We have measured high-resolution photoemission spectra of NiS, which undergoes a nonmetal-metal transition at Tₜ~260 K. Below Tₜ, a small band gap opens; the band edge is essentially a step function broadened only by {~} 15 meV. This observation is difficult to reconcile with one-electron band theory, which predicts a much broader band edge, implying a dramatic influence of electron correlation on quasiparticle excitations and on the opening of a band gap in 3D antiferromagnetic insulators.
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Nakamura et al. (1994) studied this question.
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