Optical absorption in Al and dilute alloys of Mg and Li in Al at 4.2 K

The absorptances of Al and alloys of up to 5.5 at.% of Li and Mg in Al were measured at 4.2 K in the 0.2-3.0-eV range. The theory of Ashcroft and Sturm was fitted to the data. The fit obtained is reasonably good, provided three relaxation times are used, one for the Drude term and one for each type of transition between parallel bands. The fit is slightly better for the more concentrated alloys. In all cases the discrepancy between the fit and the data is greatest in the region between the two interband peaks in the optical conductivity, and known extensions to the theory do not qualitatively improve the fit. Pseudopotential Fourier coefficients $U_{200}$ and $U_{111}$ obtained from the fits vary with solute type and concentration. The variation can be explained semiquantitatively with a virtual-crystal pseudopotential.