Osamu Kamimura

Direct Observation of Vortex Dynamics in Superconducting Films with Regular Arrays of Defects

The microscopic mechanism of the matching effect in a superconductor, which manifested itself as the production of peaks or cusps in the critical current at specific values of the applied magnetic field, was investigated with Lorentz microscopy to allow direct observation of the behavior of vortices in a niobium thin film having a regular array of artificial defects. Vortices were observed to form regular and consequently rigid lattices at the matching magnetic field, at its multiples, and at its fractions. The dynamic observation furthermore revealed that vortices were most difficult to move at the matching field, whereas excess vortices moved easily.

Observation of Dynamic Interaction of Vortices with Pinning Centers by Lorentz Microscopy

When a magnetic field penetrates a superconductor, it forms lattices of thin filaments called magnetic vortices. If a current is applied, these vortices move if not pinned down, destroying the superconductivity. The spatiotemporal behavior of the vortices was observed in a niobium film with a square lattice of defects made by ion irradiation. The vortices formed a domain of lattices. When the intensity of the applied magnetic field was decreased, the vortices were driven out of the film across its edges; when the intensity was increased, the vortices were driven into the film. The lattice exhibited brief and intermittent flow in “rivers” along domain boundaries. The rivers did not always flow along the same paths because new lattice domain configurations emerged whenever the flow stopped.

Motion of vortices in superconductors

A dissipation-free current can be achieved in a superconductor only when tiny magnetic vortices, which penetrate the superconductor when a magnetic field is applied, are pinned down against current-induced force. To investigate the mechanism by which such vortex pinning occurs, we have made real-time observations of the onset of vortex motion in high-temperature Bi2Sr2CaCu2O8+δ(Bi-2212) superconductors.

Diffraction microscopy using 20 kV electron beam for multiwall carbon nanotubes

Diffraction microscopy with iterative phase retrieval using a 20kV electron beam was carried out to explore the possibility of high-resolution imaging for radiation-sensitive materials. Fine, homogeneous, and isolated multiwall carbon nanotubes (MWCNTs) were used as specimens. To avoid lens aberrations, the diffraction patterns were recorded without a postspecimen lens. One- and two-dimensional iterative phase retrievals were executed. Images reconstructed from the diffraction pattern alone showed a characteristic structure of MWCNTs with the finest feature corresponding to a carbon wall spacing of 0.34nm.

10-kV diffractive imaging using newly developed electron diffraction microscope.

A new electron diffraction microscope based on a conventional scanning electron microscope (SEM), for obtaining atomic-level resolution images without causing serious damage to the specimen, has been developed. This microscope in the relatively low-voltage region makes it possible to observe specimens at suitable resolution and record diffraction patterns. Using the microscope we accomplished 10-kV diffractive imaging with the iterative phase retrieval and reconstructed the structure of a multi-wall carbon nanotube with its finest feature corresponding to 0.34-nm carbon wall spacing. These results demonstrate the possibility of seamless connection between observing specimens by SEM and obtaining their images at high resolution by diffractive imaging.

Low voltage electron diffractive imaging of atomic structure in single-wall carbon nanotubes

The demand for atomic-scale analysis without serious damage to the specimen has been increasing due to the spread of applications with light-element three-dimensional (3D) materials. Low voltage electron diffractive imaging has the potential possibility to clarify the atomic-scale structure of 3D materials without causing serious damage to specimens. We demonstrate low-voltage (30 kV) electron diffractive imaging of single-wall carbon nanotube at a resolution of 0.12 nm. In the reconstructed pattern, the intensity difference between single carbon atom and two overlapping atoms can be clearly distinguished. The present method can generally be applied to other materials including biologically important ones.

Direct Evidence of the Anisotropic Structure of Vortices Interacting with Columnar Defects in High-Temperature Superconductors through the Analysis of Lorentz Images

Two types of Fresnel contrasts of superconducting vortices in a Lorentz micrograph, corresponding to pinned and unpinned vortices, were obtained by a newly developed 1 MV field-emission transmission electron microscope on a Bi 2 Sr 2 CaCu 2 O 8+δ (Bi-2212) thin specimen containing tilted linear columnar defects introduced by heavy ion irradiation. The main features of the Fresnel contrasts could be consistently interpreted by assuming that the vortices are pinned along the tilted columnar defects and by using a layered or an anisotropic model to calculate the phase shift of the electron wave. The confirmed validity of both models strongly indicates that superconducting vortices in high-critical temperature (high- T c ) layered materials have an anisotropic structure.

Optical properties of a multibeam column with a single-electron source

A novel single-column multi-electron-beam system, called a beam-split array, has been developed for a high-resolution, high-throughput lithography tool. In this system, a single electron beam is divided into 1024 beams by a multisource module (MSM) composed of an aperture array (a beam-dividing aperture), a static lens array (Einzel lenses for each divided beam), and a blanker array (BLA, blanking electrode pairs for each focused beam). The MSM is used to form multiple intermediate images of the electron source at the BLA. These images are demagnified to form final images through a projection optics consisting of a double lens doublet with a blanking aperture and deflector. To align the multiple beam paths in the MSM, aligners between these arrays are used, and the aligner conditions are determined by monitoring the blanking-aperture image. Moreover, because each beam current is about 0.1% of the total beam current on the specimen, a high-contrast transmission detection method is used to detect the electr...

Foucault imaging of superconducting fluxons

The Foucault method, one of the classical Lorentz microscopy techniques, is here investigated in order to demonstrate its applicability to in-focus observations of superconducting fluxons. As the deflections involved are of the order of 10−5−10−6 rad, low-angle techniques, high brightness, coherent illumination, and a low aberration magnetic stage are needed. The first experimental results are presented and discussed in relation with a theoretical model for the fluxon which allows the interpretation of the main features of the observed patterns.

Lorentz microscopy observation of vortices inside Bi-2212 thin films with columnar defects

Abstract We observed the arrangements of vortex lines inside Bi 2 Sr 2 CaCu 2 O 8+ δ (Bi-2212) thin films with tilted columnar defects by using Lorentz microscopy with our 1 MV holography electron microscope. It was found that while vortex lines were trapped along tilted columnar defects irrespective of the direction of the applied magnetic field at temperatures above 19 K, vortex lines stood up perpendicularly to the film plane below 12 K. This unusual behavior of vortices was attributed to the different pinning mechanism due to densely distributed atomic-size defects in Bi-2212.