Direct Observation of “Hidden Order” That Has Been a Mystery in Materials Physics (Press Release)
- Release Date
- 19 Jun, 2014
- BL02B1 (Single Crystal Structure Analysis)
The University of Tokyo
Kyoto University
Japan Synchrotron Radiation Research Institute (JASRI)
Japan Atomic Energy Agency (JAEA)
Key research achievements
• “Hidden order,” a novel electronic state of a certain type of uranium compound, has been a longstanding mystery in physics.
• Direct observation of a change in the crystal structure of the hidden order electronic state in the uranium compound has been a top-priority issue because the crystal structure had been reported to be of the square order while the recent indirect evidence has analogically indicated the presence of the rhombic order.
• Direct observation of a slight change in the crystal structure was enabled for the first time by combining ultrahigh-resolution measurements using synchrotron radiation and ultrapure crystals, contributing to conclusive evidence of the presence of the rhombic order.
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Takasada Shibauchi (professor; concurrently visiting professor of the Graduate School of Science, Kyoto University), Yuta Mizukami (assistant professor) of the Graduate School of Frontier Sciences, The University of Tokyo, and Yuji Matsuda (professor) of the Graduate School of Science, Kyoto University, in collaboration with Kunihisa Sugimoto (research scientist) of the JASRI and Yoshinori Haga (chief scientist) of the JAEA, have directly observed and clarified that the crystal structure of the hidden order is slightly distorted to an orthorhombic structure in a uranium compound, URu2Si2, by ultrahigh-resolution crystallography using the synchrotron radiation at SPring-8. Hidden order, a novel electronic state of URu2Si2, has been a mystery for thirty years in materials physics. In 2011, a research group reported that indirect evidence analogically suggests the presence of the orthorhombic order. However, the crystal structure of the hidden order had been reported to be tetragonal in the past crystallography. The direct observation of the orthorhombic crystal structure in this study provided conclusive information about the space symmetry of the hidden-order electronic state. The direct observation of the hidden-order electronic state, which has been a longstanding mystery and could not be predicted using a simple method, will lead to the understanding of possible new electronic states in materials. Publication: |
《Figures》
Blue lines show the results of conventional impure samples and circles are those of ultrapure samples used in this study. The diffraction pattern shows a single peak at temperatures higher than the hidden-order phase transition temperature (black circles) but shows two peaks at lower temperatures.
and schematics of the arrangement of uranium atoms in the ab plane (bottom)
URu2Si2 has a tetragonal structure (left) and a square lattice with fourfold symmetry in the plane above the hidden-order phase transition temperature. As revealed in this study, it has an orthorhombic structure (right) and a rhombic lattice with twofold symmetry in the plane at lower temperatures.
《Glossary》
*1 Phase transition
When a material is stable at certain temperature and magnetic field, that material is in a phase. For example, the state of being solid or liquid is called a solid phase or a liquid phase. A collection of many electrons in materials can have electronic states in various phases, such as the state of being a magnet called a ferromagnetic phase and the state of being superconductive called a superconducting phase, under different conditions. The transformation of a material from one phase to another according to the changes in conditions such as temperature is called phase transition. Phase transitions are often accompanied by a change in the symmetry of a system, and the nature of the change in the symmetry determines the physical characteristics of each phase.
*2 Hidden order
There are many phase transitions exhibited by materials, and the types of phases involved in the transitions and the nature of the change in the symmetry have been clarified in most cases. The uranium compound discovered in 1985 shows an anomaly in specific heat, which is clear evidence of phase transition. However, the types of phases involved in the transition and the nature of the change in the symmetry have remained unclarified, which is why the nature of this phase transition has been called hidden order. As the change in the state at the phase transition can be understood to be the formation of an order, hidden order therefore indicates that a certain type of order is formed below the phase transition temperature. Although recently the word hidden order has become a general term for unclear phase transitions, it originally referred to the phase transition of the uranium compound that has been studied intensively in the field of materials physics and has remain a mystery to date.
*3 SPring-8
The experiments in this study were carried out at the Single Crystal Structure Analysis Beamline BL02B1.
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