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World’s First Direct Observation of Slow-Moving Electrons Derived from Cerium (Press Release)

Release Date
28 Mar, 2011
  • BL08W (High Energy Inelastic Scattering)
- Progress toward Understanding of Novel Superconductivity

University of Hyogo
Nihon University
Japan Synchrotron Radiation Research Institute

Scientists from the University of Hyogo (President, Masayoshi Kiyohara), Nihon University (President, Takeo Sakai) and Japan Synchrotron Radiation Research Institute (JASRI; President, Tetsuhisa Shirakawa) succeeded in visualizing, for the first time in the world, the previously unobservable change of "slow-moving electrons" from the localized state (staying in a certain area) to the itinerant state (moving freely) by Compton scattering*1 using the high-intensity and high-energy X-rays of SPring-8.

Electrons induce the electric conductivity of ordinary metals and they move freely in the metals. However, scientists have found some chemical compounds containing rare-earth elements, such as cerium (Ce), and actinoid elements, such as uranium (U) (namely, heavy-electron compounds), which show significant changes in their electric and magnetic properties depending on the temperature, magnetic field, pressure, and sample composition or which show novel superconductivity that cannot be explained by conventional theories. It is necessary to elucidate the behavior of the "slow-moving electrons" contained in these metals in order to clarify these novel phenomena. However, because the heavy elements such as Ce strongly absorb X-rays and reduce the measured intensity of the X-rays, it is difficult to obtain sufficient data to conduct an analysis with satisfactory accuracy. Therefore, the visualization of the changes in the states of "slow-moving electrons" had been difficult in the previous research. Such visualization was enabled by the use of the high-intensity and high-energy X-rays of SPring-8.

The advantage of Compton scattering, the method of analyzing the electron state of the materials used in this successful research, is that there are no restrictions on the experimental conditions such as temperature, magnetic field, pressure, and sample purity. This method enables the systematic research of compounds of elements such as Ce and U containing "slow-moving electrons" and will lead to a major breakthrough in the understanding of the novel superconductivity that is accompanied by magnetic characteristics. In addition, the utilization of the novel characteristics of the slow-moving electrons will contribute to the development of high-density memory devices in the future.

The results herein were achieved by Akihisa Koizumi (Associate Professor) and Gaku Motoyama (Assistant Professor) of the University of Hyogo; Yasunori Kubo (Professor) and Toshiki Tanaka (graduate student) of Nihon University; Masayoshi Ito (Associate Senior Scientist) and Yoshiharu Sakurai (Associate Chief Scientist) of JASRI, through joint research. Their findings were published online in a journal of the American Physical Society Physical Review Letters on 28 March 2011.

"f Electron Contribution to the Change of Electronic Structure in CeRu2Si2 with Temperature: A Compton Scattering Study "
A. Koizumi, G. Motoyama, Y. Kubo, T. Tanaka, M. Itou, and Y. Sakurai
Physical Review Letters 106, 136401 (2011), published online 28 March 2011

*1 Compton scattering

Light (X-ray) has the properties of particles called photons. When the X-ray photons and electrons collide with each other as billiard balls do, the photons are scattered by the electrons, and the electrons are also bumped out. It is observed that the energy of the photons is lower after the collision than before the collision. This phenomenon is called Compton scattering. In many textbooks, Compton scattering is often explained as the elastic collision between static electrons and X-ray photons. However, the electrons in real materials are always moving. Therefore, the energy distribution of Compton-scattered X-ray photons reflects the electron momentum (Doppler effect). The measured scattering intensities of the X-rays with respect to the energy are called the Compton profile. The electron state of materials can be examined using the Compton profile, which reflects the momentum of the electrons in the materials.


Fig. 1 Changes in <em>f</em> electrons
Fig. 1 Changes in f electrons

Electrons have spins underlying their magnetic properties. The spins are indicated by arrows in the figure. At a temperature higher than the Kondo temperature, the f electrons (spins) are localized and the material exhibits a magnetic order when there are interactions between the f electrons and the conduction electrons. On the other hand, at a temperature lower than the Kondo temperature, the f electrons and the conduction electrons are coupled together so that they cancel each other's spins and generate a heavy electron state, which contributes to the electric conductivity.

Fig. 2 Electron occupation number density
Fig. 2 Electron occupation number density

The figure shows the electron occupation number density obtained by the two-dimensional reconstruction analysis and the Lock-Crisp-West (LCW) analysis. When the electron occupation number density is theoretically obtained by the band calculation assuming the f electrons to be conduction electrons, the contribution of the itinerant f electrons is strongly exhibited in the area encircled by the red line in the 15th band. From the nature of the LCW analysis, the conduction electrons alone contribute to the electron occupation number density, and the electrons in the localized atomic orbits and the fully occupied electronic bands do not. Therefore, it is considered that (a), which shows the electron occupation number density at room temperature (higher than the Kondo temperature), does not exhibit the contribution of the localized f electrons but only exhibits the distribution of the conduction electrons other than f electrons. On the other hand, the intensity in the area of the red circle in (b), which shows the electron occupation number density at 5 K (lower than the Kondo temperature), shows that the f electrons and the conduction electrons are coupled together to induce the heavy electron state.

For more information, please contact:
Prof. Akihisa Koizumi (the University of Hyogo)
E-mail: mail1