SPring-8, the large synchrotron radiation facility

Release Date

20 Mar, 2009

Key research achievements
• Clarification of the transformation of a fullerene to a superconductor through the reduction of intermolecular distance and the acceleration of electron transfer by applying pressure
• Disproval of the conventional theory on fullerenes, which states that a substance cannot be an insulator and a superconductor
• Demonstration of the guiding principle that a high superconducting critical temperature is observed near the insulator phase

An international joint research team^*1 consisting of scientists of RIKEN (Ryoji Noyori, President), Tohoku University (Akihisa Inoue, President), Durham University in the UK, and other institutions investigated the structural and electronic properties of the fullerene^*3 Cs₃C₆₀, which has the highest reported superconducting critical temperature^*2 under pressure among organic superconductors (molecular substances), by a multifaceted approach. They clarified that when pressure is applied to the fullerene, the intermolecular distance is shortened, the electrons start moving, and the fullerene is transformed into a metal and exhibits superconductivity. This finding was achieved in joint research by Masaki Takata, a chief scientist of the Takata Structural Materials Science Laboratory, RIKEN SPring-8 Center; Yasuhiro Takabayashi, a postdoctoral researcher (currently at Ideal Star Inc.) and Professor Kosmas Prassides of Durham University; Professor Yoshihiro Iwasa and Taku Takano (3^rd year PhD student) of Tohoku University; Yasuo Ohishi, a senior scientist at the Japan Synchrotron Radiation Research Institute; Nao Takeshita, a researcher at the Advanced Industrial Science and Technology; and Professor Matthew J. Rosseinsky of the University of Liverpool.

The first fullerene superconductor was discovered in 1991. Its critical temperature of T_c=33 K, the highest reported for a molecular substance at that time, was demonstrated by Masami Tanigaki, a researcher at NEC (currently a professor of Tohoku University) and his coworkers. This finding attracted much attention. In 2008, Yasuhiro Takabatake, a postdoctoral researcher, and his colleagues synthesized a new cesium (Cs)-doped fullerene (Cs₃C₆₀), which exhibits superconductivity at Tc=38 K under pressure, thus breaking the record for T_c for molecular superconductors for the first time in 17 years (Nature Materials 7, p. 367, 2008). Mysteriously, Cs₃C₆₀ does not exhibit superconductivity under normal pressure and the mechanism behind its markedly different properties from conventional fullerene superconductors has remained unsolved.

As a result of this international joint research, Cs₃C₆₀ was shown to be nonconductive under normal pressure and to act as a distinctive insulator called a Mott insulator^*4. It was also clarified that when pressure is applied to Cs₃C₆₀, electrons start moving causing it to become metallic and simultaneously exhibit superconductivity with a high T_c.

The scientists were the first to clarify that the superconductivity of this fullerene with a high T_c is related to its electrical properties, which enable its transformation from an insulator to a metal, by determining the atomic arrangement of the fullerene under high pressure using the synchrotron radiation of High Pressure Research Beamline (BL10XU) at SPring-8.^*5 This behavior has also been observed in other superconductors with different characteristics from the conventional superconductivity of metals and alloys, such as organic and cuprate superconductors. The results of this research provide an important guideline for exploring molecular high-Tc superconductive materials, that is, exploring materials whose structure and composition are close to those of insulators is effective to create a superconductor with a high T_c. The results of this research were published in the online version of the American scientific journal Science on 20 March 2009.

Publication:
"The Disorder-Free Non-BCS Superconductor Cs₃C₆₀ Emerges from an Antiferromagnetic Insulator Parent State"
Yasuhiro Takabayashi, Alexey Y. Ganin, Peter Jeglic, Denis Arcon, Takumi Takano, Yoshihiro Iwasa, Yasuo Ohishi, Masaki Takata, Nao Takeshita, Kosmas Prassides, and Matthew J. Rosseinsky
Science Vol. 323. no. 5921, pp. 1585 - 1590, published online 20 March 2009

Fig. 1 Structure of new fullerene compound Cs₃C₆₀.
The grey spheres in the shape of a soccer ball indicate fullerene molecules. The red spheres indicate cesium atoms. The distance between adjacent fullerene molecules is approximately 1 nanometer (one-billionth of a meter). The crystalline structure and its changes with pressure and temperature were determined using the SPring-8 beamline (BL10XU).

Fig. 2 Change in electronic state of Cs₃C₆₀ with respect to pressure clarified in this experiment.
Electrons cannot move freely between C₆₀ molecules and are immobilized on the molecules because the adjacent C₆₀ molecules are relatively far apart under normal pressure. A substance in this state is called a Mott insulator. Electrons with opposing spin are immobilized on adjacent molecules (this phenomenon is known as antiferromagnetism). The intermolecular distance of C₆₀ is shortened when pressure is applied, which allows electrons to jump to adjacent molecules. A strong attractive force simultaneously acts between the electrons, producing Cooper pairs and causing the transition to a superconductive state.

^*1 International joint research team
This team consists of RIKEN, Tohoku University, Japan Synchrotron Radiation Research Institute, Advanced Industrial Science and Technology, Durham University in the UK, the University of Liverpool in the UK, and Jozef Stefan Institute and the University of Ljubljana in Slovenia.

^*2 Superconducting critical temperature
The superconductive state is a state in which a substance exhibits zero electrical resistance at a certain temperature or lower, which allows electricity to flow perpetually. The temperature at which the resistance becomes zero is called the critical temperature, Tc. In superconductive substances, a strong attractive force acts between the electrons, which allows paired electrons to move throughout the crystal.

^*3 Fullerene
A molecule predominantly consisting of carbon atoms with the structure of a closed hollow cage. C₆₀ is a particularly well-known fullerene because of its a soccer ball shape that has applications in various functional materials. A recent well-known application of fullerenes is in organic solar cells. American and British scientists won the Nobel Prize in Chemistry in 1996 for their discovery of C₆₀.

^*4 Mott insulator
An insulator in which electrons are immobilized on the atoms or molecules constituting the solid and thus no electricity can flow. For conventional insulators, called band insulators, no electricity flows because there are no conducting electrons, whereas Mott insulators do not allow electrons to pass even if there are conducting electrons present. These insulators are named after Nevill Mott, the theoretical physicist who first paid attention to them. Mott insulators, in most cases, exhibit magnetic properties and enter an antiferromagnetic state at low temperatures.

^*5 SPring-8 is a large synchrotron radiation facility, located in Harima Science Garden City in Hyogo Prefecture, that provides the world's most powerful synchrotron radiation. It is owned by RIKEN and managed by JASRI. SPring-8 is an abbreviation of "Super Photon ring - 8 GeV". Synchrotron radiation is a narrow and strong electromagnetic wave emitted from an electron when it is accelerated up to almost the speed of light and its path is bent using magnets. Diverse studies have been carried out using the synchrotron radiation at SPring-8 for the development of nanotechnology, biotechnology, and industrial applications.