Yong-Wei Zhang

Edge elastic properties of defect-free single-layer graphene sheets

An energetic model is proposed to describe the edge elastic properties of defect-free single-layer graphene sheets. Simulations with the adaptive intermolecular reactive empirical bond order potential are used to extract the edge stress and edge moduli for different edges structures, namely, zigzag and armchair edges, zigzag and armchair edges terminated with hydrogen, and reconstructed zigzag and armchair edges. It is found that the properties of graphene are sensitively dependent on the edge structures; armchair and zigzag edges with and without hydrogen termination are in compression, while reconstructed edges are in tension.

Phase diagrams for multi-component membrane vesicles: a coarse-grained modeling study.

We develop a multicomponent membrane coarse-grained model in which the effects of spontaneous curvature and fluidity are included. This model is used to perform computer simulations to study the phase segregation, domain coarsening, budding and budding off of multicomponent membrane vesicles. Three types of phase diagram are presented with variation of composition, spontaneous curvature, and line tension (the unlike bond energy). Various phases have been observed, including, sphere, biconcave, starfish, capsule, budding, and budding off. Our simulations show that budding off occurs only when the size of a domain is larger than a critical value, and the critical condition is closely related to the spontaneous curvature and line tension. A continuum model is used to predict the critical condition for budding off. Quantitative comparisons are made between the present simulation results and the continuum model predictions, and good agreements have been achieved.

Pressure-temperature phase diagram for shapes of vesicles: A coarse-grained molecular dynamics study

Coarse-grained molecular dynamics simulations are performed to obtain the phase diagram for shapes of a vesicle with a variation in temperature and pressure difference across the membrane. Various interesting vesicle shapes are found, in particular, a series of shape transformations are observed for a vesicle with an initial spherical shape, which changes to a prolate shape, then an oblate shape, and then a stomatocyte shape, with either increasing temperature or decreasing pressure difference across the membrane.

Dislocation junctions as barriers to threading dislocation migration

Level set simulations of dislocation dynamics in biaxially strained, heteroepitaxial films reveal interesting kinetic and thermodynamic mechanisms for blocking the migration of threading dislocations. Two dislocations on the same or on intersecting slip planes may react to form a threading dislocation segment that does not glide under the influence of the misfit strain. In the coplanar case, a kinetic barrier exists that slows down dislocation migration. For the reaction involving dislocations on intersecting planes, an energetic barrier impedes other advancing dislocations. These barriers create significant and frequent impediment to threading dislocation flow, resulting in pileups and high threading dislocation densities.

Scaling behavior of higher harmonic responses in textured (Bi,Pb)2Sr2Ca2Cu3Oy/Ag tapes

A silver-sheathed (Bi,Pb)2Sr2Ca2Cu3Oy tape was studied in terms of harmonic responses χn=χn′−iχn″ (n=3, 5, 7) to reveal the angular dependence of its physical properties. In the screening method higher harmonic responses are directly related to the nonlinear voltage–current [E(J)] relation of the sample. Therefore, the effect of the silver sheath on the harmonic measurements can be neglected. We performed measurements of the harmonic responses as a function of applied magnetic field or angle between the magnetic field and the tape plane at various temperatures. The measured data could be scaled onto a single curve by using the three-dimensional scaling relation. The derived anisotropic parameter γ ranged from 5.1 to 5.6 at the measured temperature region. Such a small γ value was attributed to the misalignment of the grains in the tape.

Fundamental and harmonic AC responses to different annealing rates of Bi2Sr2CaCu2O8 single crystals

Abstract Three Bi 2 Sr 2 CaCu 2 O 8 single crystals with different annealing processes have been measured in terms of complex AC susceptibilities χ n = χ ′ n −i χ n ′′ ( n =1,3) to reveal the effect of flux-pinning strength. The third harmonic susceptibility changes dramatically when the pinning strengths are different due to different annealing processes, although the fundamental susceptibilities are quite similar. The experimental data are explained by the calculations based on finite thickness type-II superconductors in the presence of a geometric barrier. It has been found that the temperature dependence of the third harmonic susceptibility of the crystals follows the model predictions. When the sample is slowly cooled, the geometric barrier plays a significant role in the third harmonic AC susceptibility. Its effect is suppressed when the single crystals are cooled rapidly from high temperature.

Mechanics of membrane instability in biological cells

A model is presented to show that the cell membrane supported by viscoelastic cytoplasm and driven by the periodic actions of the extensile force due to actin assembly and the contractile force due to myosin complex can create periodical mechanical instabilities, leading to giant periodic membrane lateral undulations and causing cell membrane instability.

Three-dimensional analysis of the guided-assembled growth of heteroepitaxial islands on imperfectly pre-patterned surfaces

We simulate the guided-assembled heteroepitaxial growth of quantum dot arrays on an imperfectly pre-patterned substrate, revealing the existence of different growth regimes, such as defect tolerant regimes and defect sensitive and responsive regimes. These regimes can potentially be used to grow different designed surface patterns.

Heteroepitaxial growth of quantum wire arrays through prepatterning substrate surfaces

The present work uses three-dimensional computer simulations to investigate the formation of regularly distributed quantum wire arrays via heteroepitaxial growth on prepatterned substrate surfaces. It is found that the propagation of a wavelike surface configuration due to the surface mass diffusion driven by the nonuniform stress distribution dictates the formed wire morphology. Several types of quantum wire arrays can be obtained by controlling growth and prepattern parameters. The formation of these ordered surface structures can be explained by the wavelike propagation of the surface configurations and the evolution of the surface chemical potential during growth.