Penta-hepta defect chaos in a model for rotating hexagonal convection

In a model for rotating non-Boussinesq convection with mean flow, we identify a regime of spatiotemporal chaos that is based on a hexagonal planform and is sustained by the induced nucleation of dislocations by penta-hepta defects. The probability distribution function for the number of defects deviates substantially from the usually observed Poisson-type distribution. It implies strong correlations between the defects in the form of density-dependent creation and annihilation rates of defects. We extract these rates from the distribution function and also directly from the defect dynamics.     

First-principles study of intrinsic defect properties in hexagonal BN bilayer and monolayer

The formation energies and transition energy levels of intrinsic defects in hexagonal BN (h-BN) bilayer and monolayer have been studied by first-principles calculations based on density functional theory. Our calculated results predict that excellent intrinsic p-type and n-type conductivities are very difficult to be realized in h-BN bilayer and monolayer. This is because of the high formation energies of acceptor-like defects (≥4.6 eV ) and the rather deep transition energy levels of donor-like defects (≥2.0 eV ). In order to obtain h-BN layers with more efficient p-type and n-type conductivity, extrinsic doping using foreign impurities is necessary.     

Dynamics of the field-induced formation of hexagonal zipped-chain superstructures in magnetic colloids

Combining nuclear magnetic resonance and molecular dynamics simulations, we unravel the long-time dynamics of a paradigmatic colloid with strong dipole-dipole interactions. In a homogeneous magnetic field, ionic ferrofluids exhibit a stepwise association process from ensembles of monomers over stringlike chains to bundles of hexagonal zipped-chain patches. We demonstrate that attractive van der Waals interactions due to charge-density fluctuations in the magnetic particles play the key role for the dynamical stabilization of the hexagonal superstructures against thermal dissociation. Our results give insight into the dynamics of self-organization in systems dominated by dipolar interactions.     

Atomistic simulation of dynamical and defect properties of multiferroic hexagonal YMnO_3

Atomistic simulation has been performed to investigate the dynamical and defect properties of multiferroic hexagonal YMnO3 with newly developed interaction potentials. Dynamical calculation reveals that phonon vibrations of hexagonal YMnO3 are quite different from those of orthorhombic YMnO3. Defect calculation finds that O Frenkel is the most probable intrinsic disorder, and Mn antisite defect is favorable to exist, especially for Mn ions entering the Y2 sites. It is also found that holes prefer to localiz...     

六方氮化硼单层中一种(CN)3VB缺陷的第一性原理计算

二维六方氮化硼(hBN)的点缺陷最近被发现可以实现室温下的单光子发射,而成为近年的研究热点.尽管其具有重要的基础和应用研究意义,hBN中发光缺陷的原子结构起源仍然存在争议.本文采用基于密度泛函理论的第一性原理计算,研究hBN单层中一种B空位附近3个N原子被C替代的缺陷(CN)3VB.在hBN的B空位处,3个N原子各自带...     

Transition to hexagonal pattern under the variation of intrinsic length scales of a reaction diffusion system

The intrinsic length scales of a reaction diffusion system (Gierer-Meinhardt model) is varied by quasi-statically changing the diffusion constant of the activator and a transition from rolls to hexagon is detected. The transition is hysteretic or first order like. From stability analysis, we also analytically show the possibility of such transitions.Received: 15 June 2004, Published online: 3 August 2004PACS: 87.10. + e General theory and mathematical aspects - 47.70.Fw Chemically reactive flows     

Dynamics and pattern formation in invasive tumor growth

We study the in vitro dynamics of the malignant brain tumor glioblastoma multiforme. The growing tumor consists of a dense proliferating zone and an outer less dense invasive region. Experiments with different types of cells show qualitatively different behavior: one cell line invades in a spherically symmetric manner, but another gives rise to branches. We formulate a model for this sort of growth using two coupled reaction-diffusion equations for the cell and nutrient concentrations. When the ratio of the nutrient and cell diffusion coefficients exceeds some critical value, the plane propagating front becomes unstable with respect to transversal perturbations. The instability threshold and the full phase-plane diagram in the parameter space are determined. The results are in a qualitative agreement with experimental findings for the two types of cells.     

Analytical solutions for some defect problems in 1D hexagonal and 2D octagonal quasicrystals

We study some typical defect problems in one-dimensional (1D) hexagonal and two-dimensional (2D) octagonal quasicrystals. The first part of this investigation addresses in detail a uniformly moving screw dislocation in a 1D hexagonal piezoelectric quasicrystal with point group 6mm. A general solution is derived in terms of two functions φ ₁, φ ₂, which satisfy wave equations, and another harmonic function φ ₃. Elementary expressions for the phonon and phason displacements, strains, stresses, electric potential, electric fields and electric displacements induced by the moving screw dislocation are then arrived at by employing the obtained general solution. The derived solution is verified by comparison with existing solutions. Also obtained in this part of the investigation is the total energy of the moving screw dislocation. The second part of this investigation is devoted to the study of the interaction of a straight dislocation with a semi-infinite crack in an octagonal quasicrystal. Here the crack penetrates through the solid along the period direction and the dislocation line is parallel to the period direction. We first derive a general solution in terms of four analytic functions for plane strain problem in octagonal quasicrystals by means of differential operator theory and the complex variable method. All the phonon and phason displacements and stresses can be expressed in terms of the four analytic functions. Then we derive the exact solution for a straight dislocation near a semi-infinite crack in an octagonal quasicrystal, and also present the phonon and phason stress intensity factors induced by the straight dislocation and remote loads.      

空位浓度对纤锌矿BN电子结构和光学性质影响的第一性原理研究

本文采用了第一性原理研究了空位缺陷纤锌矿BN的电子结构和光学性质.通过对能带结构、态密度分析发现：缺陷体系由于B、N的缺失,费米能级附近出现杂质能级.相较于本征体系,随着空位浓度增加,杂质能级变多,跃迁能量减小. N空位缺陷的纤锌矿BN态密度总体向低能区移动,且能级相较于B空位缺陷的纤锌矿BN明显增多.从复介电函数和光学吸收谱分析中发现,随着空位浓度的增加,缺陷体系纤锌矿BN在可见光区域的吸收逐渐增强.尤其是B₂₂N₂₄在可见光区域出现的吸收效果更好.     

氩气/空气介质阻挡放电自组织超六边形斑图实验研究

采用双水电极介质阻挡放电装置,在氩气/空气混合气体放电中,在三种边界条件下得到了一种新型的超六边形斑图.给出了超六边形斑图的傅里叶变换及其不同模强度随旋转角的变化.实验测量了超六边形斑图随空气含量和外加电压变化的相图.研究了超六边形斑图的时空动力学,发现超六边形斑图是由两套子结构嵌套而成.在四边形边界条件下,研究了放电面积的大小对斑图模式选择的影响.发现超四边形斑图的形成受边界条件影响很大,而超六边形斑图则是自组织的结果. 关键词： 介质阻挡放电 超六边形斑图 时空动力学 边界条件     

Dynamics of domain walls in pattern formation with traveling-wave forcing

We study dynamics of domain walls in pattern forming systems that are externally forced by a moving space-periodic modulation close to 2:1 spatial resonance. The motion of the forcing induces nongradient dynamics, while the wave number mismatch breaks explicitly the chiral symmetry of the domain walls. The combination of both effects yields an imperfect nonequilibrium Ising-Bloch bifurcation, where all kinks (including the Ising-like one) drift. Kink velocities and interactions are studied within the generic amplitude equation. For nonzero mismatch, a transition to traveling bound kink-antikink pairs and chaotic wave trains occurs.     

On the defect pattern evolution in sapphire irradiated by swift ions in a broad fluence range

Sapphire samples, irradiated with swift Kr (245 MeV) ions at room temperature in a broad fluence range, were investigated using a continuous and a pulsed positron beam to study the defect structure created by the passage of the ions in depths of a few micrometers. At small doses, monovacancies were identified as dominant defects and positron trapping centres. These monovacancies are assumed to be highly concentrated inside a cylindrical volume around the ion path with an estimated radius of ∼1.5 nm. For higher doses a second type of trapping centre emerges. This second class of structural imperfection was associated with the overlap of the individual ion tracks leading to the formation of larger vacancy clusters or voids.     

Tunnelling states in a potential of hexagonal symmetry

The tunnel splitting of the ground state multiplet in a potential of the form V₀cos 6φ is calculated both in the weak perturbation and in the high barrier limit. In the latter limit a true variational calculation is performed and this gives better results than obtained by previous methods.     

一类新的混沌神经放电的动力学特征的实验和数学模型研究

神经元电活动理论模型Hindmarsh-Rose(HR)模型提示有位于周期1和周期2放电模式之间的一类特殊的混沌放电,但长期以来对其没有获得足够认识.依据回归映射的确定性结构和非线性预报的短期可预报性,确认了在大鼠的实验性神经起步点的实验中发现的位于周期1和周期2放电模式之间的非周期放电是混沌放电模式,还将该混沌放电模式区分为3个不同表观样式.其中1个表观形式与HR模型的仿真结果相类似,验证了HR模型的理论预期;其余2个样式与仿真结果并不相似.进一步揭示了3个表观样式的动力学特征以及相互之间的区别与联系,并与位于周期2和周期3节律之间、周期3和周期4节律之间的混沌比较了异同,也区别了从周期1到混沌再到周期2放电模式的节律转迁历程与其他的从周期1到周期2节律的分岔过程的不同.研究结果确认了该类特殊混沌节律和相应分岔过程的新特征,丰富了混沌放电节律和节律分岔序列的种类.还对仿真该混沌的多样性和非光滑特性,以及揭示该类混沌的产生途径等进行了讨论. 关键词： 混沌 神经放电模式 分岔 节律     

Dynamics of defects and traveling waves in an interfacial finger pattern

We present the results of an experimental study of one-dimensional traveling finger patterns dominated by source and sink defects. The system studied is a driven fluid–air interface, and the traveling-wave state arises via a subcritical bifurcation. We analyze space–time images of the patterns to obtain the amplitude and local wave number of the left- and right-going traveling waves near the defects. We determine the width of the defects, and find that the width of the sinks does not vary significantly with distance above the bifurcation over the range of our experiments, while the source width increases as the bifurcation is approached from above. We also observe both localized depressions or enhancements in wave amplitude and periodic modulations of the pattern amplitude and wave number.     

Dynamics of precipitation pattern formation at geothermal hot springs

We formulate and model the dynamics of spatial patterns arising during the precipitation of calcium carbonate from a supersaturated shallow water flow. The model describes the formation of travertine deposits at geothermal hot springs and rimstone dams of calcite in caves. We find explicit solutions for travertine domes at low flow rates, identify the linear instabilities which generate dam and pond formation on sloped substrates, and present simulations of statistical landscape evolution.