Critical nature of the Ni spin state in doped NdNiO₂

Superconductivity with T_c ~ 15K was recently found in doped NdNiO₂ , heralding a new family of potentially high-temperature superconductors. According to the Zaanen-Sawatzky-Allen scheme, the Ni¹⁺O₂ layers are expected to be Mott insulators, so hole doping should produce Ni²⁺ with a total spin S=1, which is rather incompatible with robust superconductivity. Using an impurity calculation with a realistic modeling of the O bands, we show that in fact, the NiO₂ layers fall inside a critical region where the large pd hybridization favors a singlet ¹A₁ hole-doped state like in the cuprate high-temperature superconducting layers CuO₂. We also find that the superexchange is about one order of magnitude smaller than in cuprates, thus a magnon “glue” is unlikely to be responsible for this superconductivity.