Far-infrared studies of highly correlated systems

In this dissertation, thermally induced metal-insulator transitions of correlated electronic systems such as heavily doped silicons (Si:B & Si:Sb) and BaCo{dollar}sb{lcub}0.9{rcub}{dollar}Ni{dollar}sb{lcub}0.1{rcub}{dollar}S{dollar}sb{lcub}1.87{rcub}{dollar} have been investigated by studying the far-infrared dynamics of these systems.; A heavily boron-doped silicon and an antimony-doped silicon, with carrier concentrations just below the critical concentration of the metal-insulator transition, are studied. In this work, we observed thermally induced metal-insulator transition, driven by the interplay between disorder and Coulomb interaction. Upon lowering the temperature, the disordered electronic systems are gradually transfered from a marginal Fermi liquid phase to an Anderson metallic phase, and finally to an insulating phase. It turns out that the disorder involved scattering is predominant in the insulating phase of Si:B at low temperatures, leading to a Fermi glass behavior. In the insulating phase of Si:Sb, the long-range Coulomb interaction plays an important role making it behave as an electron glass.; In this thermally induced metal-insulator transition, phase separation instability plays an important role. Effects of disorder and Coulomb interaction were resolved through the analyses involving extensive fitting of far-infrared optical conductivities with theoretical models. The volume fractions and dc conductivities of each phase were also calculated as a function of temperature. The calculated dc conductivity of the Anderson metallic phase does approach to a finite value as {dollar}T to{dollar} 0 K, implying that the existence of minimum metallic conductivity is wiped out by the continuous transfer of carriers from the metallic phase into the insulating phase. Our results suggest that the phase separation is one of important factors determining the nature of the metal-insulator transition in heavily doped silicons.; Electronic states of the quenched BaCo{dollar}sb{lcub}0.9{rcub}{dollar}Ni{dollar}sb{lcub}0.1{rcub}{dollar}S{dollar}sb{lcub}1.87{rcub}{dollar} (q-BCNS) and the annealed BaCo{dollar}sb{lcub}0.9{rcub}{dollar}Ni{dollar}sb{lcub}0.1{rcub}{dollar}S{dollar}sb{lcub}1.87{rcub}{dollar} (a-BCNS) are also studied. The a-BCNS exhibits a discontinuous phase transition from an antiferromagnetic insulator to a metal upon lowering the temperature, while the q-BCNS remains an antiferromagnetic insulator for all temperatures. Optical conductivity of q-BCNS is characterized by excitation of the localized carriers, together with several phonon modes coupled to a broad electronic absorption. Optical response of a-BCNS in its insulating phase is similar to that of q-BCNS. The metallic phase of a-BCNS at temperatures below the transition is characterized by a very narrow Drude-like peak with unusually small oscillator strength. The carrier damping of this Drude-like peak remarkably smail and it increases with decreasing temperature. The temperature dependence of electronic absorption is strongly correlated with carrier dynamics. We suggest that the metallic phase of a-BCNS is dominated by depinned charge density waves formed within the (Co/Ni){dollar}sb2{dollar}S{dollar}sb2{dollar} planes.