Localization Effects in Static Disorder Metals as a Precursor of Non-Fermi-Liquid Behavior in the Strange Metal Mode: Spectroscopic Ellipsometry Study of Disordered β-Ta Films and Multilayer (FeNi–Ta)_N Structures

To refine the nature of the state in the “strange metal” (SM) mode, which occurs in a wide variety of materials from high-temperature superconductors to heavy fermion metals, we elaborate spectroscopic ellipsometry in addition to the widely used shot noise measurement technique. β-Ta and other “bad” metals can be considered as SMs in an extreme regime, since they are characterized by a negative temperature coefficient of resistivity over a wide temperature range. Using optical studies of β-Ta films and (FeNi–Ta)_N magnetic superlattices, we show that the Drude contribution of free charge carriers at the Fermi level in the 5d t_2g Ta electron band decreases with increasing disorder in β-Ta films due to the localization of free electron charge carriers. Simultaneously, optical bands, which are characteristic of systems with strong electron correlations at which the optical spectral weight is compensated, appear in the energy range ~2–4 eV. In addition, the study of (FeNi–β-Ta)_N magnetic superlattices demonstrates that the ferromagnetic interaction between the neighboring FeNi magnetic layers via a Ruderman–Kittel–Kasuya–Yosida–type (RKKY) mechanism leads to electron delocalization from localized states.