Optical absorption of Ni-based alloys

A study of the optical absorption of Ni-based dilute alloys ($\mathrm{Ni}\mathrm{Co}$, $\mathrm{Ni}\mathrm{Fe}$, $\mathrm{Ni}\mathrm{V}$, $\mathrm{Ni}\mathrm{Ti}$) by a sensitive differential technique is presented. The experiment is analyzed for the difference in the optical conductivity between the alloy and pure Ni. The structures observed in the differential optical conductivity are classified into two groups; one is sensitive to impurity species and the other is not. The former is discussed in terms of the Friedel virtual-bound-state model of transition-metal-based dilute alloys. The latter is found to be described in terms of the electronic band structure of ferromagnetic nickel as arising from disorder-induced wave-vector-nonconserving optical-absorption processes. Based on these results we also present a reinterpretation of our previous $\mathrm{Ni}\mathrm{Cu}$ data. The data appear to be consistent with the coherent-potential-approximation calculations of the density of states for Ni-rich alloys. Peaks at $\hslash \omega =1\mathrm{}$ eV ($\mathrm{Ni}\mathrm{Fe}$) and $\hslash \omega =3\mathrm{}$ eV ($\mathrm{Ni}\mathrm{Cu}$) are interpreted in terms of impurity density-of-states peaks 1 eV above, and 3 eV below the Fermi level, respectively.