Optical spectroscopy of the metal-insulator transition inNdNiO3

We report optical measurements of ${\mathrm{NdNiO}}_3$ that undergo a phase transition from a paramagnetic metal to an antiferromagnetic insulator at 200 K (=${\mathit{T}}_{\mathit{c}}$). The opening of a charge gap is observed in the optical conductivity spectrum [σ(ω)] below ${\mathit{T}}_{\mathit{c}}$, and the missing spectral weight is distributed over the energy region above 0.3 eV. The evolution of the gap below ${\mathit{T}}_{\mathit{c}}$ is rather gradual with temperature, in contrast to sharp changes of lattice parameters and resistivity at ${\mathit{T}}_{\mathit{c}}$. Such a feature in the σ(ω) spectrum of ${\mathrm{NdNiO}}_3$ 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.