Propagation behavior of elastic waves in one-dimensional piezoelectric/piezomagnetic phononic crystal with line defects

Using a stiffness matrix method, we in- vestigate the propagation behaviors of elastic waves in one-dimensional （1D） piezoelectric/piezomagnetic （PE/PM） phononic crystals （PCs） with line defects by calculating energy reflection/transmittion coefficients of quasi-pressure and quasi-shear waves. Line defects are created by the re- placement of PE or PM constituent layer. The defect modes existing in the first gap are considered and the influences on defect modes of the material properties and volume fraction of the defect layers, the type of incident waves, the location of defect layer and the number of structural layers are discussed in detail. Numerical results indicate that defect modes are the most obvious when the defect layers are inserted in the middle of the perfect PCs; the types of incidence wave and material properties of the defect layers have important effects on the numbers, the location of frequencies and the peaks of defect modes, and the defect modes are strongly de- pendent on volume fraction of the defect layers. We hope this paper will be found useful for the design of PE/PM acoustic filters or acoustic transducer with PCs structures.

Influence of line defects on focusing in a two-dimensional photonic-crystal flat lens

We investigate in detail the influence of line defects on focusing of electromagnetic waves in a two-dimensional photonic-crystal flat lens. Through simulations, we find that a focusing can always be observed when a line defect in the lens is introduced along the light transmission direction and the width of the line defect is less than λ/2. However, there appear two focusings when the width of the line defect is more than λ/2. When the line defect is introduced along the direction perpendicular to the transmission, there is always one focusing.

Line defects in monolayer TiSe_(2) with adsorption of Pt atoms potentially enable excellent catalytic activity

Two-dimensional(2D)materials with defects are desired for catalysis after the adsorption of monodispersed noble metal atoms.High-performance catalysts with the absolute value of Gibbs free energy(|△GH|)close to zero,is one of the ultimate goals in the catalytic field.Here,we report the formation of monolayer titanium selenide(TiSe2)with line defects.The low-temperature scanning tunneling microscopy/spectroscopy(STM/S)measurements revealed the structure and electronic states of the line defect.Density functional theory(DFT)calculation results confirmed that the line defects were induced by selenium vacancies and the STM simulation was in good agreement with the experimental results.Further,DFT calculations show that monolayer TiSe_(2) with line defects have good catalytic activity for hydrogen evolution reaction(HER).If the defects are decorated with single Pt atom,the HER catalytic activity will be enhanced dramatically(|△GH|=0.006 eV),which is much better than Pt metal(|△GH|=0.09 eV).Line defects in monolayer TiSe_(2)/Au(111)provide a wonderful platform for the design of high-performance catalysts.

Achieving superior performance in thermoelectric Bi_(0.4)Sb_(1.6)Te_(3.72)by enhancing texture and inducing high-density line defects

Miniaturization of efficient thermoelectric(TE)devices has long been hindered by the weak mechanical strength and insufficient heat-to-electricity conversion efficiency of zone-melted(ZM)ingots.Here,we successfully prepared a robust high-performance p-type Bi_(0.4)Sb_(1.6)Te_(3.72)bulk alloy by combining an ultrafast thermal explosion reaction with the spark plasma sintering(TER-SPS)process.It is observed that the introduced excess Te not only enhances the(00l)-oriented texture to ensure an outstanding power factor(PF)of 5 mW m^(−1)K^(−2),but also induces extremely high-density line defects of up to 10^(11)–10^(12)cm^(−2).Benefiting from such heavily dense line defects,the enhancement of the electronic thermal conductance from the increased electron mobility is fully compensated by the stronger phonon scattering,leading to an evident net reduction in total thermal conductivity.As a result,a superior ZT value of~1.4 at 350 K is achieved,which is 40%higher than that of commercial ZM ingots.Moreover,owing to the strengthening of grain refinement and highdensity line defects,the mechanical compressive stress reaches up to 94 MPa,which is 154%more than that of commercial single crystals.This research presents an effective strategy for the collaborative optimization of the texture,TE performance,and mechanical strength of Bi2Te3-based materials.As such,the present study contributes significantly to the future commercial development of miniature TE devices.

Observation of parabolic electron bands on superconductor LaRu_(2)As_(2)

Ru-based superconductor LaRu_(2)As_(2) has been discovered exhibiting the highest critical temperature of ~ 7.8 K among iron-free transition metal pnictides with the ThCr_(2)Si_(2)-type crystal structure. However, microscopic research on this novel superconducting material is still lacking. Here, we utilize scanning tunneling microscopy/spectroscopy to uncover the superconductivity and surface structure of LaRu_(2)As_(2). Two distinct terminating surfaces are identified on the cleaved crystals, namely, the As surface and the La surface. Atomic missing line defects are observed on the La surface. Both surfaces exhibit a superconducting gap of ~ 1.0 me V. By employing quasiparticle interference techniques, we observe standing wave patterns near the line defects on the La atomic plane. These patterns are attributed to quasiparticle scattering from two electron type parabolic bands.

Electron states scattering off line edges on the surface of topological insulator

We study the local density of states （LDOS） for electrons scattering off the line edge of an atomic step defect on the surface of a three-dimensional （3D） topological insulator （TI） and the line edge of a finite 3D TI, where the front surface and side surface meet with different Fermi velocities, respectively. By using a S-function potential to model the edges, we find that the bound states existed along the step line edge significantly contribute to the LDOS near the edge, but do not modify the exponential behavior away from it. In addition, the power-law decaying behavior for LDOS oscillation away from the step is understood from the spin rotation for surface states scattering off the step defect with magnitude depending on the strength of the potential. Furthermore, the electron refraction and total reflection analogous to optics occurred at the line edge where two surfaces meet with different Fermi velocities, which leads to the LDOS decaying behavior in the greater Fermi velocity side similar to that for a step line edge. However, in the smaller velocity side the LDOS shows a different decaying behavior as x-1/2, and the wavevector of LDOS oscillation is no longer equal to the diameter of the constant energy contour of surface band, but is sensitively dependent on the ratio of the two Fermi velocities. These effects may be verified by STM measurement with high precision.

Inclusion and Bubble in Steel——A Review

The type, morphology and sources of inclusion in steels, including indigenous and exogenous inclusions, were discussed and reviewed. Indigenous inclusions are deoxidation products or inclusions precipitated during cooling and solidification of steel. Exogenous inclusions arise primarily from the incidental chemical （reoxidation） and mechanical interaction of liquid steel with its surroundings （slag entrainment and erosion of lining refractory）. Types and causes for the nozzle clogging were also summarized. Reasons for bubble formation and bubble size distribution in steels were discussed thereafter. Finally, morphology and causes of inclusion-related defects in continuously cast steel products were reviewed, such as flange cracking in cans, slag spots and line defects on strips.

Domain boundaries in silicene:Density functional theory calculations on electronic properties

By using density functional theory(DFT)-based first-principles calculations, the structural stability and electronic properties for two kinds of silicene domain boundaries, forming along armchair edge and zigzag edge, have been investigated. The results indicate that a linkage of tetragonal and octagonal rings(4|8) appears along the armchair edge, while a linkage of paired pentagonal and octagonal rings(5|5|8) appears along the zigzag edge. Different from graphene, the buckling properties of silicene lead to two mirror symmetrical edges of silicene line-defect. The formation energies indicate that the 5|5|8 domain boundary is more stable than the 4|8 domain boundary. Similar to graphene, the calculated electronic properties show that the 5|5|8 domain boundaries exhibit metallic properties and the 4|8 domain boundaries are half-metal.Both domain boundaries create the perfect one-dimensional(1D) metallic wires. Due to the metallic properties, these two kinds of nanowires can be used to build the silicene-based devices.

Formation Mechanism of PPS as Antireflection Coating

The whole chemical etching process on a P-type polycrystalline silicon substrate with resistivity 1-2Ω·cm is described. The formation mechanism of porous polycrystalline silicon（PPS） microstructure was investigated. Those how the initial pits were formed and an uniform morphology of PPS was obtained are explained. Two types of etching mechanism were characterized as defect control reaction and diffusion control reaction. The morphology formed after the isotropic acidic solution etching with different etching time and HF/HNO3 concentration was compared with the effect of the same etching process after anisotropic alkaline etching. The study showed that the thickness of porous polycrystalline silicon layer with chemical acidic etching entirely depended on the existence of various types of defects.

Mass transport induced structural evolution and healing of sulfur vacancy lines and Mo chain in monolayer MoS_(2)

Defects play vital roles in tailoring structures and properties of materials including the atomically thin two-dimensional(2D)materials,and increasing demands are requested to find effective ways to realize the defect engineering,i.e.,tuning the defects and thus the materials’structure–property in a well-controlled way.Herein,we propose a novel method to tune the structures and configurations of one-dimensional(1D)line defects in monolayer MoS2 via mass transport induced structural transformation.By using atomic-resolved annular dark-field scanning transmission electron microscopy(ADF-STEM),we demonstrate in situ that sulfur vacancy line defect can be healed locally into defect-free MoS_(2)lattice via the desorption of Mo atoms from vacancy lines and adsorption into a moving Mo cluster.Furthermore,directional transport of Mo atoms(or Mo cluster)along the sulfur vacancy lines can induce the formation of Mo chains.Such a mass transport induced defect tuning provides more operational routes for the rational defect designing and property tuning in MoS_(2)as well as other related 2D materials.