Depinning Dynamics of Fluid Monolayer on a Quenched Substrate

Langevin simulations are preformed on the depinning dynamics of fluid monolayer on a quenched substrate. With increase in the strength of the substrate, we find for the first time a crossover from elastic crystal to smectic flows as well as a crossover from smectic to plastic flows above the depinning. A power-law scaling relationship can be derived between the drift velocity and the driving force for both the elastic crystal and smectic flows, but fails to be obtained for the plastic flow. The power-law exponents are found to be no larger than 1 for the elastic crystal flow and larger than 1 for the smeetic flow. The critical driving force and the averaged intensity of Bragg peaks remain invariant basically in the regime of smectic flow. A sudden increase in the critical driving force is observed within the crossover from the smeetic to plastic flows, and the averaged intensity of Bragg peaks shows sudden decreases within the crossovers both from the elastic crystal to smectic flows and from the smectic to plastic flows. The results are helpful for understanding the slip dynamics of fluids on a molecular level.     

Study of nanoscratching of polymers by using molecular dynamics simulations

Molecular dynamic simulations are performed to study the nanoscratching behavior of polymers.The effects of scratching depth,scratching velocity and indenter/polymer interaction strength are investigated.It is found that polymer material in the scratching zone around the indenter can be removed in a ductile manner as the local temperature in the scratching zone exceeds glass transition temperature Tg.The recovery of polymer can be more significant when the temperature approaches or exceeds Tg.The tangential force,normal force and friction coefficient increase as the scratching depth increases.A larger scratching velocity leads to more material deformation and higher pile-up.The tangential force and normal force are larger for a larger scratching velocity whereas the friction coefficient is almost independent of the scratching velocities studied.It is also found that stronger indenter/polymer interaction strength results in a larger tangential force and friction coefficient.     

Accurate modelling of the crush behaviour of thin tubular columns using material point method

In this paper,we apply the material point method(MPM),also known as a meshfree method,to examine the crush behaviour of thin tubular columns.Unlike the finite element method,randomly-distributed-weak-particle triggers were used to account for the deformation behaviour of collapse modes.Both symmetric and asymmetric modes of deformation and their associated mean collapse loads are determined for an elasto-plastic constitutive law describing the tubular columns.Attention was devoted to the accuracy and the convergence of the MPM simulation,which is determined by the number of the particles and the size of the background cells used in our explicit solver.Furthermore,a novel contact approach was adopted to establish the crush behaviour of the tubular columns.Two aspects of the work were accordingly examined,including three different crush velocities(5,10 and 15 m/s) and varied geometrical features of the tube(t/d and l/d) based on the deformation history.The results of our model,which were compared with existing analytical predictions and experimental findings,identify the critical geometric features of the tubular columns that would dictate the deformation mode as being either progressive collapse or following Euler’s buckling mode.     

Effect of induced shielding current transmission in longitudinal direction on levitation force of melt grown single-domain YBa_2Cu_3O_(7-x) cylindrical superconductor

YBa2Cu3O7-x (YBCO) bulk is one of the most important high temperature superconducting materials for magnetic levitation applications, because it has a high magnetic irreversibility field Hirr at liquid nitrogen temperature, and can grow into large grains. The levitation force of YBCO bulk is proportional to the critical current density (Jc) and grain radius (r)[1], so higher Jc and r are appreciated for YBCO bulks to achieve higher levitation force. Now single-domain YBCO bulks up to s…     

Nucleon Finite Volume Effect and Nuclear Matter Properties in a Relativistic Mean-Field Theory

Effects of excluded volume of nucleons on nuclear matter are studied, and the nuclear properties that follow from different relativistic mean-field model parametrizations are compared. We show that, for all tested parametrizations,the resulting volume energy a1 and the symmetry energy J are around the acceptable values of 16 MeV and 30 MeV,and the density symmetry L is around 100 Me V. On the other hand, models that consider only linear terms lead to incompressibility K0 much higher than expected. For most parameter sets there exists a critical point (ρc,δc), where the minimum and the maximum of the equation of state are coincident and the incompressibility equals zero. This critical point depends on the excluded volume parameter r. If this parameter is larger than 0.5 fm, there is no critical point and the pure neutron matter is predicted to be bound. The maximum value for neutron star mass is 1.85M⊙, which is in agreement with the mass of the heaviest observed neutron star 4U0900-40 and corresponds to r = 0.72 fm. We also show that the light neutron star mass (1.2M⊙) is obtained for r (≌) 0.9 fm.     

Stretching instability of intrinsically curved semiflexible biopolymers: A lattice model approach

We apply a Monte Carlo simulation method to lattice systems to study the effect of an intrinsic curvature on the mechanical property of a semiflexible biopolymer.We find that when the intrinsic curvature is sufficiently large,the extension of a semiflexible biopolymer can undergo a first-order transition at finite temperature.The critical force increases with increasing intrinsic curvature.However,the relationship between the critical force and the bending rigidity is structuredependent.In a triangle lattice system,when the intrinsic curvature is smaller than a critical value,the critical force increases with the increasing bending rigidity first,and then decreases with the increasing bending rigidity.In a square lattice system,however,the critical force always decreases with the increasing bending rigidity.In contrast,when the intrinsic curvature is greater than the critical value,the larger bending rigidity always results in a larger critical force in both lattice systems.     

Nanoscale domain switching mechanism of Bi_(3.15)Eu_(0.85)Ti_3O_(12) thin film under the different mechanical forces

The switching process of ferroelectric thin films in electronic devices is one of the most important requirements for their application. Especially for the different external fields acting on the film surface, the mechanism of domain switching is more complicated. Here we observe the nanoscale domain switchings of Bi3.15Eu0.85Ti3O12 thin film under different mechanical forces at a fast scan rate. As the force increases from initial state to 247.5 n N, the original bright or grey contrasts within the selected grains are all changed into dark contrasts corresponding to the polarization vectors reversed from the up state to the down state, except for the clusters. As the mechanical force increases to 495 n N, the color contrasts in all of the selected grains further turn into grey contrasts and some are even changed into grey contrasts completely showing the typical 90° domain switching. When another stronger loading force 742.5 n N is applied, the phase image becomes unclear and it indicates that the piezoelectric signal can be suppressed under a sufficiently high force, which is coincident with previous experimental results. Furthermore, we adopt the domain switching criterion from the perspective of equilibrium state free energy of ferroelectric nanodomain to explain the mechanisms of force-generated domain switchings.     

Capillary Adhesion of Microbeams： Finite Deformation Analysis

Capillary force may cause adhesion of devices at micro- and nano-scales. Considering the fact that large deformation is often involved in adhesion of microbeams, we analysed the capillary adhesion of two beams using finite deformation elasticity theory. The critical adhesion condition can be obtained from the present method as a function of the bending stiffness, Young＇s contact angle, the spacing of the two beams as well as the surface tensions of the solid and liquid phases. The solution for the capillary adhesion of a beam with a rigid substrate is also given. The results from the finite deformation analysis are compared with that of infinitesimal deformation method in order to show the necessity of accounting for the nonlinear effect associated with large deflection. The method adopted in this study can also be used to solve other adhesion problems associated with van der Waals force or electrostatic force.     

Stability of GOY Model Under Modulation of Periodic External Force

There is a phase transition between quasi-periodic state and intermittent chaos in GOY model with a critical value δ0. When we add a modulated periodic externa/force to the system, the phase transition can also be found with a critical value δe. Due to coupling between the force and the intrinsic fluctuation of the velocity on shells in GOY model, the stability of the system has been changed, which results in the variation of the critical value. For proper intensity and period of the force, δe is unequal to δ0. The critical value is a nonlinear function of amplitude of the force, and the fluctuation of the velocity can resonate with the external force for certain period Te.     

Effects of AlN nucleation layer thickness on crystal quality of AlN grown by plasma-assisted molecular beam epitaxy

In this paper,the effects of thickness of AlN nucleation layer grown at high temperature on AlN epi-layer crystalline quality are investigated.Crack-free AlN samples with various nucleation thicknesses are grown on sapphire substrates by plasma-assisted molecular beam epitaxy.The AlN crystalline quality is analysed by transmission electron microscope and x-ray diffraction (XRD) rocking curves in both (002) and (102) planes.The surface profiles of nucleation layer with different thicknesses after in-situ annealing are also analysed by atomic force microscope.A critical nucleation thickness for realising high quality AlN films is found.When the nucleation thickness is above a certain value,the (102) XRD full width at half maximum (FWHM) of AlN bulk increases with nucleation thickness increasing,whereas the (002) XRD FWHM shows an opposite trend.These phenomena can be attributed to the characteristics of nucleation islands and the evolution of crystal grains during AlN main layer growth.     

Critical temperature and condensed fraction of Bose-Einstein condensation in optical lattices

Critical temperature and condensate fraction of Bose-Einstein condensation in the optical lattice are studied. The results show that the critical temperature in optical lattices can be characterized with an equivalent critical temperature in a single lattice, which provide a fast evaluation of critical temperature and condensate fraction of Bose-Einstein condensation confined with pure optical trap. Critical temperature can be estimated with an equivalent critical temperature. It is predicted that critical temperature is proportional to q in q number lattices for superfluid state and should be equal to that in a single lattic for Mott insulate state. Required potential depth or Rabi frequency and maximum atom number in the lattices both for superfluid state and Mott state are presented based on views of thermal mechanical statistics.     

Nucleon Finite Volume Effect and Nuclear Matter Properties in a Relativistic Mean-Field Theory

Effects of excluded volume of nucleons on nuclear matter are studied, and the nuclear properties that follow from different relativistic mean-field model parametrizations are compared. We show that, for all tested parametrizations, the resulting volume energy al and the symmetry energy J are around the acceptable values of 16 MeV and 30 MeV, and the density symmetry L is around 100 MeV. On the other hand, models that consider only linear terms lead to incompressibility Ko much higher than expected. For most parameter sets there exists a critical point （pc, δc）, where the minimum and the maximum of the equation of state are coincident and the incompressibility equals zero. This critical point depends on the excluded volume parameter r. If this parameter is larger than 0.5 fm, there is no critical point and the pure neutron matter is predicted to be bound. The maximum value for neutron star mass is 1.85M⊙, which is in agreement with the mass of the heaviest observed neutron star 4U0900-40 and corresponds to r = 0.72 fm. We also show that the light neutron star mass （1.2M⊙） is obtained for r ≌ 0.9 fro.     

Formations and morphological stabilities of ultrathin CoSi₂ films

In this paper we investigate the formations and morphological stabilities of Co-silicide films using 1-8-nm thick Co layers sputter-deposited on silicon(100) substrates.These ultrathin Co-silicide films are formed via solid-state reaction of the deposited Co films with Si substrate at annealing temperatures from 450℃ to 850℃.For a Co layer with a thickness no larger than 1 nm,epitaxially aligned CoSi2 films readily grow on silicon(100) substrate and exhibit good morphological stabilities up to 600℃.For a Co layer thicker than 1 nm,polycrystalline CoSi and CoSi2 films are observed.The critical thickness below which epitaxially aligned CoSi2 film prevails is smaller than the reported critical thickness of the Ni layer for epitaxial alignment of NiSi2 on silicon(100) substrate.The larger lattice mismatch between the CoSi2 film and the silicon substrate is the root cause for the smaller critical thickness of the Co layer.     

Effects of Different Spin-Spin Couplings and Magnetic Fields on Thermal Entanglement in Heisenberg XYZ Chain

The effects of spin-spin interaction on thermed entanglement of a two-qubit Heisenberg XYZ model with different inhomogeneous magnetic fields are investigated. It is shown that the entanglement is dependent on the spin-spin interaction and the inhomogeneous magnetic fields. The larger the Ji （i-axis spin-spin interaction）, the higher critical value the Bi （i-axis uniform magnetic field） has. Moreover, in the weak-field regime, the larger Ji corresponds to more entanglement, while in the strong-field regime, different Ji correspond to the same entanglement. In addition, it is found that with the increase of Ji, the concurrence can approach the maximum value more rapidly for the smaller Bi, and can reach a larger value for the smaller bi （i-axis nonuniform magnetic field）. So we can get more entanglement by increasing the spin-spin interaction Ji, or by decreasing the uniform magnetic field Bi and the nonuniform magnetic field hi.     

Crossover from 2-dimensional to 3-dimensional aggregations of clusters on square lattice substrates

A Monte Carlo study on the crossover from 2-dimensional to 3-dimensional aggregations of clusters is presented.Based on the traditional cluster–cluster aggregation(CCA) simulation, a modified growth model is proposed. The clusters(including single particles and their aggregates) diffuse with diffusion step length l(1 ≤ l ≤ 7) and aggregate on a square lattice substrate. If the number of particles contained in a cluster is larger than a critical size s c, the particles at the edge of the cluster have a possibility to jump onto the upper layer, which results in the crossover from 2-dimensional to 3-dimensional aggregations. Our simulation results are in good agreement with the experimental findings.     

Dynamic stability of parametrically-excited linear resonant beams under periodic axial force

The parametric dynamic stability of resonant beams with various parameters under periodic axial force is studied.It is assumed that the theoretical formulations are based on Euler-Bernoulli beam theory.The governing equations of motion are derived by using the Rayleigh-Ritz method and transformed into Mathieu equations,which are formed to determine the stability criterion and stability regions for parametricallyexcited linear resonant beams.An improved stability criterion is obtained using periodic Lyapunov functions.The boundary points on the stable regions are determined by using a small parameter perturbation method.Numerical results and discussion are presented to highlight the effects of beam length,axial force and damped coefficient on the stability criterion and stability regions.While some stability rules are easy to anticipate,we draw some conclusions:with the increase of damped coefficient,stable regions arise;with the decrease of beam length,the conditions of the damped coefficient arise instead.These conclusions can provide a reference for the robust design of parametricallyexcited linear resonant sensors.     

High Refractive Index Ti_3O_5 Films for Dielectric Metasurfaces

Ti_3 O_5 films are deposited with the help of an electron beam evaporator for their applications in metasurfaces.The film of subwavelength(632 nm) thickness is deposited on a silicon substrate and annealed at 400°C. The ellipsometr.y result shows a high refractive index above 2.5 with the minimum absorption coefficient in the visible region, which is necessary for high efficiency of transparent metasurfaces. Atomic force microscopy analysis is employed to measure the roughness of the as-deposited films. It is seen from micrographs that the deposited films are very smooth with the minimum roughness to prevent scattering and absorption losses for metasurface devices. The absence of grains and cracks can be seen by scanning electron microscope analysis, which is favorable for electron beam lithography. Fourier transform infrared spectroscopy reveals the transmission and reflection obtained from the film deposited on glass substrates. The as-deposited film shows high transmission above 60%,whieh is in good agreement with metasurfaces.     

Barrier or easy-flow channel: The role of grain boundary acting on vortex motion in type-Ⅱ superconductors

Grain boundaries(GBs),as extremely anisotropic pinning defects,have a strong impact on vortex motion in type-Ⅱsuperconductors,and further on the macro level dominates the superconductivity for example the critical current density.Many previous studies indicated that mostly GB plays the role of a strong barrier for vortex motion,while an easy-flow channel just under some certain conditions.In order to thoroughly make clear of the questions of what is exactly the role of GB on vortex motion and how it works,in this article we developed a large scale molecular dynamic model and revealed the action of GB on vortex motion in type-Ⅱ superconductors.The most significant finding is that the role of GB on vortex motion can be changeable from a barrier to an easy-flow channel,and which is intrinsically determined by the competition effect correlated with its action on vortex between in the GB and no-GB regions.Such the competition effect essentially depends on the attributes of both the GB(described by the GB strength and angle θ) and no-GB pining regions(by the relative disorder strength α_p/a_v).Specifically,for a YBa_2 Cu_3 O_(7-x)(YBCO) sample,to obtain a clear knowledge of vortex motion in GB region,we visualized the three typical trajectories of vortices during the three vortex movement stages.Further,in order to understand how GB results in the macro current-carrying property,corresponding to the current-voltage relation of the YBCO conductor,we obtained the average velocity v_y of vortices varying with their driving force,which is nearly identical with the previous observations.     

Scaling of the flow field in a combustion chamber with agas——gas injector

The scaling of the flowfield in a gas--gas combustion chamber is investigated theoretically, numerically and experimentally. To obtain the scaling criterion of the gas--gas combustion flowfield, formulation analysis of the three-dimensional (3D) Navier--Stokes equations for a gaseous multi-component mixing reaction flow is conducted and dimensional analysis on the gas--gas combustion phenomena is also carried out. The criterion implies that the size and the pressure of the gas--gas combustion chamber can be changed. Based on the criterion, multi-element injector chambers with different geometric sizes and at different chamber pressures ranging from 3~MPa to 20~MPa are numerically simulated. A multi-element injector chamber is designed and hot-fire tested at five chamber pressures from 1.64~MPa to 3.68~MPa. Wall temperature measurements are used to understand the similarity of combustion flowfields in the tests. The results have verified the similarities between combustion flowfields under different chamber pressures and geometries, with the criterion applied.     

Small-scale self-focusing of divergent beam in nonlinear media with loss

<正>Bespalov-Talanov theory on small-scale self-focusing is extended to include medium loss for a divergent beam.Gain spectrum of small-scale perturbation is presented in integral form,and based on the derived equations we find that the cutoff spatial frequency for perturbation keeps a constant value.The larger the medium loss is,the smaller the fastest growing frequency and the maximum gain of perturbation with defined propagation distance are.For a given medium loss the maximum gain of perturbation becomes larger,while the fastest growing frequency becomes smaller as the propagation distance becomes longer. Furthermore,physical explanations for the appearance of these features are given.