Anomalous Magnetism of Strongly Correlated 5f Electron Compounds

利用包括高压和超高场测量在内的各种实验技术，对Uru_2Si_2及其相关化合物的低温5f电子态进行了深入的研究。主要结果如下：1.通过中子散射、核磁共振和μ-SR测量发现，施加静水压力可以使Uru_2Si_2发生从未知的“隐序”到反铁磁(AF)有序的一级相变。AF体积分数的压力变化表明，该系统长期存在的弱反铁磁性问题归因于体积分数约1%的不均匀AF团簇。2.进一步发现，沿四方基面施加的单轴应力可以诱导出AF相。这有力地表明，c/a比是控制两相竞争的相关参数。3.从Rh掺杂实验中获得了c/a比假说的另一个证据。用Rh掺杂体系研究了磁激发与两相竞争的关系。4.首次得到了U(Ru_&lt；1-x&gt；Rh_x)_2Si_2(x&lt；0.05)的精确B-T-x相图，阐明了低场Ho-AF竞争态与高场未知有序态之间的关系。5.研制了一种在极低温度(T&gt；40mK)和高压(P&lt；1.5 Gpa)下工作的高精度磁强计。通过详细的磁化强度测量，阐明了不均匀的AF演化对超导电性的影响。本研究的部分成果被选为主要国际会议的特邀讲座，如“第23届国际低温物理会议(LT23)2002年8月20-23日(日本广岛)”和“2003年美国物理学会Match 2003会议，2003年3月3-7日(美国奥斯汀)”。

Low-temperature 5f electronic States of URu_2Si_2 and related compounds were intensively investigated by using various experimental techniques including high-pressure and ultra-high-field measurements. Main results obtained are listed as follows : 1. It was found through neutron-scattering, NMR and μSR measurements that applying hydrostatic pressure induces a first-order phase transition from unknown "hidden order (HO)" to antiferromagnetic (AF) order in URu_2Si_2. The pressure variations of the AF volume fraction revealed that weak antiferromagnetism the long-standing issue of this system, is ascribed to the inhomogeneous AF clusters of 〜 1% volume fraction. 2. It was further found that the uniaxial stress applied along the tetragonal basal plane can induce the AF phase. This strongly suggests that the c/a ratio is a relevant parameter to control the two-phase competition. 3. Another evidence for the c/a-ratio hypothesis was obtained from Rh doping experiments. The relationship between magnetic excitations and the two-phase competition was also studied by using the Rh dope systems. 4. A precise B-T-x phase diagram for U(Ru_<1-x>Rh_x)_2Si_2 (x < 0.05) was obtained for the first time The relationship between the low-field HO-AF competing state and the high-field unknown order states was clarified. 5. A high-precision magnetometer operational at very low temperature (T > 40mK) and under high pressure (P < 1.5GPa) was developed. From the careful magnetization measurements using this apparatus, the influence of the inhomogeneous AF evolution on the superconductivity was clarified. Some of the results of the present research protect were selected as invited talks for major international conferences such as "The 23rd International Conference on Low Temperature Physics (LT23) August 20-23, 2002 (Hiroshima, Japan)" and "The Match 2003 Meeting of the American Physical Society, March 3-7, 2003 (Austin, USA)"