The study of heavy fermion compounds at very low temperatures

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The aim of this research project is to study (1) the magnetic ground state of uranium compounds and (2) the magnetic phase transitions in the heavy electron systems with high Kondo temperature T_K. URu_2Si_2 and CeRu_2Si_2 are typical heavy fermion compounds.URu_2Si_2 exhibits a magnetic phase transition at 17.5 K,but the antiferromagnetic ordered moment is very smal| (=0.03 mu_B).Therefore, a contribution of quadrupolar coupling between f-electrons of uranium atoms was suggested. The driving force of the phase transition is not well understood and the problem is still controversial.We studied the magnetic properties of the diluted uranium in LaRu_2Si_2 and found the Kondo screening. It seems from our study that the magnetic ground state varies with the structure of the crystalline field splitting of the f-electrons.Another important study is on the magnetic properties of spin density wave (SDW) in Ce (Ru_<1-X>Rh_X) _2Si_2 (0.05 < X < 0.3) and non-Fermi liquid behavior for x=0.4. In 0 < x < 0.4, the Kondo temperature T_K is between 20 K and 30 K and T_K >> T_N (the maximum T_N for x = 0.15 is 5.5 K). Below T_N, a partial gap opening occurs in the Fermi surface but the heavy fermion properties remain in the rest of the Fermi surface.Futhermore, the effect of quantum spin fluctuations on the magnetic phase transiton was studied for the samples around x = 0.4.