Photoemission study of the metal-nonmetal transition in NiS

We have observed small changes in the ultraviolet photoemission spectra of NiS across the temperature-induced phase transition between the Pauli-paramagnetic metal and the antiferromagnetic nonmetal. In going from the nonmetallic to the metallic phases, the main band (d8L) within 3 eV below the Fermi level EF is shifted toward EF by 0.05 eV. This observation is consistent with the closing of a S 3p Ni 3d charge-transfer gap and cannot be reconciled with that of a band gap of antiferromagnetic origin. The spectrum loses a small fraction of intensity in the vicinity of EF as the gap is collapsed. The virtually identical line shape of the main band between the two phases is consistent with the persistence of short-range antiferromagnetic correlation in the paramagnetic phase.