DEUALITY NATURE OF 5f ELECTRON SYSTEMS IN URANIUM COMPOUNDS

f电子的二象性，即流动或定域，是强相关f电子系统的一个基本问题。本项目的目的是研究铀化合物中5f电子的对偶性质所引起的物理性质。我们发现了一种新的铀化合物U_3Pd_<20>Si_6，并成功地生长出了高质量的单晶。我们还生长了晶体结构与U_3Pd_<20>Si_6相同的稀土化合物R_3(R=La, Pr, Ce, Nd)Pd_<20>Ge_6的单晶，并测量了这些化合物的物理性质和电子结构进行比较。我们对该化合物的输运、磁性和热性能进行了系统的研究。通过电阻率、磁化率和比热测量，在19K和2K处发现了两个磁跃迁。磁矩可达3.3 μ_B，释放的熵达19K，为Rln3。这些结果表明该化合物的5f电子很可能是局域化的。初步的中子实验也表明晶体场激发是可以观察到的。只有少数铀化合物是金属的，同时它们的5f电子被证明是局域的。新化合物将有助于提高我们对铀化合物中5f电子对偶性质的理解。我们还研究了UPd_2Al_3、UNi_2Al_3和U_2Rh_3Si_5等铀化合物中f电子的流动或局域性所引起的物理性质。

The duality nature of the f electrons, i.e. itinerant or localized, is an essential issue of the strongly correlated f electron systems. The purpose of the present project is to investigate the physical properties arising from the duality nature of the 5f electrons in uranium compounds.We have discovered a new uranium compound U_3Pd_<20>Si_6 and have successfully grown its high-quality single crystals. We have also grown the single crystals of the rare earth compounds R_3(R=La, Pr, Ce, Nd)Pd_<20>Ge_6 whose crystal structure is the same as that of U_3Pd_<20>Si_6, and measured the physical properties and electronic structures of these compounds for comparison.We have performed a systematic investigation on the transport, magnetic and thermal properties of this compound. Two magnetic transitions at 19K and 2K is found from the resistivity, susceptibility and specific heat measurements. The magnetic moment is found to be as large as 3.3 μ_B and the entropy released up to 19K amounts to Rln3. These results indicate that the 5f electrons of this compound is very likely to be localized. A preliminary neutron experiment also suggest that the crystal field excitation is observable. There are only a few uranium compounds which are metallic and at the same time whose 5f electrons are proved to be localized. The new compound will contribute to improve our understanding of the duality nature of the 5f electrons in the uranium compounds.We have also studied the physical properties arising from the itinerant or localized nature of the f electrons in other uranium compound like UPd_2Al_3, UNi_2Al_3 and U_2Rh_3Si_5.