Redox engineering of Ruddlesden–Popper nickelate thin films: pathways to unconventional superconductivity and new materials discovery

Abstract

The energy consumption of the technologies that underpin our modern way of life – from transistors to batteries – is fundamentally determined by the materials through which they operate. Addressing urgent challenges such as climate change and the surging demand for electrical energy calls for a new generation of materials and low-power devices. Quantum materials offer a promising avenue due to their emergent functional properties such as high-temperature superconductivity, multiferroicity, and topological order. In this thesis, I demonstrate how molecular beam epitaxy (MBE), in combination with soft-chemical synthesis methods, can be harnessed to design and discover novel nickel-based quantum materials. Leveraging the atomic precision of MBE, I first synthesize Ruddlesden-Popper (RP) nickelates thin films — compounds that have recently attracted widespread attention following the discovery of high-temperature superconductivity in the bi-layer RP phase, La${3}$Ni${2}$O$_{7}$. I employ soft-chemical reduction to selectively remove apical oxygen atoms from the RP structure to form multi-layer square-planar nickelates, a new family of nickelate superconductors and close cousins of the high-Tc copper oxides. Additionally, I show that the reverse reaction — topochemical oxidation — inserts oxygen into the rock salt spacer layers of the RP structure, leading to the discovery of a previously unknown structural family: oxygen-intercalated layered perovskites. This thesis demonstrates the remarkable power of combining the atomic precision of MBE with soft chemistry to design and discover new functional quantum materials that are inaccessible by conventional synthetic methods.