X.L. Ma

Anisotropic behavior of exchange coupling in textured Nd2Fe14B/α-Fe multilayer films

Exchange coupling has been realized in textured Nd2Fe14B/α-Fe multilayer films. A Mo spacer layer has proved to be effective for preventing interdiffusion at the interface region between the hard-magnetic Nd16Fe71B13 and the soft-magnetic α-Fe layer in these multilayer films. Anisotropic behavior of the exchange coupling in the films is observed by means of magnetic measurements. Furthermore, the effective critical correlation length is found to exhibit anisotropic behavior, which is smaller in the parallel than in the perpendicular direction at the same temperature. This anisotropic behavior of the multilayer films can be well explained by taking into account the shape anisotropy of the textured grains.

On the benefit of aberration-corrected HAADF-STEM for strain determination and its application to tailoring ferroelectric domain patterns.

Revealing strains on the unit-cell level is essential for understanding the particular performance of materials. Large-scale strain variations with a unit-cell resolution are important for studying ferroelectric materials since the spontaneous polarizations of such materials are strongly coupled with strains. Aberration-corrected high-angle-annular-dark-field scanning transmission electron microscopy (AC-HAADF-STEM) is not so sensitive to the sample thickness and therefore thickness gradients. Consequently it is extremely useful for large-scale strain determination, which can be readily extracted by geometrical phase analysis (GPA). Such a combination has various advantages: it is straightforward, accurate on the unit-cell scale, relatively insensitive to crystal orientation and therefore helpful for large-scale. We take a tetragonal ferroelectric PbTiO3 film as an example in which large-scale strains are determined. Furthermore, based on the specific relationship between lattice rotation and spontaneous polarization (Ps) at 180° domain-walls, the Ps directions are identified, which makes the investigation of ferroelectric domain structures accurate and straightforward. This method is proposed to be suitable for investigating strain-related phenomena in other ferroelectric materials.

Sensitive refractive index sensing with tunable sensing range and good operation angle-polarization-tolerance using graphene concentric ring arrays

A highly tunable refractive index sensor with excellent performance and good operation angle-polarization-tolerance is proposed and demonstrated numerically by means of the finite element method. The proposed sensor consists of a planar regular array of paired graphene concentric ring resonators sandwiched between a substrate and a sensing medium. Numerical calculation results show that a high sensitivity of 9.59 µm per refractive index unit and figure of merit of 5.82 can be reached for lower sensing medium refractive indices. The introduction of graphene in this sensor can enhance the absorption of biomolecules and make the sensing range actively tunable. Therefore, it can be conveniently used for possible detection of the refractive index variation of gases, liquids or mixed solutions. Also, we predict that a multi-channel sensor can be achieved by introducing several graphene concentric ring resonators into each unit cell of the array.

Two-band finite difference method for the bandstructure calculation with nonparabolicity effects in quantum cascade lasers

We present a two-band finite difference method for the bandstructure calculation of quantum cascade lasers (QCLs) based on the equivalent two-band model of the nonparabolic Schrodinger equation. Particular backward and forward difference forms are employed in the discretization procedure instead of the common central difference form. In comparison with the linearization approach of the nonparabolic Schrodinger equation, the method is as accurate and reliable as the linearization approach, while the velocity of the method is faster and the matrix elements are more concise, therefore making the method more practical for QCLs simulations.

Giant linear strain gradient with extremely low elastic energy in a perovskite nanostructure array

Although elastic strains, particularly inhomogeneous strains, are able to tune, enhance or create novel properties of some nanoscale functional materials, potential devices dominated by inhomogeneous strains have not been achieved so far. Here we report a fabrication of inhomogeneous strains with a linear gradient as giant as 106 per metre, featuring an extremely lower elastic energy cost compared with a uniformly strained state. The present strain gradient, resulting from the disclinations in the BiFeO3 nanostructures array grown on LaAlO3 substrates via a high deposition flux, induces a polarization of several microcoulomb per square centimetre. It leads to a large built-in electric field of several megavoltage per metre, and gives rise to a large enhancement of solar absorption. Our results indicate that it is possible to build up large-scale strain-dominated nanostructures with exotic properties, which in turn could be useful in the development of novel devices for electromechanical and photoelectric applications.

Finite difference method for analyzing band structure in semiconductor heterostructures without spurious solutions

To stably employ multiband k · p model for analyzing the band structure in semiconductor heterostructures without spurious solutions (SSs), the Hermitian forward and backward difference (HFBD) scheme for finite difference method (FDM) is presented. The HFBD is the discretization scheme that eliminates the difference instability and employs the Burt-Foreman Hermitian operator ordering without geometric asymmetry. The difference instability arises from employing Foremans strategy (FS). FS removes SSs caused by unphysical bowing in bulk dispersion curve meanwhile the HFBD is the only difference scheme that can accurately adapt for it. In comparison with other recent strategies, the proposed method in this paper is as accurate and reliable as FS, along with preserving the rapidness and simplicity of FDM. This difference scheme shows stable convergence without any SSs under variable grid size. Therefore, a wide range of experiment-determined band parameters can be applied to large-scale stable simulation with this method regardless of the SSs they originally generate.

New polytypes of LPSO structures in an Mg–Co–Y alloy

Abstract The magnesium alloys containing long-period stacking ordered (LPSO) structures exhibit excellent mechanical properties. Each LPSO structure is known to contain either AB′C′A or AB′C building block and feature its own stacking sequences. By atomic-scale high-angle annular dark field scanning transmission electron microscopy, we find the co-existence of AB′C′A and AB′C building block in a single LPSO structure of the as-cast Mg92Co2Y6 (at.%) alloy, leading to the formation of six new polytypes of the LPSO structures determined as 29H, 51R, 60H, 72R, 102R and 192R. The lattice parameter of each LPSO structure is derived as and (n presents the number of basal layers in a unit cell). The stacking sequences and the space groups of these newly identified LPSO structures are proposed based on the electron diffraction and atomic-scale aberration-corrected high-resolution images. A random distribution of Co/Y elements in the basal planes of AB′C′A and AB′C structural units is also observed and discussed.

Atomic imaging of the interface between M23C6-type carbide and matrix in a long-term ageing polycrystalline Ni-based superalloy

Besides the well-known cube-on-cube orientation relationship (OR) between M23C6 carbide and matrix, we have determined a new OR named as the twin-related OR in a long-term ageing Ni-based superalloy on the basis of the extensive and detailed electron diffraction analyses. Furthermore, by means of atomic-resolution high angle annular dark-field imaging technique which is implemented in the aberration-corrected scanning transmission electron microscope, we elucidated the interfacial characteristics between M23C6 and matrix for above two types of ORs. Taking into account of the interfacial characteristics, we propose that the twin-related OR possesses a higher total interfacial energy. Thus, its frequency of occurrence is lower than that of the cube-on-cube OR though both ORs are usually seen in the long-term ageing samples.

Strain-induced preferential dissolution at the dislocation emergences in MnS: an atomic scale study

The long-standing problem of dislocation-preferential dissolution in a crystal has been generally ascribed to the distortion energy stored in the vicinity of the dislocation core. However, due to lack of experimental means, the relationship between the local distortion state and the electrochemical behaviour of a single dislocation has not been established so far. via in situ ex-environment transmission electron microscopy (TEM), we demonstrate that the emergences of both edge and screw dislocations on MnS surfaces are the preferential sites for dissolution of the MnS inclusions within a stainless steel. In addition, we map the strain-induced variation of the standard electrode potential around the edge dislocation by a combination of the aberration-corrected high-resolution TEM and strain-analysis-based mechanochemistry theory. Significantly, our report provides a new approach to investigate the strain–corrosion correlation at an atomic scale.

Tunable and angle-insensitive plasmon resonances in graphene ribbon arrays with multispectral diffraction response

Plasmon resonances in graphene ribbon arrays are investigated numerically by means of the Finite Element Method. Numerical analysis shows that a series of multipolar resonances take place when graphene ribbon arrays are illuminated by a TM polarized electromagnetic wave. Moreover, these resonances are angle-independent, and can be tuned greatly by the width and the doping level of the graphene ribbons. Specifically, we demonstrate that for graphene arrays with several sets of graphene ribbons, which have different widths or doping levels, each of these multipolar resonances will be split into several ones. In addition, as plasmon resonances can confine electromagnetic field at the ribbon edges, graphene ribbons with different widths or doping levels offer intriguing application for electrically tunable spectral imaging.