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World’s First Imaging of Electronic State That Induces High-Temperature Superconductivity (Press Release)

Release Date
29 Apr, 2011
  • BL08W (High Energy Inelastic Scattering)
- Clue for Clarifying the Mechanism behind High-Temperature Superconductors

Japan Synchrotron Radiation Research Institute
Tohoku University
Japan Atomic Energy Agency


Scientists from Japan Synchrotron Radiation Research Institute (JASRI; President, Tetsuhisa Shirakawa), Tohoku University (President, Akihisa Inoue), and Japan Atomic Energy Agency (President, Atsuyuki Suzuki) succeeded in imaging, for the first time in the world, the electronic state that induces high- temperature superconductivity in cuprate superconductors, using the high-brilliance and high-energy X-rays of SPring-8.

Superconductivity is a natural phenomenon of zero electrical resistance observed in certain materials that are cooled to below their superconducting transition temperature (Tc). Before the discovery of high-temperature cuprate superconductors, the highest reported Tc was 23 K*1 for Nb3Ge, an alloy-based superconductor mainly composed of metal elements.

The first high-temperature superconductivity was discovered in 1986 in an insulating cuprate from which some electrons were removed. Because its Tc value was higher than those of conventional alloy-based superconductors, it was called a high-temperature superconductor. The highest Tc ever reported is 135 K. If materials with a higher Tc (ultimately, materials with Tc of room temperature, i.e., room-temperature superconductors) can be developed, the performance of medical equipment such as magnetic resonance imaging (MRI) systems can be improved and technologies for next-generation linear motor cars and large-scale power storage can be practically realized. However, many physical problems, such as the dependence of Tc on the number of electrons removed, have remained unsolved; the mechanism behind high-temperature superconductivity has not yet been clarified.

The research group succeeded in imaging, for the first time in the world, the momentum distribution*4 of holes*3 obtained as a result of removing electrons by high-resolution Compton scattering*2 using the high-brilliance and high-energy X-rays of SPring-8. Such holes play an important role in high-temperature superconductivity. This achievement is hoped to serve as a clue for clarifying the mechanism of high-temperature superconductivity, which will be essential for designing room-temperature superconductors.

The results herein were achieved by Yoshiharu Sakurai (Associate Chief Scientist) and Masayoshi Ito (Associate Senior Scientist) of JASRI; Kazuyoshi Yamada (Professor) and Masaki Fujita (Associate Professor) of Tohoku University; and Shuichi Wakimoto (Senior Scientist) of Japan Atomic Energy Agency, through joint research with scientists from four overseas universities including Northeastern State University and Ecole Centrale Paris. Their findings were published online in the American scientific journal Science Express on 28 April 2011.

"Imaging Doped Holes in a Cuprate Superconductor with High-Resolution Compton Scattering"
Y. Sakurai, M. Itou, B. Barbiellini, P.E. Mijnarends, R.S. Markiewicz, S. Kaprzyk, J.-M. Gillet, S. Wakimoto, M. Fujita, S. Basak, Yung Jui Wang, W. Al-Sawai, H. Lin, A. Bansil and K. Yamada
Science 332, 698-702 (2011), published online 28 April 2011

*1 Kelvin (K)

Absolute zero is the minimum temperature below which materials cannot be cooled. Absolute temperature is a measure of temperature using the unit of Kelvin (K), where absolute zero is 0 K. Absolute zero (0 K) is -273.15oC on the Celsius temperature scale, namely, 0oC is 273.15 K.

*2 Compton scattering
Light (X-ray) has the properties of particles called photons. When X-ray photons and electrons collide similarly to billiard balls, the photons are scattered by the electrons, and the directions of electrons are also changed. It has been observed that the energy of photons is lower after a collision than before the collision. This phenomenon is called Compton scattering. In many textbooks, Compton scattering is often explained to be the elastic collision between stationary electrons and X-ray photons. However, the electrons in actual materials are constantly moving. Therefore, the energy distribution of Compton-scattered X-ray photons reflects the electron momentum (Doppler effect). The measured scattering intensities of X-rays with respect to the energy give the Compton profile. The electronic state of materials can be examined using the Compton profile, which reflects the momentum of the electrons in materials.

*3 Holes
The electron-filled shell from which some electrons are removed is assumed to be a particle and is called a hole. Holes obtained as a result of removing negatively charged electrons are also called positive holes because they are positively charged virtual particles. The oxygen 2p orbital in La2CuO4, one of the La2-xSrxCuO4 (LSCO) superconductors examined in this study, is completely filled with electrons that cannot move; thus, La2CuO4 is an insulator that does not allow current to flow. Because three and two electrons are supplied for chemical bonding by La and Sr atoms, respectively, the replacement of one La atom with one Sr atom implies the removal of one electron; in other words, the doping of one hole. Namely, when one La atom is replaced with one Sr atom, a hole is formed on the oxygen 2p orbital and current is allowed to flow because of the movable electrons. When the material is cooled under this condition, superconductivity is exhibited.

*4 Momentum distribution
The momentum distribution is equivalent to the velocity distribution because momentum = mass × velocity.


Fig.1:Crystal structure of La2-xSrxCuO4, a high-temperature cuprate superconductor, and crystal orientation measured by Compton scattering
Fig. 1:Crystal structure of La2-xSrxCuO4, a high-temperature
cuprate superconductor, and crystal orientation measured by
Compton scattering

Fig. 2 Phase diagram of La2-xSrxCuO4
Fig. 2 Phase diagram of La2-xSrxCuO4 [abscissa: amount of Sr
(hole doping level); ordinate: temperature]

This high-temperature superconductor has an optimal hole doping level. Upon increasing the hole doping level up to x = 0.15, Tc increases; however, a much higher doping level is not effective in increasing Tc; in fact, Tc decreases with further doping. The dome-shaped graph of Tc is considered to be one of the mysteries of high-temperature cuprate superconductors.

Fig. 3 Momentum distribution of holes (experimental result)
Fig. 3 Momentum distribution of holes (experimental result)

The momentum distribution differs in underdoped and overdoped regimes. This finding indicates that the conditions of the doped holes in the two regimes are different. The red and yellow regions in the figure have a great number of holes.

Fig.4 Electron occupation density
Fig. 4 Electron occupation density

In the figure, the oxygen 2p orbital and copper 3d orbital are shown. There are two types of copper 3d orbital, i.e., (x2-y2) and (z2) orbitals. The results of this study indicate that a hole enters the oxygen 2p orbital in the underdoped regime, whereas it enters the copper 3d orbital in the overdoped regime.

For more information, please contact:
Dr. Yoshiharu Sakurai (JASRI)

Prof. Kazuyoshi Yamada (Tohoku University)