Optical investigations of polycrystalline Mg1−xB2 near metal–insulator transition

We measured reflectivity spectra of polycrystalline Mg1−xB2 samples, which show a metal–insulator transition with x. After performing the Kramers–Kronig analysis, the obtained optical conductivity spectra σ(ω) of MgB2 show a narrow Drude peak in the far-infrared region and a broad peak in the mid-infrared region. As x increases, the spectral weight of the Drude peak is strongly suppressed and that of the broad peak becomes enhanced a little. The existence of the broad mid-infrared peak in the insulating sample suggests that this peak might not be related to the free carriers in MgB2. In the far-infrared region, we also observe that the low energy dielectric constant of Mg1−xB2 diverges near the metal–insulator phase boundary (i.e. x=0.08). This result implies the possibility of a phase separation and a percolative metal–insulator transition in Mg1−xB2.