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JAPANESE JOURNAL OF APPLIED PHYSICS

Vol.47, No.1 (2008) pp.11-18 : Invited Review Paper

Development of Electron Holography
and Its Applications to Fundamental
Problems in Physics
Akira Tonomura1,2,3,*
We can now utilize the wave properties ofelectrons to conduct fundamental experimentson quantum mechanics because of the development of brighter electron beams as wellas the ability directly to image the quantum
world by utilizing the phase information ofelectrons. In this paper, we describe new possibilities that have been generated by electronphase microscopy using electron microscopes
equipped with coherent yet bright electron
beams.
KEYWORDS:electron holography, Lorentz
microscopy, superconductor,
vor tex , ferromagnet, Nb,
YBCO, Bi-2212

1. Introduction
Electron holography was devised by Gaboralmost six decades ago as a means of breaking
through the resolution limit of electron microscopes.1) He invented a two-step imaging
method, holography, to break through theresolution limit caused by the absence of aberration-free lens systems in electron microscopy.
In holography, a hologram is first formed withelectrons as an interference pattern between
the object wave and a reference wave. The
hologram is then illuminated by a reference
optical wave to reconstruct the electron wavefronts as optical wavefronts.
In electron holography, the aberrationsin the electron lens system can be opticallycompensated for in its reconstruction stage.
Gabor's original approach was in-line holography, where the electron wave passing
around the object is used as a reference
wave.1) This kind of hologram is, so to speak, adefocused electron micrograph photographed
under coherent illumination, similar to the
Fresnel fringes first reported by Boersch.2)
Although the idea of holography was
first demonstrated in optical experiments
by Gabor, 1) the first experiment on elec-tron holography was carried out by Haine
and Mulvey,3) using the in-line method. Their
reconstructed image was, however, disturbed
by the Fresnel fringes that are produced by the
conjugate image, which is always formed inaddition to the reconstructed image in holography. Hibi4) obtained similar results using apointed filament he developed as a coherent
electron source.
The reconstruction of clear images, freefrom the effects of conjugate images, was first
demonstrated by Leith and Upatnieks5) using
off-axis laser holography, where the reconstructed and conjugate images could be separated into two beams traveling in different
directions. It was also demonstrated optically by DeVelis et al.6) that image reconstruction free of conjugate-image disturbances ispossible even in an in-line holography provided
that holograms are formed in the Fraunhofer
diffraction area of the object.
Encouraged by this experiment, Tonomura et al.7) demonstrated an electron versionof this Fraunhofer holography using a 100kV
electron microscope equipped with a pointed
cathode, and clear-cut images were firstobtained. This in-line holography entailed
some limitations, such as small sizes forobjects surrounded by clear spaces. However,the coherence conditions required for the illuminating electron beam are much less stringent than those for off-axis holography.
Off-axis electron holography was firstcarried out by Möllenstedt and Wahl,8) which
however had to be made in one-dimensional
imaging to make up for the poor coherenceof the electron beam. A slit-shaped electron
source was employed to form a hologramwith many carrier fringes, and an image of aone-dimensional tungsten filament was optically reconstructed.
Image formation using electron holography was thus confirmed to be feasible. The

1 Advanced Research Laboratory, Hitachi, Ltd., Hatoyama, Saitama 350-0395, Japan
2 Frontier Research System, RIKEN, Wako, Saitama 351-0198, Japan
3 Okinawa Institute of Science and Technology, Kunigami, Okinawa 904-0411, Japan* E-mail address: akira.tonomura.zk@hitachi.com(Received May 28, 2007; accepted September 7, 2007; published online January 18, 2008)        JSAP International   No.18 (July 2008)

resolution of the reconstructed images waslow (~50 Å) and no new information was
obtained by the holography. The practical realization of electron holography had to wait forthe development of a coherent field-emission
electron source, just as optical holography had
to wait for the invention of lasers.
In this paper, we provide an overview ofthe present status of electron holography, withspecial reference to its recent applications toproblems on fundamental physics and also on
technological frontiers.

2. Developments in Holography due to Advent of Coherent
Beams
We describe the historical developmentof electron holography here with advances incoherent beam technology, since a bright electron source using field emissions was the mostdecisive factor in the development of electron
holography.
Although field emissions were alreadyused in a field-emission microscope to observe
the tip surface with atomic-scale resolution
early in the 20th century,9) it was Crewe etal.10) who developed a practical field-emission
gun for the scanning electron microscope(SEM) or scanning transmission electron microscope (STEM), and greatly improved their resolutions, particularly that of STEM, reaching
down to atomic dimensions.11) This beam isalso suitable as a coherent beam for electroninterferometery; its brightness is greater thanthat of a thermionic beam by more than threeorders of magnitude, mainly because a strong
electric field on the surface of the cathode tip
produces no space-charge effect. In addition,
the energy spread of the beam is as narrow as
0.3eV when the emission current is limited to
10µA. This source was first used only for scanning microscopes, and its acceleration voltage
was limited to 30kV.

©2008 The Japan Society of Applied Physics

We started the development of a fieldemission electron beam at Hitachi in 1967
soon after our experiments on in-line Fraunhofer electron holography.7) From our experience with the holography experiments, werealized that bright electron beams, e.g., laser
beams, would be needed to apply such holographic techniques to high-resolution microscopy for practical use. In fact, the resolution of
the reconstructed images using a pointed filament4) in our early experiments7) did not evenreach that of conventional electron microscopes due to the poor coherence of electron
beam. Our original objective to overcome theresolution limit of the electron microscopes bycompensating for the aberrations in electron
lenses was by no means achieved.
We began to develop brighter field-emission electron beams in 1967 and we havecontinued the effort up to now. Accelerated
electron-beam voltages higher than 50 kV
are needed for transmission electron microscopes. However, this development was noteasy, since the electric discharge of the highvoltage in the electron gun easily destroyed
the tip. In addition, since the extremely small
electron source, typically 50 Å in diameter,
had to be immobilized to within a fraction of
the source diameter, we had to prevent eventhe slightest mechanical vibration of the tip,
the accelerating tube, or microscope column,
or the deflection of the fine beam by strayac magnetic fields. Otherwise, the inherent
brightness or luminosity of the electron beamwould deteriorate.
After ten years of work, we developed
an 80 kV electron beam,12) which was twoof the image of the tiny electron source due toaberrations during the acceleration process, e.g.,
by adding a magnetic lens to the electron gun.
Every time we increased brightness, newpossibilities opened up as shown in Table
I. Specific examples can be found in themagnetic lines of force in the microscopic
region, which were directly and quantitatively
observed in h / e flux units13) in an interference
micrograph with an 80 kV microscope. We
were able to measure the phase shifts witha precision as small as 1 / 100 of an electron
wavelength with a 250kV microscope,14) andcarried out experiments on the Aharonov–Bohm (AB) effect. We observed the real time
dynamics of vortices in metal15) and high-Tc
superconductors16) with 350 kV17) and 1MV
microscopes.18) In general, the brightness ofan electron beam is proportional to the accelerating voltage. In reality, however, the brightness of the electron beam obtained at 1MV is2 × 1010A / (cm2 ∙ ster), which is more than one
order of magnitude higher than the expectedvalue (1.2 × 109) estimated from the brightness
at 80 kV. This is due to the decrease in theaberrations realized by introducing a magnetic
lens to the accelerating region. The maximum
number of biprism interference fringes withthis beam increased from 3,000 to 11,000.19)
We settled the controversy about the existence of the AB effect (see § 3.2.2) in 1986
through a series of experiments20–22) using
electron holography without ambiguities
about leakage magnetic fluxes from solenoids
or magnets.

Lattice images with 2.4 Å spacing werereconstructed by Tonomura et al . 24) using
off-axis holography, and spherical aberrationsof the reconstructed images were compensated for using the corresponding opticalconvex lens in the optical reconstruction
stage.25) Lichte26) numerically reconstructed
the lattice fringes of carbon black using hologram carrier fringes (0.8 Å) narrower thanlattice fringes. The reconstructed-image resolution was further improved and the aberrations were corrected by Orchowski et al .27)
Further progress was made by Lichte's groupand others.
The phase distributions of the reconstructed images were successfully used toopen up new ways of hitherto inaccessible
observations on microscopic states of materials, such as visualizing the magnetic lines
of force,13) equipotential lines,28) thickness
distributions,14) and inner potentials.29) Many
noteworthy applications are now being developed, such as the observation of magnetic
domain structures in ferromagnetic materials,30–32) dopant distributions in semiconductor devices,33,34) ferroelectric materials,35)
and vortex behaviors in superconductors.16,36)

3. Applications

orders of magnitude brighter than that of
the thermal beams used then (see Table I).
Using the beam, we were able to observe
electron interference patterns directly on a
fluorescent screen for the first time, and wewere able to take as many as 3000 interference fringes on a film, whereas 300 fringes
had been the maximum up to that time. New
information that had been inaccessible withconventional electron microscopy can now be
obtained by using electron holography. Since
the first successful development of a brightfield-emission electron beam in 1979, we havecontinued to develop even brighter electron
beams. This was mainly achieved by increasing
the beams' accelerating voltages. An electron
gun with a higher energy is larger and has
thus more space to introduce additional electron-optical systems to minimize the blurring

3.1	 High-resolution microscopy

3.2.1 Single-electron build up of interference pattern

The resolution of the reconstructed
images has been greatly improved due to theadvent of the field-emission electron beams.
For example, Munch23) used a field-emission
beam to improve the resolution up to 10 Åusing the in-line Fraunhofer holography.

An electron interference pattern is formedin such a way that a single electron exists
at one time in the “double slit” apparatus.
Feynman et al.37) once referred to this type ofexperiment as “impossible, absolutely impossible to explain in any classical way, and it has

3.2	 Fundamental problems withquantum mechanics
The relative phase of an electron wavefunction can now be precisely and directly
measured, which has enabled even “thought
experiments” to be carried out on the fundamentals of quantum mechanics.

Table I. History of development of bright electron beams.

Year

Electron microscope

Brightness[A / (cm2 ∙ster)]

1968

100kV FEEM
(Thermionic electrons)

1×106

Experimental feasibility of electron holography7)

1978

80 FEEM

1×108

Direct observation of magnetic lines of force13)

1982

250 FEEM

4 ×108Conclusive experiments of AB effect22)

1989

350 FEEM

5×109

Dynamic observation of vortices in metal superconductors15)

2000

1MV FEEM

2×1010Observation of unusual behaviors of vortices in high-Tc
superconductors16)

Application

FEEM: field-emission electron microscope

JSAP International   No.18 (July 2008)        in it the heart of quantum mechanics. This
experiment has never been done in just thisway, since the apparatus would have to bemade on an impossibly small scale”.
However, these thought experiments
have now become feasible with the progressin advanced technologies.38) Electron microscopy allows small-scale magnification, andindividual electrons can be detected with aphoton-counting detector (Hamamatsu Photonics PIAS) modified to count individual electrons with almost 100% efficiency of detection. The experiments were actually carriedout with a field-emission electron microscope
equipped with both an electron biprism anda two-dimensional position-sensitive electroncounting system. As we can observe in Fig. 1,
electrons emitted from a field-emission tip aresent to the biprism. The interference patternis then enlarged by magnifying lenses and isrecorded by the electron-counting system.
Individual electrons are displayed as brightspots on a TV monitor. When there is a small
number of detected electrons, the electron
distribution appears to be quite random as canbe observed in Figs. 2(a) and 2(b). However,an interference pattern gradually emerges[Fig. 2(c)] when the number of bright spots
increases, even when the rate of arriving electrons was as low as 10 electrons / s in theentire field of the view, meaning that at most,only a single electron existed at one time. The

fig. 2.

accumulated electrons formed the
Sourceinterference pattern
as shown in Fig.
2(d), which is formed
when the two elecElectron biprismtron waves of a single
electron pass throughboth sides of the
biprism and overlap
on the detector plane.
Detector
Even a single electron can split into twoin the form of a wavefunction on both sidesof the biprism. The
fig. 1. Double-slit experiment for electrons.
two partial electron
waves then overlap
to interfere on the observation plane and form
3.2.2 Aharonov–Bohm effectan interference pattern of probability. When
The inexplicable behavior of electrons is
detected, however, the overlapped waves are
not restricted to the double-slit experiment.
observed as a single electron, never as two.
The interaction of an electron wave with elecThis can be interpreted as the measurement
tromagnetic fields also contradicts our rational,
instantly making the extended wavefunctionsclassical concept. For example, electric andcollapse into a single point.
magnetic fields are defined as forces exerted
Our experiment described in Fig. 2 hason a charged particle. However, the absence
often been cited in physics textbooks,39) andof electric and magnetic fields does not neceswas selected and awarded as The Most Beausarily mean the absence of vector potentials.
tiful Experiments by Physics World,40,41) which
In fact, a beam of charged particles can be
was shared with an electron interference
physically affected in the form of phase shifts,
experiment that uses the actual fine multiple
even when it passes through a region free ofslits carried out by Jönsson in 1961.42)
magnetic fields outside an infinitely long sole-

Buildup process of electron interference pattern. Number of electrons: (a) 8; (b) 200; (c) 6,000; (d) 140,000.
The video clip can be seen in http://www.hqrd.hitachi.co.jp/global/fellow_tonomura.cfm

        JSAP International   No.18 (July 2008)

noid and is, therefore, not subjected to any
forces. Aharonov and Bohm43) attributed thiseffect to vector potentials exciting even in
the field-free region outside the solenoid. The
reality of vector potentials has been greatly
disputed over the past century.44) When vector
potentials were extended to gauge fields
particularly in the late 1970s, and regarded
as a fundamental physical entity in theories
unifying all fundamental forces in nature,
the AB effect received much attention as it
directly indicated the gauge principle.45) The
existence of the AB effect then began to bequestioned.46) The AB effect has been controversial ever since it was predicted. Until thelate 1970s, however, discussions were focusedon theoretical interpretations of the AB effect.
The experiments carried out in the 1960s weregenerally believed to demonstrate that the AB
effect existed.
Bocchieri and Loinger47) claimed in 1978
that the AB effect did not exist. They asserted
that the AB effect is actually gauge-dependent and is a purely mathematical concoction.
According to their analysis, a gauge func-

Invited Review Paper JAPANESE JOURNAL OF APPLIED PHYSICS

shift becomes π (mod 2π ). The phase shift is
0 for an even number of vortices. Therefore,
the occurrence of flux quantization can beused to confirm that the niobium layer actuallybecame superconductive, that the superconductor completely surrounded the magnetic
flux, and that the Meissner effect prevented
any flux from leaking out.

3.3	 Imaging microscopic objects
using electron phase information

fig. 3. Conclusive experiment of AB effect. (a) Interference pattern; (b) schematic of a sample; (c) scanning electron micrograph of toroidal ferromagnet. Electron waves passing through inside and outside the toroidal magnet are phase-shifted by
π by the quantized magnetic flux of h / (2e) though the waves never touch the magnetic fields.

tion can be chosen so that vector potentials
completely vanish outside an infinite solenoid:
consequently, there is no AB effect.47,48) They
also asserted that the Schrödinger equation
can be replaced by a set of nonlinear differential equations called hydrodynamical equations , which contain only field strengths E
and B. There is therefore no room for the AB
effect.
They also expressed doubts from experimental viewpoints about the existence of theAB effect:48) they claimed that the interference experiments must have been affected byleakage fields from solenoids or whiskers.
We carried out a series of experiments toclarify the ambiguities raised in the controversy, and we here introduce our experiment,22) which is considered to be the mostconclusive one.
We used a toroidal ferromagnet insteadof a straight solenoid. An infinitely long solenoid is experimentally unable to be attained,
but an equally ideal geometry can be achievedby the finite system of a toroidal magnetic
field. Furthermore, the toroidal ferromagnet
we used was covered with a superconducting
niobium layer to completely confine themagnetic field within the toroid.
An electron wave was incident to thetiny toroidal sample fabricated by using themost advanced lithography techniques, andthe phase difference between the two waves
passing through the hole and around thetoroid was measured in the form of an interferogram. If the phase difference is present,
it should be given by e / –h times the magnetic
flux enclosed by the two waves.
Although various magnetic flux values
were used in the measurement, the phasedifference was always either 0 or π . The
conclusions we drew are now obvious. The
photograph in Fig. 3 indicates that a relative
phase shift of π is produced even when themagnetic fields are confined within the superconductor and shielded from the electron
wave. This clearly demonstrates that there is
an AB effect. An electron wave is physicallyaffected by the vector potential.
The quantization of the relative phaseshift, in this experiment, either 0 or π , provedthat the niobium layer surrounding themagnet actually became superconductive.
When a superconductor completely surrounds
a magnetic flux, the flux is quantized to an
integral multiple of quantized flux, h / 2e .
When an odd number of vortices are enclosed
inside the superconductor, the relative phase

The AB effect can also be used to observe
the microscopic distributions of electromagnetic fields within a material. More specifically,
the thickness distribution within a specimenof a homogenous material can be observed as
thickness contours in the interference micrograph obtained by electron holography.49) Thisis because the phase of an electron wave inthis case is shifted by the line integral of theinner potential within the specimen along theelectron trajectory when the electron wave
passes through it. Although phase shifts can,in general, be detected from standard interference patterns “with” a precision of only 2π / 4,
the precision can be improved to 2π / 100 byusing a phase-amplification technique peculiar to holography. In fact, this technique has
allowed us to the detect changes in thickness
due to monatomic steps14) and carbon nanotubes.50)

3.3.1 Magnetic lines of force
Phase shift is produced by vector potentials. When the phase distribution is displayed
as an interference micrograph, the contour

Magnetization

0.1µm
(a)

(b)

fig. 4. Cobalt fine particle. (a) Schematic diagram; (b) interference micrograph. Only the triangular outline of this particle
can be observed by electron microscopy. However, two kinds of contour fringes appear in its interference micrograph:narrow fringes parallel to the edges indicate the thickness contours, and circular fringes in the inner region indicate in-planemagnetic lines of force.

JSAP International   No.18 (July 2008)        fringes indicate the magnetic lines of force inthe magnetic flux units of h / e in the case ofpure magnetic fields.
There is an example observation ofmagnetic lines of force inside a ferromagnetic
fine particle in Fig. 4. Narrow fringes parallelto the edges indicate the thickness contours.
The circular fringes in the inner region indicate
magnetic lines of force, since the thickness is
uniform there.

3.3.2 Vortices in superconductors
Vortices inside a superconducting thin film
can be visualized as black-and-white spots in a
defocused image, or a Lorentz micrograph.15)
When the film is tilted and a magnetic field
is applied, thus producing vortices, electrons
passing through the vortices in the film arephase-shifted, or deflected, by the vortices'magnetic fields. The vortices can be observedby simply defocusing the electron microscopic
image. That is, when the intensity of electrons is observed in an out-of-focus plane, thephase change is transformed into the intensity change and a vortex appears as a pair ofbright and dark contrast features.
We can thus observe the dynamics ofvortices in real time using the Lorentz microscopy. Observations include the behaviors ofvortices at pinning centers and surface steps
under various sample temperature and appliedmagnetic field conditions. In fact, vortices
move in interesting ways as if they were livingorganisms.
There is an interesting example in Fig.
5, where two kinds of vortex images, whose
contrasts are reversed, appear in a single fieldof view. They are actually vortices and anti-

vortices produced in a niobium thin film byapplying a magnetic field of 100G applied to
the film and by reversing the polarity of the
field. The original vortices would like to exit
the film, but cannot immediately do so since
they are pinned by defects, whereas the oppositely oriented vortices begin to penetrate
into the film from its edges. Where the two
streams of vortices and antivortices collide
head-on, the heads of the vortex–antivortex
of vortex streams annihilate each other. The
direct observation of this pair annihilation ofvortices can simulate that of particles and antiparticles, since vortices are elementary particles in superconductors in that they cannot be
divided any further.
High-Tc superconductors have long been
expected to be used in practice, but their low
critical currents have impeded the process.
Their critical current is low because of theirhigh-temperature operation and their layered
structure of materials, both of which enable
the vortices to move easily. We developed a
1MV field-emission electron microscope18) toinvestigate the behavior of vortices in high-Tc
superconductors. The 1 MV electrons wererequired to observe the vortices so that theelectrons can penetrate a film thicker than themagnetic radius (penetration depth) of vortices
in high-Tc superconductors. We observed theinternal behavior of vortices inside high-T c
Bi-2212 (Bi2Sr2CaCu2O8+δ ) thin films with thismicroscope.
Columnar defects were produced by irradiating the sample with high-energy heavyions, which are considered to be optimal
pinning traps for vortices in layered structurematerials. As shown in the electron micro-

fig. 5. Annihilation of vortices and antivortices in thin film of niobium. (a) Before annihilation; (b) after annihilation. When the magnetic field applied to the film is suddenly
reversed, some vortices remain at defects, whereas others begin to leave them. Antivortices begin to move in from the edges of the film. Where streams of vortices and antivortices collide head-on, the vortex–antivortex pairs of the heads of the two streams
annihilate each other.

        JSAP International   No.18 (July 2008)

graph in Fig. 6(a), they are produced in
Bi-2212 films as tilted, which can be observed
as tiny black lines. When these images aredefocused, they are blurred and eventuallydisappear completely by spreading out. Whenthey are defocused even further, however,vortex images appear, since they are producedby the phase contrast. The resulting Lorentz
micrograph of vortices is shown in Fig. 6(b).
The micrograph reveals two different
kinds of images, circular and elongated. The
elongated images indicated by the arrows in
the micrograph are only produced at the locations of the columnar defects and correspondto vortices trapped along the tilted columns.
We confirmed the validity of this interpretation in the simulation. The circular images
are produced in regions without defects, andtherefore correspond to vortices perpendicularly penetrating the film. This has enabled
us to use the vortex images to investigate
whether vortices are trapped or not undervarious conditions.51)

3.3.3 Unusual behaviors of vortices inhigh-Tc superconductors
Vortices usually form a closely packed
triangular lattice.52) This occurs even in anisotropic high-Tc superconductors, as long as themagnetic field is directed along the anisotropy c-axis. When the magnetic field is steeply
tilted away from the c -axis, however, Bitter
images reveal that the vortices no longer forma triangular lattice. Instead, they form arrays oflinear chains along the direction of the tilted
field as observed in YBCO (YBaCu3O7.8),53) oralternating domains of chains and triangularlattices as in Bi-2212.54,55) While the chain

fig. 6. Comparison of columnar-defect image and vortices in Bi-2212 thin film. (a) Electron
micrograph; (b) Lorentz micrograph. Some vortices are trapped at columnar defects andothers are untrapped. The images of untrapped vortex lines perpendicular to the film plane
are circular spots having bright and dark regions. Vortex images located at the positions ofcolumnar defects are elongated spots with lower contrast indicated by the arrows, since these
vortex lines are trapped at columnar defects tilted at 70°.

Invited Review Paper JAPANESE JOURNAL OF APPLIED PHYSICS

B

2µm

(a)

(b)

Tilted Vortex

(c)

(d)

fig. 7. Lorentz micrographs of vortices in YBCO film sample at tilted magnetic fields (T = 30K; Bp = 3G). (a) θ = 75°, (b) θ =82°, (c) θ = 83°. (d) Schematic of tilted vortex lines. When the tilt angle θ becomes larger than 75°, the vortex images begin
to elongate and, at the same time, form arrays of linear chains.

state in YBCO can be explained by the tiltingof vortex lines within the framework of theanisotropic London theory, the chain-latticestate in Bi-2212 has long been an objectof discussion. For example, Grigorieva andSteeds55) concluded from both Bitter observations that the chain vortices and lattice vortices
are both tilted, but in different directions. On
the other hand, Huse56) suggested two sets ofvortices, one set running parallel to the layers
and the other set running normal to the layers.
However, this model was not able to explain
why the two perpendicular vortices cross eachother to form stable chains, since no interaction takes place between the two perpendicular magnetic fields.
In 1999, Koshelev57) proposed a solution
for the chain-lattice state; Josephson vortices
penetrate between the layer planes and thepancake vortices that perpendicularly intersect
the Josephson vortices form chains; the rest of
the vortices form triangular lattices. However,
Koshelev considered the second-order approximation and determined that there wasenergy reduction in this vortex arrangementby assuming that a vertical vortex line winds
slightly in opposite directions just above andbelow the crossing Josephson vortex as aresult of the interaction with the Josephson
vortex; the circulating supercurrent of theJosephson vortex exerts Lorentz forces on thevertical vortex line.
No direct evidence for such mechanisms,
however, was found through experiments
because of the lack of methods of directly
observing the arrangements of vortex lines
inside superconductors. Lorentz microscopy
with our 1MV electron microscope was used
to determine whether the vortex lines in the
chain states inside high-T c superconductors
were tilted or not.
We found that vortex lines in YBCO aretilted together in the direction of the appliedmagnetic field, as is evident from the Lorentz

fig. 8. Series of Lorentz micrographs of vortices in field-cooled Bi-2212 film sample whenmagnetic field Bp perpendicular to layer plane begins to be applied and increases at fixed in-planemagnetic field of 50G at T = 50K. (a) Bp = 0; (b) Bp = 0.2G; (c) Bp = 1G.

micrographs in Fig. 7, where the vortex
images became more elongated and formed
linear chains together as the tilting angle ofthe magnetic field increased. In the case of
Bi-2212, our observations by Lorentz microscopy revealed that neither chain vortices nor
lattice vortices tilted, but both stood perpendicular to the layer plane.58) In our present
experiment, we were not able to detect theslight windings of the pancake vortex lines
Koshelev predicted. If vortex lines were tilted
at an angle comparable to that of the appliedmagnetic field, the vortex images in Fig. 8
should have been elongated.
Our finding that both chain vortices andlattice vortices in Bi-2212 stand straight almost
perpendicular to the film plane and do not tilt
is clear evidence of the Koshelev mechanism.
Although the clearest evidence for this model
would of course be given if Josephson vortices
were observed along chain vortices. However,
the magnetic field of a Josephson vortex
extends widely between the layers, therefore
making it difficult to detect with our method.
When a magnetic field is applied parallelto the layer plane, no vertical vortices areproduced; therefore, no vortex images can beobserved in Fig. 8(a). Even though Josephson
vortices parallel to the layer plane should exist,
these vortices cannot be observed by Lorentz
microscopy because of the wide distributionof the vortex magnetic field. When the verticalmagnetic field Bp increases, vertical vortices
begin to appear along straight lines indicatedby the white arrows in Fig. 8(b), which areconsidered to be determined by Josephson
vortices. Since vortices are arranged along
straight lines, we could find no other reason
for the production of chain vortices otherthan assuming that vertical vortices crossing
Josephson vortices form chains. Above B p= 1G, vertical vortices also appear between
chain vortices, as shown in (c).
The vortices do not form a closely packed
triangular lattice, but they are located along
straight lines [Figs. 8(b) and 8(c)]. The reason
they appear along straight lines is that perpendicular vortices tend to line up densely along
the Josephson vortices.

JSAP International   No.18 (July 2008)        Invited Review Paper JAPANESE JOURNAL OF APPLIED PHYSICS

4. Conclusions
Some experiments that were once
regarded as “thought experiments” cannow be carried out because of recent developments in advanced technologies such as
coherent electron beams, highly sensitive elec-1 ) D. Gabor: Proc. R. Soc. London, Ser. A 197(1949) 454.
2 ) H. Boersch: Naturwissenschaften 28 (1940) 711[in German].
3 ) M. E. Haine and T. Mulvey: J. Opt. Soc. Am. 42(1952) 763.
4 ) T. Hibi: J. Electron Microsc. 4 (1956) 10.
5 ) E. N. Leith and J. Upatnieks: J. Opt. Soc. Am. 52(1962) 1123.
6 ) J. B. DeVelis, G. B. Parrent, and B. J. Thompson:
J. Opt. Soc. Am. 56 (1966) 423.
7 ) A. Tonomura, A. Fukuhara, H. Watanabe, andT. Komoda: Jpn. J. Appl. Phys. 7 (1968) 295.
8 ) G. Möllenstedt and H. Wahl: Naturwissenschaften 55 (1968) 340 [in German].
9 ) E. W. Müller: Z. Phys. 106 (1937) 541 [in German].
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N. Osakabe, H. Umezaki, J. Endo, K. Shinagawa,tron detectors, and photolithography. In addition, the wave nature of electrons can now
be utilized to observe microscopic objects
that were previously unobservable. Examples
are the quantitative observation of both themicroscopic distribution of magnetic lines

of force in h / e units and the dynamics ofquantized vortices in superconductors. Thismeasurement and observation technique is
expected to play an important role in future
research and development in nanoscience andrelated technology.

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Akira Tonomura is a fellow of Hitachi, Ltd., a group director of RIKEN, anda principal investigator of Okinawa Institute of Science and Technology. He
graduated from the University of Tokyo, Department of Physics, in 1965
and joined Hitachi, Ltd. Since then, he has been engaged in experimental
researches on electron physics using electron microscopes. From 1973 hestayed for one year at Tübingen University to research on electron interferometry. He served concurrently as the director of the national “Electron
Wavefront Project” (ERATO) from 1989 to 94. He was awarded the Nishina
Memorial Prize in 1982, the Japan Academy Prize and Imperial Prize in 1991,
and the Benjamin Franklin Medal in physics in 1999. In 2000, he was elected10        JSAP International   No.18 (July 2008)as a foreign associate of the National Academy of Sciences in the UnitedStates. Since 2005, he has been also elected as a member of Science Council of Japan. He is now a member of the Japan Academy.
He is recognized for his pioneering work in the new field of electron holography which was made possible by the development of “coherent” fieldemission electron beams. This method is used for the direct observation ofboth microscopic electromagnetic fields and magnetic vortices in superconductors, and the verification of fundamental phenomena in quantum mechanics such as Aharonov–Bohm effect and single-electron build-up of aninterference pattern.