INTERPLAY OF INTERSITE AND INTRASITE INTERACTIONS IN HEAVY FERMION AND KONDO LATTICE SYSTEMS

The purpose of this work is an investigation of the competition between RKKY intersite interaction that may lead to a magnetically ordered ground state, and the Kondo effect resulting in a suppression of local moments. Both are mediated through an indirect exchange interaction with conduction electrons. Magnetic and non-magnetic ground states in heavy fermion and Kondo lattice systems are close energetically and study of the materials that are near a magnetic instability proved to be very productive. In the course of this work four systems are examined: CePtSi, CeRuSi, CePtGe and CeRuGe. The two former compounds are heavy fermion systems with electronic specific heat coefficient of {dollar}sim{dollar}850 mJ/mol*K{dollar}sb2{dollar} and {dollar}sim{dollar}210 mJ/mol*K{dollar}sb2{dollar} respectively. These systems do not order magnetically, though CePtSi experiences an onset of short range magnetic order at T{dollar}sb{lcub}rm m{rcub}sim{dollar} 2.8 K, possibly with a partial ferromagnetic component, as inferred from magnetic measurements. This fact is unusual, as ferromagnetic interaction is not, generally, considered to be coexistent with the Kondo effect. In CeRuSi no magnetic correlations can be seen in the range of temperatures studied (.6 K-300 K), instead Fermi liquid behavior was observed in electrical resistivity (R(T) {dollar}sim{dollar} T{dollar}sb2{dollar}) at low temperatures. In the same time magnetic susceptibility (magnetization) and specific heat measurements hint for a possibility of developing such correlations at lower temperatures.; In CePtGe and CeRuGe, as Silicon is replaced by Germanium, pronounced magnetic effects are observed. Both systems order antiferromagnetically, CePtGe at T{dollar}sb{lcub}rm N{rcub}sim{dollar} 3.5 K and CeRuGe at T{dollar}sb{lcub}rm N{rcub}sim{dollar} 6.5 K. A possibility that a short range magnetic interactions may lead to a precipitous decline of the electrical resistivity in CePtGe and some other magnetically ordering compounds (CeAl{dollar}sb2{dollar}, CePt{dollar}sb2{dollar}) is discussed. The response of CePtGe and CeRuGe to applied external pressure is analyzed. Pressure has little effect on the magnetic ordering temperature of CePtGe, a fact that places this compound into "weak exchange interaction" region, i.e. the Kondo temperature is small compared to the Neel temperature. In CeRuGe large pressure effects are observed, with a large reduction of T{dollar}sb{lcub}rm N{rcub}{dollar} as pressure is increased, the Kondo temperature is comparable to the Neel temperature.