Professor Ichiro Terasaki et al. of the Department of Applied Physics Department of Science and Engineering at Waseda University, in conjunction with the University of Tokyo, the Tokyo Institute of Technology, Tohoku University, Okayama University, and the Japan Synchrotron Radiation Research Institute, discovered how to produce alternating current from direct-current (similar to an inverter) using a monocrystal organic conductor θ-(BEDT-TTF)2CsCo(SCN)4 cooled with liquid helium (４．２Ｋ＝absolute temperature, -269℃), displaying the same current-voltage characteristics as a thyristor. This is the first time ever that a thyristor effect has been demonstrated with a simple substance, hinting at possibilities for new organic electronics element design. The results were published in the September 22 issue of the British science journal “Nature” (title: “An organic thyristor”)
（note）θ-(BEDT-TTF)2CsCo(SCN)4 ＝ a derivative (complex) of BEDT-TTF (bis(ethylenedithio)tetrathiafulvalene）. BEDT-TTF derivative displays electrical conductivity and superconductitivty.
The research group was led by Professor Terasaki and included Bunsho Sawano (Graduate Student, Department of Science and Engineering, Waseda University), Hatsumi Mori (Assistant Professor, Institute for Solid State Physics, University of Tokyo), Takehiko Mori (Assistant Professor, Graduate School of Science and Engineering, Tokyo Institute of Technology), Masashi Watanabe (Research Associate, Institute of Multidisciplinary Research for Advanced Materials, Division of Materials Analysis, Tohoku University), Yukio Noda (Professor, Institute of Multidisciplinary Research for Advanced Materials, Division of Materials Analysis, Tohoku University), Sunao Ikeda (Senior Researcher, Japan Synchrotron Radiation Research Institute), Yoshio Nogami (Professor, Department of Physics, Okayama University) and others. The result of examining the solid state properties at super cold temperatures (using liquid helium to cool) of a mono-crystal θ-(BEDT-TTF)2CsCo(SCN)4 that is electrically conductive at normal temperatures confirmed that direct current changed into alternating current.
Upon examining the cause of this phenomenon, it was discovered that thawing occurs as a result of the current, changing the charge ordering state within the crystal, thus changing its properties. The charge order is such that the electrons are in a frozen state and, in the absence of current, are unable to move. When current is applied, however, the electrons in the frozen state begin to thaw, allowing electricity to flow rapidly. This is the reason why it possesses the same current-voltage characteristics as a thyristor. Diffraction experiments with radiant light clearly revealed this thawing phenomenon in the charge order in response to the peculiar current-voltage characteristics.
This phenomenon of thawing occurring in the charge state is, physically, an intrinsic non-equilibrium phenomenon. One example of non-equilibrium phenomena is water in a lake in winter that does not freeze despite the surface of the lake having frozen over. Modern physics is still unable to satisfactorily explain this phenomenon. The current discovery is potentially the electron version of this, and, as more about it is understood, the contributions to the development of non-equilibrium physics are thought to be great.
Thyristors are necessary electron devices incorporated in inverter circuits used to save energy and stabilize power sources. These inverter circuits are frequently used in air conditioners and refrigerators. Traditional thyristors, however, are devices which use the electrical properties of the PN-junction of semiconductors and are made through the micro-fabrication of mono-crystals using semiconductor technology.
In the case of the organic thyristor material that Professor Terasaki et al. discovered, however, the mono-crystal itself has the same current-voltage characteristics as a thyristor. This means that it is possible to manufacture without using the traditional process. The drawback is that the phenomenon in question only exhibits itself at super cold temperatures (achieved with liquid helium). Like a superconductor, it does not work at normal temperatures, and, thus, does not have immediate, practical applications. The tremendous potential of organic compounds as electronics materials, however, is well supported.
This research conducted by Professor Terasaki et al. was carried out with the support of the 21st Century COE Program of the Japan Society for the Promotion of Science, the subsidy for basic research and grant-in-aid Scientific Research on Priority Areas, and the Basic Research Programs of the Japan Science and Technology Agency.
