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

Superconductivity is one of the most intriguing properties of matter characterized by zero electrical resistance and described on the basis of electrons bound into Cooper pairs by an attractive interaction. The most widely studied superconductors are the cuprates which have the highest critical temperature (T_c) at ambient conditions. In the search for compounds analogous to cuprates, nickel oxides were long proposed to show superconductivity [PRB 59, 7901 (1999); PRL 100, 016404 (2008)]. This research crystalized in 2019 with its discovery in infinite layer [Nature 572, 624 (2019)] and reduced Ruddlesden-Popper [Nature Mat. 21, 160 (2021)] nickelate thin films (T_c=9-15 K), albeit without clarifying the underpinning mechanism.

 

Here we use Density Functional Theory (DFT) simulations to show that superconductivity in nickelates is driven by an electron-phonon coupling originating from a charge ordered state. Due to an intrinsic electronic instability in half-doped compounds, Ni^1.5+ cations dismutate into more stable Ni¹⁺ and Ni²⁺ cations, which is accompanied by bond disproportionation producing a checkerboard of NiO₄ complexes. Once doping suppresses bond disproportionation, its vibration helps forming Cooper pairs with a T_c=7-14 K. Despite the presence of correlation effects inherent to 3d elements, nickelates superconductors appear similar to non-magnetic bismuth oxide superconductors. Our results further suggest that undoped materials are not necessarily a proper starting point for understanding superconductivity and the search for instabilities in the doped phase diagram thus appears crucial. Finally, all these phenomena can be identified if relevant degrees of freedom as well as an exchange correlation functional that sufficiently amends self-interaction errors inherent to practiced DFT are involved in the simulations.