The critical nature of the Ni spin state in doped NdNiO$_2$

TL;DR

This paper investigates the electronic and magnetic properties of doped NdNiO$_2$, revealing that Ni spin states and hybridization effects are crucial for understanding its superconductivity, which differs from cuprates due to weaker superexchange interactions.

Contribution

It demonstrates that NiO$_2$ layers are in a critical hybridization region favoring a singlet state, challenging previous assumptions about Ni$^{2+}$ states and superexchange mechanisms in doped NdNiO$_2$.

Findings

NiO$_2$ layers are in a critical hybridization region.

Superexchange in NdNiO$_2$ is much weaker than in cuprates.

A different pairing mechanism than magnon-mediated is likely.

Abstract

Superconductivity with $T_c \approx 15K$ was recently found in doped NdNiO$_2$. The Ni$^{1+}$O$_2$ layers are expected to be Mott insulators so hole doping should produce Ni$^{2+}$ with $S=1$, incompatible with robust superconductivity. We show that the NiO$_2$ layers fall inside a ``critical'' region where the large $pd$ hybridization favors a singlet $^1\!A_1$ hole-doped state like in CuO$_2$. However, we find that the superexchange is about one order smaller than in cuprates, thus a magnon ``glue'' is very unlikely and another mechanism needs to be found.