Charles Ahn, Yale University (Principal Investigator)

Frederick Walker, Yale University (Co-Investigator)

The infinite-layer nickelates represent a novel class of unconventional superconductors that are spurring intense research interest due to their electronic and structural similarities to high temperature cuprate superconductors. Recently, an in-situ synthesis thin film processing technique was developed to induce superconductivity in a new nickelate composition using the element europium (Eu) as a dopant, resulting in thin films with atomically smooth surfaces, high transition temperature, and large upper magnetic critical field. In this project, we will leverage this materials advance, applying a combination of thin film synthesis and advanced characterization techniques to measure key experimental variables that serve as inputs for theories of superconductivity in transition metal oxides.  Molecular beam epitaxy (MBE) will be applied for the design of new nickelate superconductors, including layered square planar structures, such as La₃Ni₂O₇, recently discovered to be a high temperature superconductor under high pressure. Innovative reduction methodologies will be developed for infinite-layer, square planar NdNiO₂ that preserve pristine film surfaces that are amenable to surface sensitive spectroscopy techniques, such as in-situ angle resolved photoemission spectroscopy (ARPES). The electronic structure of films synthesized with these methods will be measured using optical and advanced synchrotron-based spectroscopy techniques, such as resonant inelastic x-ray scattering (RIXS), diffraction x-ray absorption near edge structure (dXANES), and x-ray absorption spectroscopy (XAS). This approach will develop an understanding of the effect of Eu doping on electronic and magnetic structure and open pathways to manipulate both electronic and magnetic properties of nickelate superconductors. The discovery of Eu as a dopant also opens new avenues of superconductor development through the ability to control local magnetic moments in the crystal structure and tune the interplay between valence, charge, and magnetic doping. These microscopic parameters are expected to influence unusual behavior found in these materials at high magnetic fields. Measurements at such high fields may also help reveal the nature of the superconducting order parameter in nickelates and explore the potential role of magnetism in nickelate superconductivity. This approach is targeted towards addressing key questions pertaining to the link between electronic structure and a microscopic mechanism of superconductivity. The understanding and discoveries made in this project will motivate the development of new nickelate superconductors with high upper magnetic critical fields and transition temperatures.