| Research Abstract |
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)"
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