Observation of Aharonov-Bohm Effect by Electron Holography
Akira Tonomura · Tsuyoshi Matsuda · Ryo Suzuki · Akira Fukuhara · Nobuyuki Osakabe · Hiroshi Umezaki · Junji Endo · Kohsei Shinagawa · Yutaka Sugita · Hideo Fujiwara
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In one page
Akira Tonomura and his colleagues at Hitachi’s Central Research Laboratory answered a twenty-three-year argument with a photograph. The Aharonov–Bohm effect says an electron beam that never touches a magnetic field still picks up a phase shift set by the flux it encircles: what the electron responds to is the vector potential, not the field. Critics replied that the earlier tests used iron whiskers and solenoids whose flux leaked into the electron path. Tonomura’s team removed that objection by making the magnet a tiny ring of permalloy, so the flux closes on itself inside the metal, and by reading the phase with electron holography — record a hologram in a field-emission electron microscope, then reconstruct it with a helium–neon laser so that phase becomes visible contour lines. The rings produced the predicted shift, 5.5 wavelength units measured against 6.0 predicted, and the shift reversed when the magnetisation reversed. Because one contour interval is one flux quantum, the same picture also proves the leakage was far too small to fake the result.
Why it matters hereChapter 10 rests on this: the vector potential is a real, measurable influence on matter in a region where the magnetic field is zero, and it acts by controlling phase. Chapter 1 keeps it as the model of how a contested result is closed — not by argument but by an apparatus built so that the leading objection cannot apply, and by a measurement that shows the leakage itself.
What it claims
01Holographic interferometry with small toroidal ferromagnets, each forming a closed magnetic flux circuit, shows that the phase difference between two electron beams which have passed through field-free regions agrees well with the fundamental Aharonov–Bohm relation.Abstract
Settled physics02The measured phase difference is 5.5 in wavelength units against a theoretical 6.0, an agreement within 20 percent, computed from a saturation magnetisation of 9500 plus or minus 500 oersted, a film thickness of 400 plus or minus 30 ångström and a toroid width of 6400 plus or minus 500 ångström.Page 1444, the paragraph beginning ‘The photographs reveal’, with Figures 1 and 4
Settled physics03A second toroid on the same carbon film, magnetised the opposite way, retards the phase in its inner space instead of advancing it, and the number of contour lines rises in proportion to the width of the magnet; across toroids of various sizes the deviation from that proportionality was measured to be less than 1 percent.Page 1444 to 1445, Figure 5 and the following paragraph
Settled physics04The magnetic flux between two adjacent contour lines equals the flux quantum h over e whatever the electron energy, so leaking field shows up directly as contour lines escaping the toroid; the contour maps of the two magnets used here put the leakage flux below one flux quantum, too small to conceal the effect.Page 1445, the paragraph on leakage fields, with Figure 6
Settled physics05Changing the accelerating voltage between 80, 100 and 125 kilovolts changes how far the electrons penetrate the magnet appreciably but leaves the phase difference unchanged — the control that separates a genuine potential effect from any field the beam might have touched on the way through.Page 1445, the paragraph beginning ‘The different electron energy’
Settled physics06The observed interference patterns are fully explained by the Stokes vector potential, and could not be expected if the vector potential were zero everywhere outside the toroid, as Bocchieri and colleagues had proposed for a solenoid.Page 1446, closing paragraph
Settled physics
The way in
https://doi.org/10.1103/PhysRevLett.48.1443The Letter is closed access under the APS default licence, so no text is reproduced on this page. The full text was read for this sheet from the American Physical Society’s own harvest service at harvest.aps.org (the article PDF for this DOI), and every claim below is located to a passage or figure of that text. The work was done at the Central Research Laboratory of Hitachi Ltd, Kokubunji, Tokyo; the authors thank C. N. Yang for discussions at the planning stage.
How to cite it
Akira Tonomura, Tsuyoshi Matsuda, Ryo Suzuki, Akira Fukuhara, Nobuyuki Osakabe, Hiroshi Umezaki, Junji Endo, Kohsei Shinagawa, Yutaka Sugita, Hideo Fujiwara (1982) Observation of Aharonov-Bohm Effect by Electron Holography. doi:10.1103/PhysRevLett.48.1443
Where it sits in the curriculum
Scalar waves and the field behind the fieldsThe evidence ladder