The authors have measured the optical reflectivity (0.5-4 eV) of metallic Si in the primitive hexagonal (PH) phase at pressures of 16 to 35 GPa. The dominant optical absorption near 2.5 eV is explained in terms of interband excitations between parallel bands, which are characteristic for polyvalent nearly-free-electron metals. The interband absorption has been calculated from an ab initio band structure of PH-Si and is found to be in good agreement with the experimental results. Optical reflection spectra for metallic Ge in the beta -tin-type phase are also reported.
 

Semiconductor Science and Technology, 4 250-1, 1989.