  The magnetically driven metal-insulator transition (MIT) was predicted by
Slater in the fifties. Here a long-range antiferromagnetic (AF) order can open
up a gap at the Brillouin electronic band boundary regardless of the Coulomb
repulsion magnitude. However, while many low-dimensional organic conductors
display evidence for an AF driven MIT, in three-dimensional (3D) systems the
Slater MIT still remains elusive. We employ terahertz and infrared spectroscopy
to investigate the MIT in the NaOsO3 3D antiferromagnet. From the optical
conductivity analysis we find evidence for a continuous opening of the energy
gap, whose temperature dependence can be well described in terms of a second
order phase transition. The comparison between the experimental Drude spectral
weight and the one calculated through Local Density Approximation (LDA) shows
that electronic correlations play a limited role in the MIT. All the
experimental evidence demonstrates that NaOsO3 is the first known 3D Slater
insulator.
