The study of heavy fermion compounds at very low temperatures

本研究的目的是研究(1)铀化合物的磁性基态和(2)具有较高近藤温度的重电子体系中的磁性相变。URU_2Si_2和CeU_2Si_2是典型的重费米子化合物。在17.5K时，URU_2Si_2具有磁性相变，但反铁磁有序矩很小(=0.03mU_B)。因此，提出了铀原子f电子之间的四极耦合的贡献。相变的驱动力还不是很清楚，这一问题仍然存在争议。我们研究了LaRu2Si2中稀铀的磁性，发现了近藤屏蔽。另一个重要的研究是Ce(Ru_&lt；1-X&gt；Rh_X)_2Si2(0.05&lt；X&lt；0.3)中的自旋密度波(SDW)的磁性以及x=0.4时的非费米液体行为。在0&lt；x&lt；0.4中，近藤温度T_K介于20-30K之间，T_K&gt；&gt；T_N(x=0.15时的最大T_N为5.5K)。在T_N以下，费米面出现了部分能隙开口，但费米面的其余部分仍保持着重费米子性质。此外，对于x=0.4附近的样品，研究了量子自旋涨落对磁相变的影响。

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.