We report optical measurements of NdNiO3 that undergo a phase transition from a paramagnetic metal to an antiferromagnetic insulator at 200 K (=Tc). The opening of a charge gap is observed in the optical conductivity spectrum [σ(ω)] below Tc, and the missing spectral weight is distributed over the energy region above 0.3 eV. The evolution of the gap below Tc is rather gradual with temperature, in contrast to sharp changes of lattice parameters and resistivity at Tc. Such a feature in the σ(ω) spectrum of NdNiO3 is characteristic of a spin-density-wave (SDW) state, though the peak energy (∼0.4 eV) of the σ(ω) spectrum is considerably larger than that predicted by a simple SDW theory. The effect of electron correlation is discussed on the basis of the σ(ω) spectra.
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