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Direct Observation of Fermi Surface Consisting of "Heavy Electrons" for the First Time in the World -Toward Clarifying the Mechanism of the Mysterious Coexistence of Superconductivity and Magnetism- (Press Release)

Release Date
26 May, 2009
  • BL23SU (JAEA Actinide Science)
A research group consisting of scientists led by Tetsuo Okane, Associate Senior Scientist of the Quantum Beam Science Directorate of the Japan Atomic Energy Agency, Haruyoshi Aoki, Professor of the Department of Physics, Graduate School of Science, Tohoku University, Atsushi Fujimori, Professor of the Department of Physics, Graduate School of Science, The University of Tokyo, and Hiroshi Yamagami, Professor of the Department of Physics, Faculty of Science, Kyoto Sangyo University, succeeded in directly observing a Fermi surface consisting of "heavy electrons" with an extremely large apparent mass for the first time in the world. This achievement makes it possible to determine the electric-conduction properties of metals caused by heavy electrons, which depend on the type of metal.

Japan Atomic Energy Agency (JAEA)
Tohoku University
University of Tokyo
Kyoto Sangyo University

A research group consisting of scientists led by Tetsuo Okane, Associate Senior Scientist of the Quantum Beam Science Directorate of the Japan Atomic Energy Agency (JAEA; Toshio Okazaki, President), Haruyoshi Aoki, Professor of the Department of Physics, Graduate School of Science, Tohoku University (Akihisa Inoue, President), Atsushi Fujimori, Professor of the Department of Physics, Graduate School of Science, The University of Tokyo (Junichi Hamada, President), and Hiroshi Yamagami, Professor of the Department of Physics, Faculty of Science, Kyoto Sangyo University (Toyoh Sakai, President), succeeded in directly observing a Fermi surface*1 consisting of "heavy electrons"*2 with an extremely large apparent mass for the first time in the world. This achievement makes it possible to determine the electric-conduction properties of metals caused by heavy electrons, which depend on the type of metal.

The electrons in a metal are classified into two groups: itinerant electrons, which move around to induce electron conduction, and localized electrons, which induce magnetism and do not move. When these two types of electrons are mixed and the electrons are allowed to strongly interact with each other, heavy electrons with an apparent mass 10-1000 times that of usual electrons appear. It is considered that by determining the properties of these heavy electrons, the mechanism behind the superconductivity coexisting with magnetism*3 can be clarified. Each metal has a Fermi surface with a unique shape that depends on the electric-conduction properties. Therefore, the Fermi surface is called the "face of a metal." If we can observe a Fermi surface consisting of heavy electrons, the electric-conduction properties caused by the heavy electrons can be studied in detail for each metal; however, there have been no reports on the direct observation of Fermi surfaces consisting of heavy electrons.

In this research, a Fermi surface consisting of heavy electrons was successfully observed, for the first time in the world, by the selective observation of specific electron orbits by angle-resolved resonant photoemission spectroscopy*4 using the soft X-ray at JAEA Actinide Science Beamline BL23SU of SPring-8. Such an observation is impossible by conventional methods. It is expected that the mechanism behind superconductivity coexisting with magnetism, which has been observed in metals having heavy electrons, will be further clarified in the future by systematically determining the relationship between the shape of the Fermi surface consisting of heavy electrons and the expression of superconductivity, as well as magnetism, by angle-resolved resonant photoemission spectroscopy.

The achievement was published in Physical Review Letters (online version), a journal of the American Institute of Physics, on 27 May 2009.

Publication:
"4f-derived Fermi Surfaces of CeRu2(Si1-xGex)2 near the Quantum Critical Point: Resonant Soft X-ray ARPES Study"
T. Okane, T. Ohkochi, Y. Takeda, S.-i. Fujimori, A. Yasui, Y. Saitoh, H. Yamagami, Y. Matsumoto, M. Sugi, N. Kimura, T. Komatsubara, and H. Aoki
Physical Review Letters 102, 216401 (2009), published online 27 May 2009.

<Figure>

Fig. 1 Schematic image of change of Fermi surface depending on the difference in the relationships between itinerant and localized electrons. Fig. 1 Schematic image of change of Fermi surface depending on the difference in the relationships between itinerant and localized electrons.
The Fermi surfaces of itinerant electrons (blue circles) and localized electrons (red circles) are drawn as blue and red closed curves, respectively.

Fig. 2 Angle-resolved photoemission spectra of LaRu2Si2 and CeRu2Si2 Fig. 2 Angle-resolved photoemission spectra of LaRu2Si2 and CeRu2Si2.
The red dashed line shows the energy dispersion of the electron band structure. The Fermi surface is obtained by depicting the points where the electron band structure crosses the zero-binding energy line (Fermi level) in the momentum space.

Fig. 3 Fermi surface obtained from the angle-resolved photoemission spectrum measured in the nonresonant and resonant regions of CeRu2Si2. Fig. 3 Fermi surface obtained from the angle-resolved photoemission spectrum measured in the nonresonant and resonant regions of CeRu2Si2.
In the nonresonant region, a Fermi surface corresponding to the blue closed curve in Fig. 1 is observed, whereas in the resonant region, a Fermi surface corresponding to the red closed curve in Fig. 1 is observed.

<Glossary>

*1 Fermi surface
A curve representing the relationship between the energy and momentum of electrons in a solid is called the electron band structure. A material in which a lower part of the available energy levels are filled with electrons is a metal. The momentum of electrons existing at the boundary between the occupied and unoccupied areas (Fermi level) can be three-dimensionally represented in momentum space. This representation is called the Fermi surface. Because the shape of the Fermi surface characterizes the electric-conduction properties and depends on the type of metal, the Fermi surface can be called the "face of a metal."

*2 Heavy electrons
In certain types of metal compounds, when localized electrons, which do not move around but merely exist near the atomic nucleus, mix and interact with itinerant electrons, which move around and induce electron conduction, they also play a role in conductivity. This mixing means that electrons that rarely move are included among itinerant electrons that induce electron conduction. Because of this difficulty in movement, such electrons have an apparent mass (effective mass) 10-1000 times that of usual electrons. These electrons are called heavy electrons.

*3 Superconductivity coexisting with magnetism
Superconductivity is a phenomenon in which the electric resistance of the material becomes zero at or below a temperature unique to the material. It is widely accepted that the appearance of superconductivity can be explained by BCS theory, in which the formation of electron pairs through the mediation of lattice vibration is the fundamental mechanism. However, since the discovery of high-temperature superconductivity in 1986, materials whose superconductivity cannot be explained by BCS theory have been extensively explored. Such new superconducting materials are often discovered near the boundary of the region where magnetism is expressed, and there is a possibility that superconductivity can coexist with magnetism. The formation of electron pairs, which is the foundation of the superconducting mechanism, originates from the attractive force between electrons; in contrast, magnetism originates from the repulsive force between electrons. In this sense, the coexistence of superconductivity with magnetism is an extremely mysterious phenomenon and has been attracting attention in cutting-edge research on physical phenomena related to both conductivity and magnetism.

*4 Angle-resolved resonant photoemission spectroscopy
When high-energy light is irradiated onto a material, electrons inside the material can be knocked out. Photoemission spectroscopy is an experimental method by which the electron state in the material is investigated through the relationship between the number of electrons (photoelectrons) emitted and the energy. The method to observe the relationship between the momentum and energy of electrons (electron band structure) by measuring the distribution of the emission angle of photoelectrons is called angle-resolved photoemission spectroscopy. The Fermi surface is obtained by depicting the points where the obtained electron band structure crosses the Fermi level in momentum space. If the light energy is set to a value at which absorption to a specific inner shell level occurs, only the intensity of emission of photoelectrons from certain orbits, which are selected as the target orbits of transition by the inner shell absorption, is selectively intensified. In angle-resolved resonant photoemission spectroscopy, the spectral data of angle-resolved photoemission are measured using light with such a specific energy. By this method, the electron band structures and Fermi surfaces, to which only certain electron orbits are closely related, can be selectively observed.

For more information, please contact:
Dr. Tetsuo Okane, Japan Atomic Energy Agency (JAEA),
e-mail: Fig. 1 Schematic image of change of Fermi surface depending on the difference in the relationships between itinerant and localized electrons..