Measurement of charge density in nanoscale materials using off-axis electron holography

Abstract Three approaches for the measurement of charge density distributions in nanoscale materials from electron optical phase images recorded using off-axis electron holography are illustrated through the study of an electrically biased needle-shaped sample. We highlight the advantages of using a model-based iterative algorithm, which allows a priori information, such as the shape of the object and the influence of charges that are located outside the field of view, to be taken into account. The recovered charge density can be used to infer the electric field and electrostatic potential

Direct imaging of a zero-field target skyrmion and its polarity switch in a chiral magnetic nanodisk

A target skyrmion is a flux-closed spin texture that has two-fold degeneracy and is promising as a binary state in next generation universal memories. Although its formation in nanopatterned chiral magnets has been predicted, its observation has remained challenging. Here, we use off-axis electron holography to record images of target skyrmions in a 160-nm-diameter nanodisk of the chiral magnet FeGe. We compare experimental measurements with numerical simulations, demonstrate switching between two stable degenerate target skyrmion ground states that have opposite polarities and rotation senses and discuss the observed switching mechanism.Comment: 18 pages, 4 figure

The Spatially Resolved Properties of the GW170817 Host Galaxy

GW170817 is the unique gravitational-wave (GW) event that is associated to the electromagnetic (EM) counterpart GRB 170817A. NGC 4993 is identified as the host galaxy of GW170817/GRB 170817A. In this paper, we particularly focus on the spatially resolved properties of NGC 4993. We present the photometric results from the comprehensive data analysis of the high spatial-resolution images in the different optical bands. The morphological analysis reveals that NGC 4993 is a typical early-type galaxy without significant remnants of major galaxy merger. The spatially resolved stellar population properties of NGC 4993 suggest that the galaxy center has passive evolution with the outskirt formed by gas accretion. We derive the merging rate of the compact object per galaxy by a co-evolution scenario of supermassive black hole and its host galaxy. If the galaxy formation is at redshift 1.0, the merging rate per galaxy is $3.2\times 10^{-4}$ to $7.7\times 10^{-5}$ within the merging decay time from 1.0 to 5.0 Gyr. The results provide the vital information for the ongoing GW EM counterpart detections. The HST data analysis presented in this paper can be also applied for the Chinese Space Station Telescope (CSST) research in the future.Comment: RAA accepte

Studies of radiation absorption on flame speed and flammability limit of CO2 diluted methane flames at elevated pressures

Peer reviewed: YesNRC publication: Ye

Experimental observation of magnetic bobbers for a new concept of magnetic solid-state memory

The use of chiral skyrmions, which are nanoscale vortex-like spin textures, as movable data bit carriers forms the basis of a recently proposed concept for magnetic solid-state memory. In this concept, skyrmions are considered to be unique localized spin textures, which are used to encode data through the quantization of different distances between identical skyrmions on a guiding nanostripe. However, the conservation of distances between highly mobile and interacting skyrmions is difficult to implement in practice. Here, we report the direct observation of another type of theoretically-predicted localized magnetic state, which is referred to as a chiral bobber (ChB), using quantitative off-axis electron holography. We show that ChBs can coexist together with skyrmions. Our results suggest a novel approach for data encoding, whereby a stream of binary data representing a sequence of ones and zeros can be encoded via a sequence of skyrmions and bobbers. The need to maintain defined distances between data bit carriers is then not required. The proposed concept of data encoding promises to expedite the realization of a new generation of magnetic solid-state memory

High spatial resolution and three-dimensional measurement of charge density and electric field in nanoscale materials using off-axis electron holography

The ability to make local measurements of charge density in nanoscale materials and devices is essential for understanding many material properties. The charge density can then be used to infer the electric field or electrostatic potential within and around the specimen. This information is important for scientists working on subjects such as field electron emissionand atom probe tomography. Off-axis electron holography is a powerful technique that can be used to record the phase shift of a high-energy electron wave travelling through an electron-transparent specimenin a transmission electron microscope. Information about the charge density within the specimen can be retrieved from the measured phase with high spatial resolution. In this thesis, charge density and electric field measurements are performed, both in projection and in three dimensions, with a primary focus on samples that have a needle-shaped geometry. Three approaches are used: an analytical model-dependent approach, a model-independentapproach and an approach based on numerical model-based iterative reconstruction. The model-based iterative approach allows $\textit{a priori}$ information, such as the shape of the object and the influence of charges that are located outside the field of view, to be taken into account. More importantly, it also allows for the reconstruction of three-dimensional charge density distributions from incomplete tomographic tilt of phase images without the artefacts that would be present if conventional tomographic reconstruction algorithms were used. In this thesis, a W$_{5}$O$_{14}$ nanowire is investigated experimentally in the presence of anapplied electrical bias and the charge distribution along it is evaluated. A carbon fibre needle-shaped specimen is then studied, in order to demonstrate the capability of the modelbased iterative approach to measure the three-dimensional charge density, electric field and electrostatic potential both inside and around it. Finally, a systematic investigation of electron-beam-induced charging in a needle-shaped specimen with an insulating Al$_{2}$O$_{3}$ apex is presented, including the dependence of the results on electron dose rate, total dose, temperature, primary electron energy and the surface state of the sample. Great care is required with the acquisition and interpretation of the results, in particular because charging phenomena are sensitive to the electrical conductivity of the sample, the presence of contact potentials and the presence of (unknown) surface states

High spatial resolution and three-dimensional measurement of charge density and electric field in nanoscale materials using off-axis electron holography

The ability to make local measurements of charge density in nanoscale materials and devices is essential for understanding many material properties. The charge density can then be used to infer the electric field or electrostatic potential within and around the specimen. This information is important for scientists working on subjects such as field electron emission and atom probe tomography. Off-axis electron holography is a powerful technique that can be used to record the phase shift of a high-energy electron wave travelling through an electron-transparent specimen in a transmission electron microscope. Information about the charge density within the specimen can be retrieved from the measured phase with high spatial resolution. In this thesis, charge density and electric field measurements are performed, both in projection and in three dimensions, with a primary focus on samples that have a needle-shaped geometry. Three approaches are used: an analytical model-dependent approach, a model-independent approach and an approach based on numerical model-based iterative reconstruction. The model-based iterative approach allows a priori information, such as the shape of the object and the influence of charges that are located outside the field of view, to be taken into account. More importantly, it also allows for the reconstruction of three-dimensional charge density distributions from incomplete tomographic tilt of phase images without the artefacts that would be present if conventional tomographic reconstruction algorithms were used. In this thesis, a W5O14 nanowire is investigated experimentally in the presence of an applied electrical bias and the charge distribution along it is evaluated. A carbon fibre needle-shaped specimen is then studied, in order to demonstrate the capability of the model- based iterative approach to measure the three-dimensional charge density, electric field and electrostatic potential both inside and around it. Finally, a systematic investigation of electron-beam-induced charging in a needle-shaped specimen with an insulating Al2O3 apex is presented, including the dependence of the results on electron dose rate, total dose, temperature, primary electron energy and the surface state of the sample. Great care is required with the acquisition and interpretation of the results, in particular because charging phenomena are sensitive to the electrical conductivity of the sample, the presence of contact potentials and the presence of (unknown) surface states

Prospect for measuring two-dimensional van der Waals magnets by electron magnetic chiral dichroism

Two-dimensional (2D) van der Waals magnets have drawn considerable attention in recent years triggered by the huge interest in novel magnetism and spintronic devices. Magnetic measurement of 2D van der Waals (vdW) magnets is crucial to understand the physical origin of magnetism in 2D limits. Therefore, advanced magnetic characterization techniques are highly required. However, only a limited number of such techniques are available due to the extremely small volume of 2D vdW magnets. Here, we introduce the electron magnetic chiral dichroism (EMCD) technique in transmission electron microscope (TEM) to measure 2D vdW crystals. In comparison with some other already-employed techniques in 2D magnets, EMCD is able to quantitatively measure magnetic parameters in three orthogonal directions at nanometer or even at atomic scale. We then perform EMCD simulations on several typical 2D vdW magnets with respect to the accelerating voltage, the number of atomic layers and beam tilt under zone axial orientation. The intensity and distribution of EMCD signals in three orthogonal directions are given in the diffraction plane, thereby providing an optimized design to achieve EMCD measurements. Finally, we discuss the signal-to-noise-ratio and required electron dose in order to obtain a measurable EMCD signal for 2D vdW magnets. Our results provide a feasibility analysis and guideline to measure 2D vdW magnets in future experiments