Solid state reduction of nickelate thin films

The square-planar nickelates are a class of superconductors analogous to the cuprates and promise to provide insight into the pairing mechanism in high-temperature superconducting oxides. The parent phase of doped superconducting films is NdNiO2, which is prepared by reducing 3(+) Ni in NdNiO3 films. In this paper, we develop an ultrahigh vacuum reduction method using aluminum deposited on top of the 3(+) nickelates and monitor the reduction process in real time through in situ crystal truncation rod measurements and diffraction x-ray absorption near edge spectroscopy measurements across the Ni K edge. We establish a relation between Ni valence and the lattice constant of NdNiO3-x and show that the process can precisely control the oxygen content in the films. Finally, we extend the reduction process to quintuple square-planar nickelates.