Critical Nature of the Ni Spin State in Doped NdNiO2

Superconductivity with T-c approximate to 15 K was recently found in doped NdNiO2. The Ni1+O2 layers are expected to be Mott insulators, so hole doping should produce Ni2+ with S = 1, incompatible with robust superconductivity. We show that the NiO2 layers fall inside a critical region where the large pd hybridization favors a singlet (1)A(1) hole-doped state like in CuO2. 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.