JAPAN NANONET BULLETIN - 9th Issue - January 8, 2004

NANONET INTERVIEW

Akira TONOMURA Fellow, Hitachi Ltd. and Group Director, The Institute of Physical and Chemical Research (Riken) 1965 B. Sc. in Physics, The University of Tokyo joined the Central Research Laboratory of Hitachi, Ltd. 1990 -  99 Senior Chief Research Scientist, Advanced Research Laboratory, Hitachi, Ltd. 1999 Fellow, Hitachi Ltd.   1975 Dr. of Engineering, Nagoya University 1993 Ph. D., Gakushuin University 1989 -  94 Project Leader, Electron Wavefront Project, Exploratory Research for Advanced Technology (ERATO) organized by Research Development Corporation of Japan (JRDC) 2001 - Group Director, Single Quantum Dynamics Research Group, Frontier Research System, The Institute of Physical and Chemical Research (RIKEN)   HONORS AND AWARDS 1982 Nishina Memorial Prize (Nishina Memorial Foundation) 1991 Japan Academy Prize and Imperial Prize (Japan Academy) 1999 Benjamin Franklin Medal (Franklin Institute, Philadelphia) 2000 Foreign Associate of the National Academy of Sciences in the United States 2002 Person of Cultural Merits (Ministry of Education, Culture, Sports, Science and Technology, Japan) etc.

Fig. 1 Large Image Nanocrystalline silicon particles Single-crystal silicon particles with a diameter of 10nm are covered with silicon oxide film. They exhibit new functions such as single-electron devices and high efficiency visible light emission. Fig. 2 Large Image Double-slit experiments with single electrons The electrons sent one by one through the double-slit (electron biprism) are detected as particles by the detector (a and b). Each electron must pass through either of the slits, but once a large number of electrons are accumulated, interference fringes are seen, which are formed only when the electrons pass through both sides of the double-slit simultaneously. It must be passing through the both slits as a wave. Fig. 3 Large Image Verification of Aharonov-Bohm effect When electron beams pass through the inside and outside of the hole of the doughnut-shaped ferromagnet completely covered with superconductor, phase difference is produced between the electron beam passing through the hole of the doughnut and the electron beam passing outside the doughnut (a). Despite the fact that electrons are passing through regions free of any electromagnetic field, an observable effect is produced due to the existence of vector potentials. Fig. 4 Large Image 1MV holography electron microscope The electron microscope with the world record in lattice resolution. This microscope is equipped with a high-brightness field-emission electron gun, an electron biprism, and a low-temperature specimen stage. Fig. 5 a.b Large Image    c Large Image Chains of magnetic flux quanta in Bi-2212 When a tilted magnetic field is applied to a layered high-temperature superconductor, a tight array or a “chain” of magnetic flux quanta is formed. In case of Bi-2212, chains and, triangular lattices of magnetic flux quanta appear alternately (a), which had long been a mystery. The development of the 1MV holography electron microscope made it possible to observe the magnetic flux quanta inside the superconductor. The observation clarified that magnetic flux quanta are not tilted. If tilted the images of flux quanta elongated as shown in Figure (b). These results lead to the verification of the Koshelev model shown in Figure (c).

Going beyond quantum mechanics
— the challenge of holography electron microscopy —

What is “the most beautiful experiment in physics”? When British magazine Physics World asked this question, its readers chose the double-slit experiments with single electrons, Dr. Tonomura and others carried out. “Following our experiments, you could see big names like Galileo and Newton in the top five,” Dr. Tonomura says with a smile. Richard Feynman once called the double-slit experiments “the heart of quantum mechanics but impossible to perform.” Dr. Tonomura succeeded in carrying out the experiment to show, for the first time, the essence of quantum mechanics--that single electrons can have characteristics of both waves and particle.

“I had always admired the ripples of water since I was a child. And when I went to university and studied quantum mechanics, I was taught that electrons are also waves. This motivated me to want to see the ripples of electrons with my own eyes, and made me decide to go into this area.” In 1968, Dr. Tonomura developed the holography electron microscope and continued to increase the brightness of electrons. Since then, he has led the world in research utilizing holography electron microscopy. In 1978, he succeeded in observing the magnetic lines of force. In 1982, he conclusively verified the existence of the Aharonov-Bohm effect, and demonstrated that vector potential actually exists. Until then, vector potential had been regarded as a quantity used for mathematical convenience, without any physical meaning. However, Dr. Tonomura verified that the vector potential is a more fundamental physical entity that also affects electrons than electric or magnetic fields.

Furthermore, in 1989, he achieved the first dynamic observation of the magnetic flux quanta inside superconductors; sometimes it was a river-like motion. In order to keep the superconductive state, it is necessary to pin down the magnetic flux quanta. It was known empirically that this could be done where there were defects in the superconductor. By his dynamic observation, Dr. Tonomura succeeded in catching the figure for the first time. “Direct observation of magnetic flux quanta had long been a dream for specialists in electron microscopy like myself, but at last we succeeded in doing this. Technical development and research are inseparable, like a pair of wheels of a carriage. If you can obtain the best technology, it would become possible to do research on what was seen as impossible before.” In the year 2000, Dr. Tonomura succeeded in developing a 1 million-volt holography electron microscope with the world’s highest resolution and beam brightness. The line-resolution was better than 0.5Å, with a brightness level of 2x10¹⁰ (A/cm² · sr), which was 4 digits higher than that of the electron beam used in his initial microscope.

Having obtained the world’s best technology, Dr. Tonomura is now using it to observe magnetic flux quanta in a “high-temperature” superconductor. Because the superconductor has a layered structure, the behavior of magnetic flux quanta becomes much more complicated than in conventional metallic superconductors. One of the unique phenomena observed in high-temperature superconductors is the chain of magnetic flux quanta. When a magnetic field is applied perpendicular to the layer plane of a high-temperature superconductor, the magnetic flux quanta form a closely-packed triangular lattice. However, with a tilted magnetic field, a tight array or “chain” of magnetic flux quanta is formed within the magnetic flux quanta forming the triangular lattice. It has been guessed that the flux quanta run obliquely, inside the superconductors, but Dr. Tonomura revealed in his research that Josephson quanta penetrate between the layers, and magnetic flux quanta perpendicularly crossing them form the chains. These results are considered to include the keys to clarifying the mysteries of high-temperature superconductors.

“I regard myself as a specialist on photography using the electron microscopy. Photographs include a variety of information. Beauty of the image as well as theory is an important element in judging whether the information shown on a photograph is true or not. With our effort, we would be able to obtain beautiful images that would allow us to see new things. This is what attracts me so much to pursue the nano-order world.” Dr. Tonomura’s curiosity seems to be endless. But it all started from his admiration of water ripples. Dr. Tonomura expects young people to have the same feeling he does. “I want them to first of all find out what they like to do. It is much easier to persuade others if you try to start research that is already an area of interest by everybody else. But you won’t be able to surprise people that way. On the contrary, if you try to go into an unconventional field of research, you need to persuade people first, which would require a lot of effort. But if that is what you want to do, it shouldn’t be too painful.” This is the road that Dr. Tonomura himself has been following.

Dr. Tonomura did not forget to add that researchers alone cannot bring development to science and technology. “It is necessary for the people to pay respect to those that have achieved outstanding results. When Rikidozan, the star professional wrestler in Japan back in the 1950’s, was fighting a match, every single person in the nation was watching him on the TV. If we can grow an amazing person like that in the world of science, the entire nation will turn their eyes to science. If everybody’s watching, researchers will be able to do a much better job. People’s attention and researchers’ efforts are both necessary for the development of science and technology.” One of the projects that Dr. Tonomura is paying attention to at the moment is the Super-Kamiokande project. Just like Dr. Tonomura himself, the project is aimed at pursuing the truth of the unseen. Even when succeeding in making an invisible thing visible, there is always the next unseen object to confront. Dr. Tonomura says, “As a matter of fact, I sometimes feel that observation of living organisms may also be interesting. But there are still so many compelling phenomena in the world of physics.” “If we continue with our observations using electron microscopy, we may come against a phenomenon that cannot be explained with quantum mechanics. Quantum mechanics has always been successful since it was founded, but this doesn’t mean it will keep on being successful in the future. We humans haven’t revealed everything.” Quantum mechanics came after classical mechanics. What then will come after quantum mechanics? “In order to find that out, we need to have some research to start off with. Who knows? It may be research on nanotechnology that will trigger the finding.”