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Demonstration of Synchrotron Radiation Technique That Can Reveal the Hidden World of Electrons (Press Release)

Release Date
14 Jun, 2011
  • BL11XU (JAEA Quantum Dynamics)
- Success in distinguishing orbital states of excited electrons by analyzing polarization characteristics of scattered X-rays

Japan Atomic Energy Agency
High Energy Accelerator Research Organization

Key research achievements
• Developing a technique for examining the polarization characteristics of inelastically scattered X-rays (the regularity of X-rays in the vibration direction)
• Demonstrating that the analysis of polarization characteristics is effective for distinguishing orbital states of electrons in excited states (diffusion pattern of electrons)
• Accelerating the clarification of the mechanisms behind properties and functions such as superconductivity

cientists of the Quantum Beam Science Directorate of the Japan Atomic Energy Agency (Atsuyuki Suzuki, President), including Kenji Ishii (Associate Senior Scientist), jointly with those of Tohoku University (Akihisa Inoue, President), the High Energy Accelerator Research Organization (Atsuto Suzuki, Director General), and the International Institute for Advanced Studies (Kazuo Oike, Director), have developed a technique for measuring the diffusion pattern of electrons in excited states (states with energy higher than the usual ground state) using synchrotron radiation X-rays. They also verified the validity of the technique at SPring-8.

For transition-metal compounds,*1 such as high-temperature cuprate superconductors, the diffusion pattern of electrons inside a material, i.e., the orbital state,*2 is considered to determine the mobility of electrons and the direction of propagation of their interaction. Resonant inelastic X-ray scattering*3 is an experimental technique for observing excitations that change the orbital state of electrons; however, it was considered difficult to distinguish such excitations because excitations to various orbital states occur simultaneously in transition-metal compounds.

To solve this problem, the scientists of the research group focused on polarization,*4 a property of X-rays, and developed a system that can separate multiple polarized components of X-rays that are scattered at a sample. Using this system, polarization characteristics can be examined in resonant inelastic X-ray scattering experiments. An experiment was carried out by this method at SPring-8 using a transition-metal compound, KCuF3,*5 and it was demonstrated that this method can distinguish different orbital states to which electrons are excited.

Thanks to the development of the above technique and the results of the demonstration experiment, it has become possible to determine, without the use of theoretical models, the type of electron orbital excitation related to various properties and functions, such as superconductivity, magnetism, and dielectric properties particularly observed in strongly correlated electron systems, by resonant inelastic X-ray scattering with the analysis of polarization characteristics. This is expected to accelerate the elucidation of the mechanisms behind such functions.

This research was partly supported by the “Research of Innovative Material and Process for Creation of Next-Generation Electronic Devices” project of the Core Research for Evolutional Science and Technology (CREST) program of the Japan Science and Technology Agency (JST). The results were published online as a Rapid Communication and an Editors' Suggestion paper in the journal of the American Institute of Physics Physical Review B on 14 June 2011.

"Polarization-analyzed resonant inelastic x-ray scattering of the orbital excitations in KCuF3"
K. Ishii, S. Ishihara, Y. Murakami, K. Ikeuchi, K. Kuzushita, T. Inami, K. Ohwada, M. Yoshida, I. Jarrige, N. Tatami, S. Niioka, D. Bizen, Y. Ando, J. Mizuki, S. Maekawa, and Y. Endoh
Physical Review B: Rapid Communications 83, 241101(R) (2011), published online 16 June 2011

*1 Transition-metal compound

Compounds that contain transition-metal elements, which belong to the third to eleventh columns in the periodic table, such as copper (Cu) and nickel (Ni), are called transition-metal compounds. In these compounds, d-electrons, unpaired electrons in transition-metal elements, mainly determine properties such as electric conductivity and magnetism.

*2 Orbital state
Electrons in an atom are diffused in various patterns in accordance with their energy and angular momentum, and such patterns are called orbital states. It is known that d-electrons, which play a key role in the properties of transition-metal compounds, can be in any of the orbital states shown in Fig. 1.

*3 Resonant inelastic X-ray scattering
When X-rays irradiated onto a sample are scattered at the sample and cause the exchange of energy between the sample and X-rays, this is called X-ray inelastic scattering. During X-ray inelastic scattering, momentum, as well as energy, can be exchanged because X-rays have momentum equivalent to that of the electrons in a sample, making it possible to measure the state of motion of electrons. When using X-rays with an energy corresponding to the gap between electron levels of the elements constituting a sample, the scattering intensity increases because of the resonance effect, which is called resonant inelastic X-ray scattering. In this study, the d-orbital state of Cu was selectively observed by using X-rays with an energy corresponding to the gap between electron levels of Cu.

*4 Polarization
WElectromagnetic waves, such as X-rays, propagate while electric and magnetic fields vibrate perpendicular to the direction of propagation. Polarization refers to the direction of vibration of the electric field. In general, SPring-8 synchrotron radiation X-rays are polarized in the horizontal direction. Such horizontally polarized X-rays were irradiated onto a sample without modification in this study.

*5 KCuF3
KCuF3 is known as a typical compound with an orbital order in which two types of d-orbital state are alternately and periodically arranged. This orbital order causes the propagation of magnetic interaction to be uniaxial and intensified. Hence, many scientists are also studying KCuF3 as a unidirectional magnetic material.


Fig. 1	d-orbital states of transition-metal compounds
Fig. 1 d-orbital states of transition-metal compounds

The electron energy depends on the orbital state. In X-ray inelastic scattering, the orbital state can be changed by supplying electrons with the energy from X-rays.

Fig. 2 	Schematic of polarization analysis experiment
Fig. 2 Schematic of polarization analysis experiment

Because polarized X-rays emitted from the storage ring at SPring-8 are well aligned, as indicated by the black arrows, they are irradiated onto a sample without modification. In contrast, X-rays scattered at the sample in this study contain two polarized components (red and blue arrows), which are separated and detected using a polarization analysis system.

Fig. 3 Resonant inelastic X-ray scattering spectrum of KCuF3
Fig. 3 Resonant inelastic X-ray scattering spectrum of KCuF3

The measurement results for the two polarized components of fscattered X-rays (red and blue) shown in Fig. 2 are plotted with circles in the corresponding colors. Also, d-orbital states are shown at the positions of corresponding energy. The red and blue arrows represent the change in the orbital state observed for the respective polarized components. Scattering that changes the orbital state of KCuF3 is observed between 1.0 and 1.5 eV in the spectrum. The polarized component shown in red has a peak at 1.4 eV, corresponding to the change in the orbital state indicated by the red arrow. In contrast, the polarized component shown in blue has scattering intensity peaks at 1.0 eV as well as at 1.4 eV, revealing that two types of excitation are simultaneously observed, as indicated by the two blue arrows. Namely, excitations at 1.0 and 1.4 eV have different polarization characteristics in resonant inelastic X-ray scattering. Therefore, two different electron excitations can be distinguished by analyzing such polarization characteristics. In addition, it was found that polarization characteristics can be well explained using a theoretical model of the resonant inelastic X-ray scattering process.

For more information, please contact:
Dr. Jun' ichiro Mizuki (Japan Atomic Energy Agency)

Dr. Kenji Ishii (Japan Atomic Energy Agency)

Prof. Youichi Murakami (KEK)

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