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176 results on '"Nagai, Takuro"'

1. Atomic-Scale Observation of Moire potential in Twisted Hexagonal Boron Nitride Layers by Electron Microscopy

Author

Mishima, Rina, Nagai, Takuro, Segawa, Hiroyo, Ehara, Masahiro, and Uchino, Takashi

Subjects
Condensed Matter - Materials Science, Condensed Matter - Mesoscale and Nanoscale Physics
Abstract

Moire superlattices (MSLs) are an emerging class of two-dimensional functional materials whose electronic states can be tuned by the twist angle between two van der Waals layers and/or the relative placement of the layers. The intriguing properties of MSLs are closely correlated to the moir\'e potential, which is the electrostatic potential induced by interlayer coupling. Intensive efforts have been made to understand the nature and distribution of the moire potential by using various experimental and theoretical techniques. However, the experimental observation of the moir\'e potential is still challenging because of the possible presence of the surface and/or interlayer contaminants. In this work, we develop a method to obtain hexagonal boron nitride (hBN) nanolayers (with or without twist) using a specially designed chemical exfoliation technique. The resulting hBN nanolayers are atomically clean and strain free, hence providing ideal MSLs for the investigation of their moir\'e potential. Aberration-corrected high resolution transmission electron microscopy measurements on the twisted hBN nanolayers allow us to observe moir\'e diffraction spots in Fourier space. Then, the moire potential is reconstructed by the inverse fast Fourier transform of the moire diffraction spots. It has been revealed that the local interlayer atomic overlap plays a decisive role in determining the periodicity and distribution of the moir\'e potential, as supported by density functional theory calculations. This work not only provides a general strategy to observe the moire potential in MSLs, but it also expands the application of electron microscopy to the further study of MSLs with atomic resolution., Comment: 30 pages, 10 figures and 12 supporting figures

Published
2024

3. Tunable N\'eel-Bloch magnetic twists in Fe3GeTe2 with van der Waals structure

Author

Peng, Licong, Yasin, Fehmi S., Park, Tae-Eon, Kim, Sung Jong, Zhang, Xichao, Nagai, Takuro, Kimoto, Koji, Woo, Seonghoon, and Yu, Xiuzhen

Subjects
Condensed Matter - Materials Science
Abstract

The advent of ferromagnetism in two-dimensional (2D) van der Waals (vdW) magnets has stimulated high interest in exploring topological magnetic textures, such as skyrmions for use in future skyrmion-based spintronic devices. To engineer skyrmions in vdW magnets by transforming Bloch-type magnetic bubbles into N\'eel-type skyrmions, the heterostructure of heavy metal/vdW magnetic thin film has been made to induce interfacial Dzyaloshinskii-Moriya interaction (DMI). However, the unambiguous identification of the magnetic textures inherent to vdW magnets, e.g., whether the magnetic twists (skyrmions/domain walls) are N\'eel- or Bloch-type, is unclear. Here we demonstrate that the N\'eel- or Bloch-type magnetic twists can be tuned in the vdW magnet Fe3GeTe2 (FGT) with/without interfacial DMI. We use an in-plane magnetic field to align the modulation wavevector q of the magnetizations in order to distinguish the N\'eel- or Bloch-type magnetic twists. We observe that q is perpendicular to the in-plane field in the heterostructure (Pt/oxidized-FGT/FGT/oxidized-FGT), while q aligns at a rotated angle with respect to the field direction in the thin plate by thinning bulk FGT. We find that the aligned domain wall twists hold fan-like modulations, coinciding qualitatively with our computational results.

Published
2021
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20. Real-Space Observation of Topological Defects in Extended Skyrmion-Strings

Author

Yu, Xiuzhen, primary, Masell, Jan, additional, Yasin, Fehmi S., additional, Karube, Kosuke, additional, Kanazawa, Naoya, additional, Nakajima, Kiyomi, additional, Nagai, Takuro, additional, Kimoto, Koji, additional, Koshibae, Wataru, additional, Taguchi, Yasujiro, additional, Nagaosa, Naoto, additional, and Tokura, Yoshinori, additional

Published
2020
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24. Tunable Néel–Bloch Magnetic Twists in Fe3GeTe2 with van der Waals Structure.

Author

Peng, Licong, Yasin, Fehmi S., Park, Tae‐Eon, Kim, Sung Jong, Zhang, Xichao, Nagai, Takuro, Kimoto, Koji, Woo, Seonghoon, and Yu, Xiuzhen

Subjects
MAGNETIC films, MAGNETIC fields, SKYRMIONS, THIN films, HEAVY metals
Abstract

The advent of ferromagnetism in 2D van der Waals (vdW) magnets has stimulated high interest in exploring topological magnetic textures, such as skyrmions for use in future skyrmion‐based spintronic devices. To engineer skyrmions in vdW magnets by transforming Bloch‐type magnetic bubbles into Néel‐type skyrmions, a heavy metal/vdW magnetic thin film heterostructure has been made to induce interfacial Dzyaloshinskii–Moriya interaction (DMI). However, the unambiguous identification of the magnetic textures inherent to vdW magnets, for example, whether the magnetic twists (skyrmions/domain walls) are Néel‐ or Bloch‐type, is unclear. Here we demonstrate that the magnetic twists can be tuned between Néel and Bloch‐type in the vdW magnet Fe3GeTe2 (FGT) with/without interfacial DMI. We use an in‐plane magnetic field to align the modulation wavevector q of the magnetizations in order to distinguish the Néel‐ or Bloch‐type magnetic twists. We observe that q is perpendicular to the in‐plane field in the heterostructure (Pt/oxidized‐FGT/FGT/oxidized‐FGT), while q aligns at a rotated angle with respect to the field direction in the FGT thin plate thinned from bulk. We find that the aligned domain wall twists hold fan‐like modulations, coinciding qualitatively with our computational results. [ABSTRACT FROM AUTHOR]

Published
2021
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25. Cross‐sectional structural characterization of the surface of exfoliated HOPG using HRTEM‐EELS.

Author

Miyazawa, Kun'ichi, Nagai, Takuro, Kimoto, Koji, Yoshitake, Masaru, and Tanaka, Yumi

Subjects
*SCANNING transmission electron microscopy, *HIGH resolution electron microscopy, *ELECTRON energy loss spectroscopy, *SURFACE analysis, *ATMOSPHERIC oxygen, *FOCUSED ion beams
Abstract

We analyzed the surface atomic structure of highly oriented pyrolytic graphite (HOPG) substrate exfoliated with adhesive tape, using high‐resolution transmission electron microscopy and scanning transmission electron microscopy‐electron energy‐loss spectroscopy (STEM‐EELS). The surface step height of the exfoliated HOPG substrate was determined using high‐angle annular dark‐field‐scanning transmission electron microscopy (HAADF‐STEM) images and the depth profiles of the EELS spectra of a cross‐sectioned thin foil specimen prepared via focused ion beam milling. The exfoliated surface of the HOPG substrate presented disordered and curved graphene layers. The STEM‐EELS measurements indicated that upon exfoliation, the surface of the HOPG substrate reacted with atmospheric water and oxygen molecules. [ABSTRACT FROM AUTHOR]

Published
2021
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28. A Rhombic Dodecahedral Honeycomb Structure with Cation Vacancy Ordering in a γ-Ga2O3 Crystal

Author

Mitome, Masanori, Kohiki, Shigemi, Nagai, Takuro, Kurashima, Keiji, Kimoto, Koji, and Bando, Yoshio

Subjects
Cation vacancy ordering, Condensed Matter::Materials Science, defect spinel structure, Rhombic dodecahedral honeycomb, γ-Ga2O3, Epitaxial growth
Abstract

The crystal structure of a γ-Ga2O3 layer grown epitaxially on an MgO substrate by a vapor phase transport method was investigated by transmission electron microscopy, electron diffraction, and scanning transmission electron microscopy with aberration correctors. Some forbidden reflections were excited in electron diffraction patterns by double reflection from the vicinity of the substrate interface. Phase boundaries are observed in atomic column images using high-angle annular dark field images. A structure model is proposed to explain the experimental results. Cation vacancy ordering is introduced in the structure model to distort the γ-Ga2O3 crystal lattice along one axis and reduce the lattice mismatch with the substrate. Some grains are formed and alter the directions to reduce the distortion for the other axis. The grains are stacked with {110} phase boundaries and form a rhombic dodecahedral honeycomb. The rhombic dodecahedral honeycomb structure model with cation vacancy ordering is stabilized by the lattice mismatch between the γ-Ga2O3 crystal and the MgO substrate, and it disappears at a depth of 170 nm from the interface.

Published
2013

29. Spinel-to-Ca[Fe.sub.2][O.sub.4]-type structural transformation in Li[Mn.sub.2][O.sub.4] under high pressure

Author

Yamaura, Kazunari, Huang, Qingzhen, Lianqi Zhang, Takada, Kazunori, Baba, Yuji, Nagai, Takuro, Kosuda, Kosuke, Matsui, Yoshio, and Takayama-Muromachi, Eiji

Subjects
Lithium compounds -- Structure, Lithium compounds -- Chemical properties, High pressure chemistry -- Research, Chemistry
Abstract

A structural transformation of the Li[Mn.sub.2][O.sub.4] spinel is reported, by which the spinel structure is found to transform under high pressure of 6GPa, during heating at 1100 degrees Celsius or above. The compound is identified to be the polycrystalline Ca[Fe.sub.2][O.sub.4] (calcium ferrite) type, is 6% denser than the spinel, and exhibits semiconducting-like characteristics above 5 K.

Published
2006

33. HRTEM‐EELS cross‐sectional structural analyses of glassy carbon substrate irradiated by platinum ions using a coaxial arc plasma gun.

Author

Miyazawa, Kun'ichi, Nagai, Takuro, Kimoto, Koji, Yoshitake, Masaru, and Tanaka, Yumi

Subjects
*PLASMA torch, *PLATINUM, *ELECTRON energy loss spectroscopy, *VACUUM arcs, *FOCUSED ion beams, *CARBON analysis, *CROSS-sectional method
Abstract

A glassy carbon (GC) substrate was irradiated by Pt ions using a coaxial arc plasma gun in vacuum. The structure of the substrate was studied in the atomic scale using high‐resolution transmission electron microscopy (HRTEM) and electron energy loss spectroscopy (EELS) with a thin foil specimen prepared by cross‐sectioning using focused ion beam milling. HRTEM combined with the EELS measurements of the cross‐sectioned sample indicated a disordering of the GC substrate surface and the detection of phenol‐type species, carbon atoms in aliphatic chain molecules, and carboxyl groups, which are considered to have been formed by chemical reaction between the carbon atoms of the GC substrate surface and hydrogen or oxygen atoms. Penetrated Pt atoms were observed inside the GC substrate, and the surface position of the GC substrate was located from the depth profiles of the EELS spectra. STEM‐EELS analyses of a pristine GC substrate without Pt ion irradiation were also conducted for reference. [ABSTRACT FROM AUTHOR]

Published
2020
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40. BiScO(sub 3): Centrosymmetric BiMnO(sub 3-) type oxide

Author

Belik, Alexei A., Likubo, Satoshi, Kodama, Katsuaki, Igawa, Naoki, Shamoto, Shin-ichi, Maie, Makoto, Nagai, Takuro, Matsui, Yoshio, Stefanovich, Sergey Yu., Lazoryak, Bogdan I., and Takayama-Muromachi, Eiji

Subjects
Scandium -- Chemical properties, Bismuth -- Chemical properties, Oxides -- Chemical properties, Chemistry
Abstract

Solid solutions including BiM'O(sub 3) as the end members, for example, PbTiO(sub 3)-BiScO(sub 3) and PbTiO(sub 3)-BiGaO(sub 3), were studied in an attempt to improve ferroelectric properties of PbTiO(sub 3) and reduce the amount of lead. There is good indication that the atomic arrangements in the bulk BiMnO(sub 3) are very close to centrosymmetric, if not centrosymmetric at all and therefore structural distortion cannot be the origin of ferroelectricity in bulk BiMnO(sub 3), as generally accepted.

Published
2006

44. Single adatom dynamics at monatomic steps of free-standing few-layer reduced graphene

Author

Chang, Haixin, Saito, Mitsuhiro, Nagai, Takuro, Liang, Yunye, Kawazoe, Yoshiyuki, Wang, Zhongchang, Wu, Hongkai, Kimoto, Koji, Ikuhara, Yuichi, Chang, Haixin, Saito, Mitsuhiro, Nagai, Takuro, Liang, Yunye, Kawazoe, Yoshiyuki, Wang, Zhongchang, Wu, Hongkai, Kimoto, Koji, and Ikuhara, Yuichi

Abstract

Steps and their associated adatoms extensively exist and play prominent roles in affecting surface properties of materials. Such impacts should be especially pronounced in two-dimensional, atomically-thin membranes like graphene. However, how single adatom behaves at monatomic steps of few-layer graphene is still illusive. Here, we report dynamics of individual adatom at monatomic steps of free-standing few-layer reduced graphene under the electron beam radiations, and demonstrate the prevalent existence of monatomic steps even down to unexpectedly ultrasmall lateral size of a circular diameter of ~5 Å. Single adatom prefers to stay at the edges of the atomic steps of few-layer reduced graphene and evolve with the steps. Moreover, we also find that how the single adatom behaves at atomic step edges can be remarkably influenced by the type of adatoms and step edges. Such single adatoms at monatomic steps and ultrasmall atomic steps open up a new window for surface physics and chemistry for graphene-based as well as other two-dimensional materials.

Published
2014

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