Doping effects on the Kondo lattice materials: Iron silicide, cerium-cobalt-indium, ytterbium-indium-copper

Three doping studies on Kondo lattices are investigated in this thesis: FeSi1-xGex, Ce1-xLaxCoIn 5, and Yb1-xY xInCu4.;For FeSi1-xGe x, we constructed the phase diagram through the analysis of magnetic, thermal and transport measurements on single crystals. The phase diagram shows a first-order transition from a Kondo insulator (exponentially activated properties) to a ferromagnetic metal at a critical concentration, xc ∼ 0.25. The field dependence of the magnetization (M(H)) shows that the saturation moment of x = 0.27 is 10 times larger than that of x = 0.24. The spin gap of x = 0.24, 167K, is quite close to the transition temperature of x = 0.27, 150K, indicating that the characteristic energies of the two competing phases, i.e. the Curie temperature and the spin gap of the Kondo insulator, are essentially equal at the critical concentration.;For x < xc, spin gap, transport gap and resistivity minimum systematically decrease with increasing x. Saturation moments and specific heat coefficients are almost zero for x < xc. The temperature dependence of magnetic susceptibility (X(T)) for x = 0.2 shows a broad maximum around 200K, indicating that the broad maximum temperature decreases with x for x < xc. The variable range hopping analysis suggests the existence of the localized state for this region.;For x > xc, the data break into two distinct regimes: xc < x < ∼0.5 and ∼0.5 < x ≤ 1. For xc < x < ∼0.5, chi( T) does not displays a sharp transition at T c and M(H) increases with increasing fields. The temperature dependence of the resistivity (rho( T)) shows metallic behavior. However, it does not have any kink at Tc. In contrast, for ∼0.5 < x ≤ 1, chi(T) displays a sharp transition at T c and M(H) saturates at H ∼0.3T. rho(T) has a kink at Tc. Based on the Kondo insulator picture, we can explain the specific heat coefficient gamma evolution with x.;The transition from a Kondo insulator to a ferromagnetic metal can be explained as the consequence of the changes in hybridization between Fe 3d electrons and Si/Ge p conduction electrons in conjunction with disorder on the Si/Ge ligand site. (Abstract shortened by UMI.)...