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Revealing the origin of the �Hidden order� in uranium compound

Jan. 28, 2011


Joint Research group of Kyoto university, JAEA and Osaka university has revealed a longstanding problem in uranium compounds by detecting the rotational symmetry breaking in the �hidden order� state in URu2Si2.


A uranium compound URu2Si2 is known to exhibit two phase transitions at low temperature. One of them is a superconducting transition at 1.4 K. Another one takes place at 17.5 K. However, the origin of the latter phase transition has not been revealed. In general, a phase transition takes place when a matter changes its state by reducing the degree of freedom. A typical example is the phase transition between solid, liquid and gas, where the free motion of atoms in gaseous phase is not allowed in solid state. In uranium compounds, 5f electrons can be the origin of the phase transition because they have spin-, charge- and orbital-degree of freedom. However, any experimental investigations have failed to detect the change in the degree of freedom in URu2Si2 for more than 25 years after its discovery.


The research group conducted ultra-high-sensitivity magnetic torque measurements to detect a possible change in magnetic anisotropy across the phase transition using a high-quality single crystal of URu2Si2. In the high temperature disordered state above 17.5 K, magnetic anisotropy is dominated by its tetragonal crystal structure with four-fold symmetry. When URu2Si2 enters in the �hidden order� state below 17.5 K, two-fold magnetic anisotropy emerges, violating the crystal symmetry. This discovery demonstrates that the �hidden-order� state breaks the rotational symmetry of the crystal structure. The present results suggest that the 5f electron distribution changes its symmetry to a highly anisotropic one in the �hidden order� state.


This work is published in the American science journal �Science� (online, January 28, 2011): �Rotational Symmetry Breaking in the Hiddend-Order Phase of URu2Si2�, R. Okazaki, T. Shibauchi, H. J. Shi, Y. Haga, T. D. Matsuda, E. Yamamoto, Y. Onuki, H. Ikeda, Y. Matsuda, Science 331 (2010) 439.


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