a Study of Ytterbium Heavy Fermion Systems.

Specific heat, magnetic susceptibility and resistivity data of YbSi indicate a Kondo lattice and heavy fermion antiferromagnet with T_{rm N } = 1.6^circK. It has one of the largest gamma-values found to date for any Yb compound. Above 25^ circK an anomalously large resistivity which is about 8-10 times larger than typical (Ce or U) heavy fermion materials was observed. The explanation for this high resistivity is still not clear. Upon dilution of Yb by Y (in the pseudobinary compounds of Yb_{rm 1-x} Y_{rm x} Si), there is a gradual decrease of the electronic specific heat coefficient gamma as the concentration of Y increases. This indicates that the heavy fermion antiferromagnetic behavior of YbSi (x = 0) is suppressed by the dilution of Yb by Y from x = 0.2 toward a so called "light heavy fermion" system and then becomes a normal metallic system at YSi (x = 1). Upon the substitution of Ge for Si (in the pseudobinary compounds of YbSi_{rm 1-x} Ge_{rm x}), magnetic susceptibility, specific heat and resistivity data indicate similar behavior with slightly smaller high temperature gamma-values than that of YbSi compound. There is also an indication of increasing Neel temperature of about 45% as the Ge concentration is increased up to 30%. At x > 0.35, the substitution of Si by Ge causes a change in the structural stability from orthorhombic system as observed from the room temperature X-ray diffraction data.