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

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.

5 pages, 4 figures (to appear in PRL)