Charge ordering as the driving mechanism for superconductivity in rare-earth nickel oxides

Abstract

Superconductivity is one of the most intriguing properties of matter described by an attractive interaction that bounds electrons into Cooper pairs. To date, the highest critical temperature at ambient conditions is achieved in copper oxides. While layered nickel oxides were long proposed to be analogous to cuprates, superconductivity was only demonstrated in 2019 albeit without clarifying the pairing mechanism. Here we use Density Functional Theory (DFT) to show that superconductivity in nickelates is driven by an electron-phonon coupling originating from a charge ordering. Due to an intrinsic electronic instability in half-doped compounds, Ni$^{1.5+}$ cations dismutate into more stable Ni$^+$ and Ni$^{2+}$ cations, which is accompanied by a bond disproportionation of NiO$_4$ complexes producing an insulating charge ordered state. Once doping suppresses the instability, the bond disproportionation vibration is sufficient to reproduce the key characteristic of nickelates observed experimentally, notably the dome of T$_c$ as a function of doping content. These phenomena are identified if relevant degrees of freedom as well as an exchange correlation functional that sufficiently amends self-interaction errors are involved in the simulations. Finally, despite the presence of correlation effects inherent to $3d$ elements, nickelates superconductors appear similar to non-magnetic bismuth oxide superconductors.