Solid state reduction of nickelate thin films
Wenzheng Wei, Kidae Shin, Hawoong Hong, Yeongjae Shin, Arashdeep Singh Thind, Yingjie Yang, Robert F. Klie, F. J. Walker, Charles Ahn
Abstract
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 ${\mathrm{NdNiO}}_{2}$, which is prepared by reducing ${3}^{+}$ Ni in ${\mathrm{NdNiO}}_{3}$ 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 $\mathrm{NdNi}{\mathrm{O}}_{3\ensuremath{-}x}$ 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.