Spin-orbit driven terahertz optical response in ferromagnetic Fe-Co-Al alloys

We study the optical conductivity response of simple ferromagnetic alloys driven by spin-orbit coupling. The spin-orbit coupling leads to a band splitting near the Fermi surface in the range of 10-100 THz. This band-splitting energy range provides an opportunity to study the optical conductivity in the THz regime. The optical conductivity response is sensitive to the details of the structure and chemical composition. For simple alloys, like Fe-Co alloys, we find that there is not much dependence of optical conductivity response in the THz range. However, we find the optical conductivity response changes sign from positive in Fe to negative in Fe-Al alloys. Furthermore, those features are washed away at 10 THz but remain only at 100 THz because of the short electron lifetime.