To reuse heat waste generated at power plants or incinerators as electric energy needs so-called thermoelectric materials that have a high thermal electromotive force, carry electric current efficiently, and conduct little heat. Although these three properties are incompatible in general, some materials have gathered attention for more than 10 years. Among them is known a family of clathrate compounds with periodically-arranged polyhedrons (called a basket hereafter) made of elements such as germanium. The inner space of the basket encapsules one foreign atom as a guest. When electrons move from the guest atom to the host basket, the guest atom becomes ionic, and the basket is negatively charged. The guest ion vibrates and rattles in the basket just like "Rattle," a baby toy. "Rattling" in the title is an adjective of the sound of the toy.

In this study, a joint research group led by Professor Naoki Toyota at Department of Physics, Tohoku University, Professor Toshiro Takabatake at Graduate School of Advanced Sciences of Matter, and Professor Masayuki Udagawa at Graduate School of Integrated Arts and Sciences, Hiroshima University has succeeded in photographing the rattling vibration of barium ions in a type-I clathrate (Ba8Ga16Ge30) by terahertz light. Here terahertz light has an intermediate frequency between microwave and infrared light. The method in this study is based on terahertz time-domain spectroscopy, which might be said to photographing with a superhigh-speed camera to track and record a time evolution of an electric field vector of lights that transmit the crystal. This technique makes it possible to determine excited vibrational states from the time-dependent correlation of local current carried by rattling barium ions. To analyze vibrational spectra in a wide thermal domain (through ambient temperature down to around absolute zero), the research group has also calculated the spectra based on an anharmonic vibration model. In the present on-center rattling, the spectral form is asymmetrically sharpened, and the vibrational energy decreases, with decreasing temperature. It is because that the energy intervals between excited levels are exaxctly equal in a harmonic vibration of small atomic displacement, but not equal at all in an anharmonic vibration of large displacement just as in the present case.

From experience, we know that crystalline materials such as copper consisted of regularly-arranged atoms, conduct electricity and heat efficiently, but also that glassy materials that lose periodicity by irregularly-arranged atom conduct both inefficiently. In a periodical space as crystal, both electronic waves that conduct electricity and atomic vibrational waves (called acoustic waves) that conduct heat and sound can propagate a long way. In a nonperiodical space such as glass, however, these waves become scattered and hence attenuated. How about introducing some nonperiodical elements like glass in a periodical crystal? How efficient and effective are rattling vibrations and its anharmonicity as a nonperiodical element? How about for electric conduction? It is just a start to solve problems. This is not only academic matters, and not that there is no relation to technologies to reuse energy mentioned in the opening sentence.

The research results have been published in “Physical Review B” as two papers on June 12 and 30, 2009. The experimental study has been conducted actively by undergraduate and graduate students including Tatsuya Mori, Kei Iwamoto, Shohei Goshima, Syunsuke Kushibiki, Koichiro Suekuni, while the theoretical study has been made by Fellow Dr. Hideki Matsumoto and his collaborators. The Ministry of Education, Culture, Sports, Science and Technology has supported Global CEO Program (Materials Integration), Tohoku University, and provided a research grant for scientific researches.

[Contact]
Professor Naoki Toyota
Low Dimensional Quantum Physics Laboratory, Department of Physics,
Tohoku University
Tel: +81 70-6622-4568, +81 22-795-6467(6604)
E-mail: toyota-n@ldp.phys.tohoku.ac.jp
URL: http://ldp.phys.tohoku.ac.jp/DotNetNuke/

Fellow Dr. Hideki Matsumoto
Tel: +81 22-795-6604
E-mail: matumoto@ldp.phys.tohoku.ac.jp