Broken symmetries in URu₂Si₂

Abstract

To resolve the nature of the hidden order below 17.5 K in the heavy-fermion compound URuSi, identifying which symmetries are broken below the hidden-order transition is one of the most important steps. Several recent experiments on the electronic structure have shown that the Fermi surface in the hidden-order phase is quite close to the result of band-structure calculations within the framework of itinerant electron picture assuming the antiferromagnetism. This provides strong evidence for the band folding along the axis with the ordering vector of , corresponding to broken translational symmetry. In addition to this, there is growing evidence for fourfold rotational symmetry breaking in the hidden-order phase from measurements of the in-plane magnetic anisotropy and the effective mass anisotropy in the electronic structure, as well as the orthorhombic lattice distortion. This broken fourfold symmetry gives a stringent constraint that the symmetry of the hidden-order parameter should belong to the degenerate -type irreducible representation. We also discuss a possibility that time reversal symmetry is also broken, which further narrows down the order parameter that characterizes the hidden order.

Keywords:

• crystal symmetry
• heavy-fermion metals
• electronic properties
• magnetic anisotropy
• phase transitions
• strongly correlated electrons

Acknowledgements

This review is based on the work in collaboration with T. Fukuda, Y. Haga, K. Hashimoto, K. Ikada, S. Kasahara, Y.-H. Lin, T. D. Matsuda, Y. Mizukami, R. Okazaki, Y. Onuki, H. J. Shi, H. Shishido, K. Sugimoto, S. Tonegawa, Y. Tsuruhara, D. Watanabe, E. Yamamoto and N. Yasuda. We thank D. Aoki, A. V. Balatsky, K. Behnia, F. Bourdarot, P. Chandra, P. Coleman, R. Flint, S. Fujimoto, K. Ishida, H. Harima, S. Kambe, A. Kaptulnik, Y. Kasahara, I. Kawasaki, G. Knebel, G. Kotliar, Y. Kuramoto, H. Kusunose, J. A. Mydosh, P. M. Oppeneer, P. S. Riseborough, A. F. Santander-Syro, M. Sigrist, S. Takagi, T. Takimoto, P. Thalmeier, C. M. Varma, H. Yamagami, Y. Yanase and Y. Zaanen for helpful discussions.

Notes

1 This work was supported by Grant-in-Aid for the Global COE programme ‘The Next Generation of Physics, Spun from Universality and Emergence’, Grants-in-Aid for Scientific Research on Innovative Areas ‘Heavy Electrons’ [grant numbers: 20102002; 20102006; 23102713] and ‘Topological Quantum Phenomena’ [grant number 25103713] from the Ministry of Education, Culture, Sports, Science and Technology (MEXT) of Japan, and KAKENHI from the Japan Society for the Promotion of Science (JSPS).