Uncovering the quarter-century mystery of phase transition in a Uranium compound
2011. 5. 16

 In nature, we have many different states of matter. The most familiar examples are solid, liquid and gas states. A state can transform into a different state through a 'phase transition', which is accompanied by a change of symmetries. In physics, it is the most essential in understanding the phase transition to elucidate this 'symmetry breaking'.

 Electrons in solids also exhibit many different states. In heavy-fermion compound URu2Si2, three groups have discovered independently in 1985 a new phase transition of electronic system at a low temperature of 17.5 K (approximately -256 deg C). Despite 25 years of intensive experimental and theoretical studies, however, the nature of the phase transition remains unresolved, and the low-temperature phase is called as the 'hidden order' phase. Especially, even the most fundamental issue of 'what symmetry is broken' has not been clarified, and it is one of the most important problems in condensed matter physics today.

 A Kyoto-University team of Ryuji Okazaki (a graduate student who is moved to Nagoya University), Takasada Shibauchi (Associate Professor), Hirokai Ikeda (Assistant Professor), and Yuji Matsuda (Professor) has conducted precise measurements of magnetic anisotropy in high-quality single crystals of URu2Si2, which have been grown by the group of Japan Atomic Energy Agency. They use magnetic torque as a probe, which was measured by using very sensitive micro-cantilevers. They found the emergence of an in-plane anisotropy of the magnetic susceptibility below 17.5 K, which shows two-fold symmetry under 360 deg rotation of in-plane magnetic field. This is not expected from the tetragonal crystal structure of URu2Si2 and breaks the four-fold rotational symmetry of the lattice. This surprising result directly indicates that the hidden-order state breaks the rotational symmetry.

 More than 20 theoretical models have been proposed for this mysterious hidden order. The `rotational symmetry breaking' uncovered in the present study, has not been considered as a fundamental premise in these models, and should impose strong constraints on theories of the hidden order in URu2Si2. Such an electronic state with broken rotational symmetry can be called as an electronic `nematic' state (see figure), which may lead to new understanding of novel states in condensed matter.

 This result has been published in the international journal SCIENCE on January 28, 2011. Free Reprint as published in SCIENCE Online can be accessed through the link below.

(Figure) Schematic illustration of electronic states in a plane including Uranium atoms of URu2Si2. Isotropic state at high temperatures becomes nematic with two-fold rotational symmetry below the transition temperature.