PROFILE Profile of Akira Tonomura ne of the most useful scien- lines of force and the electric lines of tific instruments of the 20th equipotentials. century, the electron micro- ‘‘The improved electron microscope scope, continues to be used could also use the phase information to Oworldwide to examine the atomic world. quantitatively record minute magnetic The first transmission electron micro- fields inside a sample that are otherwise scopes were developed in the 1930s and undetectable,’’ he says. Traditional tech- began seeing institutional use in the niques use the information about the 1940s. In the 1970s, the microscope was intensity distribution of electrons, developed further into a device that whereas in Tonomura’s method, the could record not just the intensity of the phase information of electrons is used in electrons, but their phase as well, by addition to the intensity distribution, using coherent field-emission electron which has independent information. beams. Physicist Akira Tonomura, ‘‘Therefore, using this technique, we can elected to the National Academy of Sci- observe the material state in addition to ences in 2000 as a Foreign Associate, material structures down to atomic di- was responsible for much of this techno- mensions,’’ he says. ‘‘This technique is logical development. useful for magnetism. We can directly Transmission electron microscopes observe microscopic magnetic lines of beam electrons through a thin sample of force.’’ Tonomura explains that his material and record changes in their method can be used to observe the be- trajectories upon passage. The intensity havior of vortices in superconductors, or the particle and wave natures of elec- of the electrons that pass through are Akira Tonomura recorded and assembled into an image. trons when looking at the fundamentals Because of wave͞particle duality, the of quantum mechanics. electrons have a wavelength that can be In 1968, Tonomura and his colleagues used to probe the sample, as with a light created the world’s first electron holo- Seeking Aharonov–Bohm microscope. However, because the elec- gram (3), for which Tonomura later won Tonomura’s enhanced electron micro- trons have a much shorter wavelength the Setoh Prize from the Japanese Soci- scope technique came to the forefront than a photon of light, much higher res- ety of Electron Microscopy in 1980. The when it was used to search for the olutions are possible as compared with resolution for this technology was not as Aharonov–Bohm (AB) effect. In 1959, light microscopy. high as current electron microscopes, Yakir Aharonov and David Bohm pro- Tonomura invented a technique to and he temporarily set aside that path posed that a moving electron can have produce more coherent electron beams to concentrate on scanning electron mi- its phase altered by the vector potential for greater accuracy and to precisely croscopes equipped with field-emission of the electromagnetic field of a nearby record electron phases, providing even electron guns, which provide a source object, without actually encountering the more atomic information than previ- for electrons. During a fellowship at object or its magnetic field (5). James ously possible (1). Such phase informa- the University of Tu¨bingen (Tu¨bingen, Clerk Maxwell used electromagnetic tion, when added to traditional pictures Germany) in 1973 and 1974, Tonomura potentials as physical quantities in his of intensity, generates an electron holo- worked with Gottfried Mo¨llenstedt, who union of electricity and magnetism, but gram. This technique of electron holog- was the first researcher to observe elec- the potentials came to be regarded raphy has opened up the ‘‘pico-world,’’ tron interference patterns by developing merely as mathematical auxiliaries. helping to reveal and study various phys- electron biprism interferometers. In 1981, when Tonomura was plan- ical quantum phenomena. In his Inaugu- Tonomura returned to Hitachi in 1974 ning to make decisive experiments con- ral Article in this issue of PNAS (2), and turned to developing brighter elec- cerning the existence of the AB effect, Tonomura reviews the development of tron sources through field-emission guns he wrote a letter to Chen Ning Yang at more coherent electron beams and de- in order to increase the practicality of the State University of New York, scribes the direct observation of theoret- electron holography. This research led Stony Brook, a complete stranger to ical quantum effects and the motion of to his team’s development of a high- Tonomura at that time. He asked Yang magnetic vortices in superconductors, coherence electron microscope (4). The whether this kind of experiment in fun- as afforded by these more powerful images Tonomura and his group were damental physics was worth doing. ‘‘To microscopes. producing by 1978, which recorded in- my surprise, just one month after that, tensity and phase information, were he was visiting Tokyo University and Electron Holography to the Fore detailed enough to challenge conven- kindly visited our laboratory and dis- Tonomura graduated from the Univer- tional electron microscopes. The key to cussed our planning for the experi- sity of Tokyo in 1965 with a bachelor’s the resolution was the high coherence ments,’’ says Tonomura. ‘‘We were very degree in physics and has spent most of of the electron beam, which was two encouraged by his passion and enthusi- his career in Japan, working at Hitachi orders of magnitude above conventional asm for the experiment. I admired his Laboratories. Joining Hitachi soon after sources. When the electron wave passes passion for physics, warm and good per- graduating and working first in the through a sample, the phase is modu- sonality, and his ways of explaining Kokubunji area of Tokyo and then later lated by the sample’s electrostatic field in the Hatoyama area of Saitama, Ja- or magnetic field. If there is a magnetic This is a Profile of an elected member of the National pan, Tonomura has studied electron- or electrostatic field inside the material, Academy of Sciences to accompany the member’s Inaugural beam physics and electron microscopy the phase is shifted and can be used to Article on page 14952. for four decades. detect and visualize the magnetic field © 2005 by The National Academy of Sciences of the USA www.pnas.org͞cgi͞doi͞10.1073͞pnas.0506215102 PNAS ͉ October 18, 2005 ͉ vol. 102 ͉ no. 42 ͉ 14949–14951 Downloaded by guest on September 29, 2021 physics in easy terms for laymen like as particles hitting the detector one by 49.5 pm, both world records. A con- me,’’ he says. Their friendship has con- one at seemingly random positions at ventional 1-MeV electron microscope tinued to this day. first. with a thermal electron beam has a The experiment that Tonomura and brightness of 108 AcmϪ2strϪ1 and his colleagues designed to detect the AB Swirling Vortices of Superconductors 100-pm lattice resolution. effect used a toroidal ferromagnet with In the 1990s, Tonomura began to study In his PNAS Inaugural Article (2), a completed magnetic circuit. If the AB magnetic vortices in superconductors Tonomura describes the use of his effect existed, two parallel streams of because of their physical properties, the 1-MeV electron microscope to record electrons should generate a phase differ- understanding of which is needed to de- detailed real-time observations of vor- ence that shows up in their interference velop practical superconducting applica- tices inside superconductors, revealing pattern as one stream passes outside of tions. He and his group first looked at unusual behavior. ‘‘The vortices move the ferromagnet and one stream passes very cold metal superconductors and around like living creatures,’’ he says. through its central hole. This difference later at high-temperature superconduc- His team has seen the vortices drift could be recorded by electron hologra- tors. Superconductors develop swirling through a superconductor with a lattice phy. Tonomura and his groups indeed magnetic vortices, which can be tempo- of point defects. The vortices can be- found that the interference pattern rarily ‘‘pinned’’ or trapped at defects come trapped at these defects and shifted, demonstrating the existence of and impurities. The vortices are affected then, when these locations are filled, the AB effect (6). The existence of the by currents, externally applied magnetic fill the spaces in between. His team AB effect provided strong support for fields, and temperature. has also observed many more vortices the popular gauge theories of the day. Using electron holography, Tonomura with opposite polarizations than ex- Not all physicists were convinced by observed the magnetic field lines ema- pected by theory. this experiment, however. So Tonomu- nating outside the superconductor Because vortices and antivortices col- ra’s team designed another test, again surface from vortices inside lead super- lide and merge, releasing small bursts with Yang’s advice. This time, they of thermal energy, Tonomura suggests ␮ conductors in 1989 (9). In 1992, he used fabricated a 6- m-wide toroidal ferro- Lorentz microscopy, where phase shifts that this phenomenon may be useful magnet coated with niobium. At 5 K, due to vortices are transformed into in- for studying matter–antimatter colli- niobium is a superconductor, and be- tensity variations by image defocusing, sions because the pair-annihilation pro- cause of the Meissner effect, the mag- to dynamically observe vortices inside cess of vortices and antivortices can be netic field is completely contained observed in real time. ‘‘They are ele- thin films of superconducting niobium within the toroid, but any vector poten- mentary particles in the sense they (10). The key to the latter observation tial would still exist outside the toroid. cannot be divided into two,’’ he says. was his team’s development of a 350-kV Again, in 1986, Tonomura was able to These mergers are undetectable by electron holography microscope, the show a displacement of the interference macroscopic magnetic field measure- bright beam of which had the required fringes, conclusively demonstrating that ments, because the opposite polariza- the AB effect was real (7).