Coherent Electron Beam Research Articles

Observation of quantized vortices in superconductors by electron waves

Coherent field-emission electron beams and electron holography techniques have made it possible to produce interference micrographs from which the phase distribution of the wavefunction of an electron beam transmitted through an object relative to that of a reference beam can be measured to within 1/100th of the electron wavelength. This has opened up a new way to make ultrafine measurements of material structures and of electromagnetic field distributions. Magnetic lines of force of quantized vortices in a superconducting thin film, for example, have been directly observed by electron-holographic interference microscopy. In addition, the dynamics of individual vortices interacting with pinning centers has been observed in real time by Lorentz microscopy.

Observation of quantized vortices in superconductors by electron waves

Coherent field-emission electron beams and electron holography techniques have made it possible to produce interference micrographs from which the phase distribution of the wave function of an electron beam transmitted through an object relative to that of a reference beam can be measured to within 1 100 th of the electron wavelength. This has opened up a new way to make ultrafine measurements of material structures and of electromagnetic field distributions. Magnetic lines of force of quantized vortices in a superconducting thin film, for example, have been directly observed by electron-holographic interference microscopy. In addition, the dynamics of individual vortices interacting with pinning centers has been observed in real time by Lorentz microscopy.

Coherent Electron Acceleration by Subcycle Laser Pulses

Analytic three dimensional solutions for electromagnetic waves correct even for subcyclic pulses have been obtained. It is demonstrated analytically and via 1--2/2 dimensional PIC (particle in cell) simulations that the irradiation of an intense subcyclic pulse on a thin plasma layer gives rise to a pickup of all plasma electrons on the spot, accelerates them over a small distance, and leads to a short, coherent, and ultracold electron beam.

Electron holography of p-n junctions

A wide holographic field of view (up to 15 μm in the Hitachi-HF2000) is achieved in a TEM by switching off the objective lens and imaging the sample by the first intermediate lens. Fig.1 shows the corresponding ray diagram for low magnification image plane off-axis holography. A coherent electron beam modulated by the sample in its amplitude and its phase is superimposed on a plane reference wave by a negatively biased Möllenstedt-type biprism.Our holograms are acquired utilizing a Hitachi HF-2000 field emission electron microscope at 200 kV. Essential for holography are a field emission gun and an electron biprism. At low magnification, the excitation of each lens must be appropriately adjusted by the free lens control mode of the microscope. The holograms are acquired by a 1024 by 1024 slow-scan CCD-camera and processed by the “Holoworks” software. The hologram fringes indicate positively and negatively charged areas in a sample by the direction of the fringe bending (Fig.2).

Practical aspects of hollow-cone imaging

Hollow-cone imaging (HCI) was proposed in the late 1940s in order to improve the resolution of TEM. This method reduces the influence of the spherical aberration of the objective lens on the image quality, leading to a theoretical improvement of resolution up to 100%. However, it is shown that the HCI contrast decreases with increasing cone radius and in effect, with increasing resolution. Another difficulty in applying this technique lies in the required accurate alignment of the microscope. The limitations in hollow-cone imaging are outlined and optimum imaging parameters are determined. Considering all the limitations and assuming a coherent electron beam, HCI may lead to an improvement of the resolution by 20–30% compared to the point-to-point resolution at axial illumination. The effects of the limited coherence of the electron beam upon HCI are discussed, and experimentally obtained images with a Philips CM30 ST electron microscope (where the information limit lies close to the point-to-point resolution) are shown. Image simulations show that substantial improvement in resolution can be obtained only using such a microscope fitted with a field emission electron source.

Field Emission Phenomena and Applications. Some Trials for Developing Coherent Electron Beam.

Field Emission Phenomena and Applications. Some Trials for Developing Coherent Electron Beam.

The brightest beam in science: New directions in electron microscopy and interferometry

An ultrasharp electron field-emission source, with emission region on the order of atomic size (i.e., fraction of a nanometer), produces a bright, strongly self-focused, highly coherent electron beam—indeed the brightest particle beam currently known to science. Employed in the configuration of a point-production microscope, this ‘‘nanotip’’ source facilitates low-energy electron imaging of fragile structures at atomic-scale resolution. When slightly out of focus, the microscope serves as perhaps the world’s simplest electron interferometer providing Fresnel diffraction patterns from which important information like effective source size, source brightness, and beam degeneracy can be determined. One remarkable feature of point sources is the perfect (i.e., aberration-free), lensless imaging of periodic structures at a denumerable set of focal planes with complete suppression of nonperiodic detail. The high degeneracy and coherent emission of an electron pointlike source can be exploited in new types of quantum interferometry involving correlated electrons.

The influence of the objective lens current in low magnification electron holography

In order to align an electron microscope for low magnification holography we usually completely switch off the objective lens and image the sample by the first intermediate lens. In addition, to achieve a highly coherent electron beam we highly excite the condensor lens resulting in a divergent illumination of the sample and the intermediate lens. Now negatively biasing the fiber of a Möllenstedt type biprism placed between the first an second intermediate lenses of our Hitachi HF-2000 field emission electron microscope creates two virtual sources below the back focal plane of the first intermediate lens. These two sources are necessary to form off-axis holograms. Slightly exciting the objective lens and still imaging the sample by the first intermediate lens results in two major changes in our holograms.First: Due to an electron beam less divergent or even convergent illuminating the first intermediate lens when exciting the objective lens (compare Fig. 1 to Fig.2) the angle β at which object wave and reference wave are superimposed decreases.

Studies of Mo/Si multilayers with coherent electron beams

The Mo/Si multilayer system has been used to evaluate different methods employing coherent electrons from field-emission sources, in particular nanodiffraction and high-spatial-resolution parallel electron-energy-loss spectroscopy. Nanodiffraction patterns from the individual multilayer interfaces exhibited strong streaking on the transmitted disk, directed towards the Mo layer. The streaking increased with specimen thickness and reached a saturation value of ∼7 mrad. This effect can be interpreted as due to refraction of high-energy electrons at the interface, yielding a measured mean inner potential difference between Mo and Si of 8 ± 2 V. PEELS t/λ studies show that specimen thickness profiles are variable for the same multilayer sample. Core-loss integrated intensity profiles show asymmetry in interface width of sputtered multilayers, but proble deconvolution is needed to quantify the width of the intermixed interface. The nanodiffraction and PEELS methods are compared with previous studies with off-axis electron holography, shadow imaging, and with more established incoherent imaging methods.

Phase-contrast visualization of an undecagold cluster by in-line electron holography

This paper reports the use of Fraunhofer in-line electron holography to visualize an undecagold cluster, which has been virtually invisible by conventional bright-field transmission electron microscopy (TEM), by taking advantage of the coherent electron beam of high brightness from a 200 kV cold field-emission gun and thus making the full use of phase contrast and avoiding radiation damage. Theoretical and experimental investigations of contrast of an in-line hologram show that phase contrast can be maximized by using an appropriate defocusing distance according to the wavelength of illuminating electrons and the size of the object. A parameter termed Fresnel number is found to be important in considering the optimum experimental conditions, especially when in-line electron holography is applied to weak-phase objects.

Electron holography of ferroelectric materials

Electron holography gives to the electron microscope the ability the visualize, and measure, electric and magnetic fields at high spatial resolution . The principle of the method is shown in figure (1). Some fraction of a coherent electron beam, typically produced from a cold field emission electron gun, is incident on the specimen while the rest passes through vacuum, or through some other region of the sample, to form a reference wave. An electrostatic bi-prism converges these two components to form an interferogram or ‘hologram’ at the viewing screen. The hologram consists of a fringe pattern, with a spacing determined by the potential on the bi-prism, modulated by the intensity variations of the transmitted bright-field signal. Variations in the phase of the transmitted signal are stored as shifts in the fringe pattern. The hologram can then be reconstructed, either optically or by using a computer and appropriate software, to yield two images, one - the 'Amplitude Image' - containing normal intensity contrast (bright field) information, and a second- the 'Phase Image'- which displays variations in the relative phase of the signal as changes in grey level. The phase of the transmitted beam will be shifted by an amount proportional to the mean inner potential of the specimen multiplied by the thickness, and also by the potential that the electron encounters as it travels through the sample.

Observation of buried interfaces with low energy electron microscopy.

In this Letter we show that a coherent low energy electron beam (100 eV) can be used to obtain real space images of structures and defects buried deep below the surface of the sample. The elastic strain fields of such buried structures, extending to the free surface, are found to give rise to localized phase shifts in the reflected electron waves, resulting in excellent image contrast under slight objective lens defocus conditions. We can now image the formation and evolution of buried interfaces and defects in situ, and in real time. Because of the very low electron energies used, this imaging method is nondestructive.

Observation of flux lines by electron holography

The wave nature of electrons is utilized through electron holography to directly observe magnetic flux lines, as an electron interference micrograph, with a coherent field-emission electron beam. This technique is applied to the magnetic domain structure observation of a ferromagnetic thin film and also to the direct observation of flux lines penetrating a superconducting thin film. >

NON-TRADITIONAL AHARONOV-BOHM EFFECTS IN CONDENSED MATTER

In 1959, Aharonov and Bohm proposed an elegant experiment demonstrating observability of electromagnetic potentials (or, which is the same, the non-locality of the wave function of charged particles) in quantum mechanics. This paper discusses the Aharonov-Bohm effect, based on the fundamental principles of quantum theory, as the superposition principles, the quantum character of motion of particles and locality of the interaction of a charge with an electromagnetic potential L{sub int} = j{sub {mu}}A{sup {mu}}. It is thus no wonder that the Aharonov-Bohm's paper aroused much dispute which is still ongoing. Originally, the Aharonov-Bohm effect (ABE) means the dependence of the interference pattern on the magnetic fluid flux {phi} in a Gendaken experiment on a coherent electron beam in the field of an infinitely thin solenoid. Later, however, it became common to refer to the Aharonov-Bohm phenomenon wherever the characteristics of systems under study appear to depend on the flux {phi} in the absence of electric and magnetic fields. In this sense, it was highly interesting to analyze the ABE in condensed media (the many-particle Aharonov-Bohm effect), in particular to study the dependence of the thermodynamic and kinetic characteristics, e.g., of metal on the flux. Such a problem was first discussedmore » by Byers and Yang who formulated the general theorems related to the ABE in conducting condensed media. The next important step was the work of Kulik who formulated a concrete model and calculated the flux-dependent contribution to the metal free energy and provided a first clear formulation of the requirements to reveal.« less

Electron-holographic interference microscopy

Abstract Recent developments in electron interference microscopy are introduced based on holography principles. This technique has been given a major boost due to the development of a coherent field-emission electron beam. This has facilitated the measurement of the phase distribution of an electron beam transmitted through or reflected from an object to within 1/100 of the electron wavelength. Phase distribution can be displayed as a phase contour map in an electron micrograph. There the contour fringes directly and quantitatively indicate the thickness contours of a uniform sample, magnetic lines of force of a magnetic sample and equipotential lines of an electrostatic sample. Microscopic objects or fields have made their appearance in amplified interference micrographs through the use of this technique. Specific examples are the static and even dynamic observation of magnetic fluxons penetrating a superconductor and the quantitative measurement of specimen thickness or topography in atomic dimensions.

Utilization of STEM nanodiffraction data

Abstract A scanning transmission electron microscope (STEM) designed to simultaneously record all of the diffraction data at each beam position is necessary to fully utilize the imaging potential of a STEM. However, analysis of this large quantity of data requires considerable computation that should be undertaken only with the assurance that the data were collected under near optimum conditions from the desired region of the sample. This paper develops equations for the total elastic scattering produced with a small, coherent electron beam incident on a thin sample. These equations are used as the basis for simulations which indicate that high-angle annular-dark-field (ADF) images may be used to indicate the quality of the diffraction data. When the microscope is adjusted such that the portion of the data falling within a high-angle annulus forms a real-time high-resolution image with 2.5A˚resolution or better, the simultaneously recorded nanodiffraction data are suitable for the reconstruction of images possessing 1A˚resolution or better. While the resolution of the ADF image is determined by the electron beam size, the coherent nanodiffraction data contain structural information for each beam position to the resolution limit of the diffraction pattern.

Image resolution in conventional transmission electron microscopy

The image resolution in bright-field (BF) and dark-field (DF) conventional transmission electron microscopy (TEM) is given by: r = 0.66 CS¼¾¾, where Csand ¾ are the spherical aberration coefficient of the objective lens and electron wavelength, respectively. Based on this formula, it should be possible to resolve single atoms or clusters of atoms by phase contrast imaging with a highly coherent electron beam and a properly defocused objective lens; this has been demonstrated for both BF and DF imaging. However, for most situations encountered in conventional TEM, the type of information that can be obtained about the specimen is the most important, rather than the instrumental resolution. Atomicresolution microscopy of crystalline specimens relies on phase contrast produced when two or more beams interfere to form an image and this is discussed elsewhere in this symposium. This paper discusses the contrast and resolution when either a single beam or diffuse scattering is used to form an image.

Divergent-beam holography with a 200keV FE TEM

A hologram, first described and named by Gabor (1949), permits a medium such as photographic film, which responds only to intensity, to store the complete amplitude and phase information which characterizes an electron wavefront. The hologram is formed by allowing some fraction of a coherent electron wave which has interacted with a specimen to interact again with original incident wave so as to generate an interference pattern. If the hologram is then itself illuminated by a coherent light source and optical system which mimic the original electron-optical system then a pair of images -one real and the other virtual -can be reconstructed and viewed. Because the hologram contains both the amplitude and the phase data of the wavefront, errors and distortions in either component due to aberrations in the objective lens can be corrected by optical manipulates before the image is reconstructed. With the advent of commercial field emission transmission electron microscopes capable of generating both high resolution images and highly coherent electron beams, these holographic techniques are now available as practical tools to improve TEM performance as well as to create new modes of images (Tonomura 1987).

Electron holography for magnetic property study of materials

An electron beam is often modulated, only in phase and not in intensity. When the beam passes through a magnetic field or a ferromagnetic thin film which can be regarded as pure phase objects. Therefore, great defocusing is indispensable for magnetic domain observation in Lorentz microscopy. Electron interference microscopy, however, can directly display the phase distribution of an electron beam in an in-focus electron micrograph, and consequently should provide direct information about magnetic fields.The development of a coherent field emission electron beam has opened a way to such a possibility using electron holography in which the phase-amplified phase distribution can be displayed as an interference micrograph. In addition, contour fringes have been interpreted as in-plane magnetic lines of force. Such an intuitive interpretation is possible for the following reason (Fig.1). An incident wavefront is displaced by the vector potential circulating around a magnetic field.

Electron interferometry using a coherent beam from point electron source

Using the electron holography principle, a coherent field-emission electron beam is employed to quantitatively measure to a precision of 2π/100 the phase distribution of an electron wave function transmitted through or reflected from a sample. This technique opens up the possibility of previously impossible high-precision measurements such as the quantitative measurement of magnetic lines of force in a microscopic region, and thickness measurement in atomic dimensions.