Probing hybridization: Understanding the many faces of magnetism through optical spectroscopy

A long-standing problem in condensed matter physics is to understand the multifaceted role of local moments in various materials. This work has focused on understanding how the hybridization between local moments and extended states affects the dynamics of the free carriers as well as the emergence and details of the magnetic state in various materials. In particular Infrared Sprectroscopy (IR) and Ellipsometry were employed to explore the band structure of thin films (as grown & annealed) and digitally doped superlattices of Ga1-xM xAs. These results clearly established the important role played by the Mn induced impurity band in the band structure and ferromagnetism of Ga1-xMxAs. These studies clearly demonstrate that the impurity band remains, despite the high carrier density, calling into question leading theories of ferromagnetism in Ga1-xMxAs. A combined optical, transport and thermodynamic study of Yb14MnSb 11 is discussed that provided three "firsts", a d-electron system with a Kondo-resonance and ferromagnetism, Jahn-Teller and Kondo effects, and Kondo lattice in the under-screened limit. Lastly, optical studies are being used to reveal hybridization as key to the underlying physics of the heavy fermion superconductor CeTmIn5.