Competition between Stepwise Polarization Switching and Chirality Coupling in Ferroelectric GeS Nanotubes

Ferroelectricity of group-Ⅳ chalcogenides MX(M = Ge,Sn;X = Se,S) monolayers has been extensively investigated.However,how the ferroelectricity evolves in their one-dimensional nanotubes remains largely unclear.Employing an accurate deep-learning interatomic potential of first-principles precision,we uncover a general stepwise mechanism for polarization switching in zigzag and chiral Ge S nanotubes,which has an energy barrier that is substantially lower than the one associated with the conventional one-step switching mechanism.The switching barrier(per atom) gradually decreases with increasing the number of intermediate steps and converges to a value that is almost independent of the tube diameter.In the chiral Ge S nanotubes,the switching path of polarization with chirality coupling is preferred at less intermediate steps.This study unveils novel ferroelectric switching behaviors in one-dimensional nanotubes,which is critical to coupling ferroelectricity and chirality.

Electron-Exciton Coupling in 1T-TiSe_(2)Bilayer

Excitons in solid state are bosons generated by electron-hole pairs as the Coulomb screening is sufficiently reduced.The exciton condensation can result in exotic physics such as super-fluidity and insulating state.In charge density wave(CDW)state,1T-TiSe_(2) is one of the candidates that may host the exciton condensation.However,to envision its excitonic effect is still challenging,particularly at the two-dimensional limit,which is applicable to future devices.Here,we realize the epitaxial 1T-TiSe_(2) bilayer,the two-dimensional limit for its 2×2×2 CDW order,to explore the exciton-associated effect.By means of high-resolution scanning tunneling spectroscopy and quasiparticle interference,we discover an unexpected state residing below the conduction band and right within the CDW gap region.As corroborated by our theoretical analysis,this mysterious phenomenon is in good agreement with the electron-exciton coupling.Our study provides a material platform to explore exciton-based electronics and opto-electronics.

Performance Evaluation for Medium Voltage MIMO-OFDM Power Line Communication System

Power line communication(PLC)provides intelligent electrical functions such as power quality measurement,fault surveys,and remote control of electrical network.Most of research works have been done in low voltage(LV)scenario due to the fast development of in-home PLC.The aim of this paper is to evaluate the link-level performance of a medium voltage(MV)MIMO-OFDM communication system based on transmission link under underground power line channel.The MIMO channel is modeled as a modified multipath model in the presence of impulsive noise and background noise.We first perform a measurement on the practical MV MIMO channel parameters for a section of buried cable of 1 km long in Ganzhou city,Jiangxi province,China.Based on the measured channel,we design the frame structure based on an IEEE standard for broadband over power line networks[1]to support MV MIMO-OFDM transmission.According to designed frame structure,we design an encoder and a decoder for a dual binary tail-biting turbo code and optimize some key decoder parameters for low bit error rate performance.Finally,the link-level performance for both spatial multiplexing and spatial diversity are evaluated.Numeral results show that MV MIMO-OFDM is a promising approach to provide both high data rate and link reliability for PLC.

Strain transfer behavior and crystallographic mechanism of crack initiation during cyclic deformation in zirconium

Fully-reversed cyclic deformation of a pure Zr(a thickness of 17 mm)was conducted at two different strain amplitudes(0.4%and 0.8%)to investigate the deformation and crack initiation behaviors based on slip trace analysis.It was found that prismaticaslip with a higher Schmid Factor(m>0.4)was the dominant deformation mode.The grains containing persistent slip bands(PSBs)tended to go towards[1210]pole and the Schmid Factor had a critical value of 0.4 above which prismatic and pyramidal slip were dominant.Fatigue cracks were mainly initiated at PSBs and grain boundaries(GBs).It showed that 61.1%of the cracks were PSB cracks under a strain amplitude of 0.4%,while it decreased to 53.5%at a strain amplitude of 0.8%.PSB cracks were mainly parallel to prismaticaslip with higher Schmid Factor while some cracks tended to be initiated at GBs with higher misorientation angles.The interaction of PSBs with GBs would result in strain transferring to the neighboring grain.Strain transfer was more likely to occur at the condition of the higher geometrical compatibility factor m,and lower residual Burgers vectorb,which could reduce strain localization.

Slip behavior of Bi-modal structure in a metastable β titanium alloy during tensile deformation

β titanium alloys with bi-modal structure which exhibit improved strength-ductility combination and fatigue property are widely used in aviation and aerospace industry.However,owing to the small size of primary α(αp) and nano-scaled multi variant distribution of secondary α platelets(αs),investigating the deformation behavior is really a challenging work.In this work,by applying transmission electron microscopy(TEM),the slip behavior in αp and transformed β matrix with different tensile strain was studied.After α/β solution treatment,the initial dislocation slips on {110} plane with <1 1 1> direction in β matrix.During furtherdeformation,(110),(101) and(1 1 2) multi slip is generated which shows a long straight cro s sing configuration.Dislocation cell is exhibited in β matrix at tensile strain above 20 %.Diffe rent from the solid solution treated sample,high density wavy dislocations are generated in transformedβ matrix.High fraction fine as hinders dislocation motion in β matrix effectively which in turn dominates the strength of the alloy.In primary α phase(αp),a core-shell structure is formed during deformation.Both pyramidal a+c slip and prismatic/basal a slip are generated in the shell layer.In core region,plastic deformation is governed by prismatic/basal a slip.Formation of the core-shell structure is the physical origin of the improved ductility.On one hand,the work hardening layer(shell) improves the strength of αp,which could deform compatibly with the hard transformed β matrix.Meanwhile,the center area(core) deforms homogeneously which will sustain plastic strain effectively and increase the ductility.This paper studies the slip behavior and reveals the origin of the improved strength-ductility combination in Bi-modal structure on a microscopic way,which will give theoretical advises for developing the next generation high strength β titanium alloys.

Designing high-strength titanium alloy using pseudo-spinodal mechanism through diffusion multiple experiment and CALPHAD calculation

This study used the pseudo-spinodal mechanism to obtain the ultrafineαphase for designing highstrength titanium alloy.Diffusion multiple experiments were designed to find the composition range of TM-x Mo-y V alloy(TM:Ti-4.5 Al-2 Cr-2.5 Nb-2 Zr-1 Sn)for obtaining ultrafineαphase.CALPHAD results confirm that when the alloy composition is located near the intersection of theαandβphase free energy curves,the alloy will undergo pseudo-spinodal transformation and obtain the ultrafineαphase.The designed TM-6 Mo-3 V alloy has a yield strength of 1411 MPa and an elongation of 6.5%.The strength of the alloy depends on the thickness,fraction of theαphase and the solid solution strengthening effect of the alloying elements.The deformation mechanism of the alloy is the dislocation slip of theαandβphases and the twin deformation of theαphase.The large number ofα/βinterfaces produced by the fineαphase is the main reason for limiting the ductility of the alloy.The use of the pseudo-spinodal mechanism combined with diffusion multiple experiments and CALPHAD is an effective method for designing high-strength titanium alloys.

二维铁电材料对单分子磁性调控的第一性原理研究

基于磁电效应,通过电场调控材料的磁性能有利于低能耗、高密度存储设备的应用和发展.以前的相关研究大多为通过压电效应或外加电场进行材料磁性的易失性调控,即材料的磁性改变无法在撤掉电场后维持.虽然可以产生自发极化且能在外电场作用下改变极化方向的铁电材料为非易失调控提供了优异的平台,但随着材料尺度的减小,其自发极化逐渐减小甚至消失,很难在原子尺度实现电控磁性.二维铁电材料在实验上的成功制备,为实现原子尺度下的非易失调控提供了新的机遇.本文基于第一性原理提出了一种通过二维铁电基底调节分子磁性的方法.研究发现,不仅磁矩大小可以在零或非零之间转换,其易轴也可以在面内或面外之间反转,同时较大的磁各向异性能使其有望实现在室温以上的应用.该结果为二维铁电调控材料磁性的研究提供了新思路.

2Hα-In_(2)Se_(3)铁电体中极序工程控制的光电流和多态存储器

控制铁电材料的极序可以丰富其特性和功能的多样性,为设计新型电子和光电子器件提供新的机会.本文报道了一种基于二维(2D)范德瓦尔斯(vdW)层状铁电材料2Hα-In_(2)Se_(3)的面内多态存储器件.通过施加面内电场调控铁电α-In_(2)Se_(3)层的极序,实现了可以相互切换的三种电阻态,其具有快的切换速度、良好的耐久性和保持性.值得注意的是,中间阻态(MRS)和低阻态(LRS)之间可以通过比其他基于2D铁电材料的存储器小1–2个数量级的超低电场来实现可逆切换.此外,还发现这三种不同的极序态表现出了特有的光响应.以上结果表明,α-In_(2)Se_(3)在非易失性高密度存储器和新型光电器件中具有很大的应用潜力.

Prediction of protected band edge states and dielectric tunable quasiparticle and excitonic properties of monolayer MoSi_(2)N_(4)

The electronic structure of two-dimensional(2D)materials are inherently prone to environmental perturbations,which may pose significant challenges to their applications in electronic or optoelectronic devices.A 2D material couples with its environment through two mechanisms:local chemical coupling and nonlocal dielectric screening effects.The local chemical coupling is often difficult to predict or control experimentally.Nonlocal dielectric screening,on the other hand,can be tuned by choosing the substrates or layer thickness in a controllable manner.Therefore,a compelling 2D electronic material should offer band edge states that are robust against local chemical coupling effects.Here it is demonstrated that the recently synthesized MoSi_(2)N_(4)is an ideal 2D semiconductor with robust band edge states protected from capricious environmental chemical coupling effects.Detailed many-body perturbation theory calculations are carried out to illustrate how the band edge states of MoSi_(2)N_(4)are shielded from the direct chemical coupling effects,but its quasiparticle and excitonic properties can be modulated through the nonlocal dielectric screening effects.This unique property,together with the moderate band gap and the thermodynamic and mechanical stability of this material,paves the way for a range of applications of MoSi_(2)N_(4)in areas including energy,2D electronics,and optoelectronics.