavior by varying the RP family dimensionality (n) rather than tuning the degree of electron filling through cation substitution. The 3D structural disorder caused by chemical substitution can naturally be avoided in such structures, making the multilayer nickelate, e.g. Nd₆Ni₅O₁₂, an ideal candidate for studying the intrinsic properties of the superconducting nickelates. However, it is still a formidable experimental challenge in the thin film synthesis and the subsequent chemical reduction to attain superconductivity in nickelate heterostructures. Moreover, while superconductivity was observed in a Nd_0.8Sr_0.2NiO₂ thin film, it was found that Tc monotonically decreased with film thickness, completely suppressing it for below 4-5 nm. This led many to question whether this stems from extrinsic effects or whether Tc intrinsically depends on thickness and vanishes in the film dimensional limit.

In this talk, we demonstrate the appearance of superconductivity in single-unit-cell multilayer nickelate, Nd₆Ni₅O₁₂ grown on NdGaO₃, by molecular beam epitaxy, exhibiting a thickness invariant superconducting transition as compared to thicker films. We utilize in situ synchrotron X-ray scattering performed during growth of the parent phase, e.g. Nd₆Ni₅O₁₆, to reveal that the necessary layer-by-layer deposition sequence does not follow the sequence of the formula unit but an alternate order due to the occurrence of dynamic layer rearrangement, a phenomenon initially discovered for Sr₂TiO₄ and La₃Ni₂O₇. We find that ultrathin Nd₆Ni₅O₁₆ must be grown by an alternate sequence accounting for interdiffusion during growth. We exploit this insight to grow ultrathin heterostructures and report the appearance of superconductivity in single unit cell Nd₆Ni₅O₁₂ after topotactic chemical reduction, exhibiting a transition temperature as high as that of thicker films. Our subsequent in situ X-ray studies of topotactic reduction find that formation of the square-planar phase occurs rapidly and is highly sensitive to reduction temperature, with non-ideal conditions leading to defective regions and the loss of superconductivity. Cross-sectional structural snapshots of the 1-UC Nd₆Ni₅O_x film during the reduction, performed with X-ray phase retrieval techniques, indicate that the reduction occurs in two stages, with the upper half of the Nd₆Ni₅O_x unit cell forming square-planar layers prior to the bottom half, suggesting that the reduction of thicker films may result in a complete reduction only in layers closer to the surface.

Our findings provide insight into growth of the Ruddlesden-Popper nickelates, highlighting the need for in situ X-ray studies of the metastable phases key to superconductivity. The in situ X-ray results and structural details discussed in this talk provide much needed information for quantitative calculations of the electronic structure as well as a method for the synthesis of other possible layered superconductors, e.g., those with other cations and / or higher-order Ruddlesden-Popper sequences.