二维胶体玻璃中玻色峰与结构无序度的关联

本文从实验上研究了胶体玻璃在相同面密度下随着体系结构无序程度的增加, 振动态密度和玻色峰的变化规律. 通过调制两种不同粒径的温敏性水凝胶的数量比来改变体系的无序程度. 通过分析无序体系的声子模式得到体系的振动特性. 研究发现, 随着无序度的增加, 态密度在低频区域增强、玻色峰增高、玻色峰的峰值向低频区域移动. 不同无序程度的样品引起玻色峰的低频声子模式都表现出准局域的特点, 且低频准局域声子模式与样品中无序结构存在关联.     

Vibrational modes identify soft spots in a sheared disordered packing

We analyze low-frequency vibrational modes in a two-dimensional, zero-temperature, quasistatically sheared model glass to identify a population of structural "soft spots" where particle rearrangements are initiated. The population of spots evolves slowly compared to the interval between particle rearrangements, and the soft spots are structurally different from the rest of the system. Our results suggest that disordered solids flow via localized rearrangements that tend to occur at soft spots, which are analogous to dislocations in crystalline solids.     

Connecting structural relaxation with the low frequency modes in a hard-sphere colloidal glass

Structural relaxation in hard-sphere colloidal glasses has been studied using confocal microscopy. The motion of individual particles is followed over long time scales to detect the rearranging regions in the system. We have used normal mode analysis to understand the origin of the rearranging regions. The low-frequency modes, obtained over short time scales, show strong spatial correlation with the rearrangements that happen on long time scales.     

Short-time dynamics of a packing of polyhedral grains under horizontal vibrations

We analyze the dynamics of a 3D granular packing composed of particles of irregular polyhedral shape confined inside a rectangular box with a retaining wall subjected to horizontal harmonic forcing. The simulations are performed by means of the contact dynamics method for a broad set of loading parameters. We explore the vibrational dynamics of the packing, the evolution of solid fraction and the scaling of dynamics with the loading parameters. We show that the motion of the retaining wall is strongly anharmonic as a result of jamming and grain rearrangements. It is found that the mean particle displacement scales with inverse square of frequency, the inverse of the force amplitude and the square of gravity. The short-time compaction rate grows in proportion to frequency up to a characteristic frequency, corresponding to collective particle rearrangements between equilibrium states, and then it declines in inverse proportion to frequency.     

线性时滞反馈引起的周期性振动共振分析

研究了高频信号和微弱低频信号同时激励下线性时滞反馈对过阻尼双稳系统和Duffing振子系统中振动共振现象的影响. 解析分析和数值结果都表明, 系统对低频信号的响应幅值增益随时滞参数的变化同时呈现两种不同的周期性关系, 其周期分别为输入的高频信号和低频信号的周期. 数值结果还表明, 对不存在经典振动共振现象的单稳Duffing系统, 通过调节时滞参数也可以引发振动共振现象. 使用时滞反馈不仅可以有效地控制振动共振, 还可以进一步增强系统对微弱低频信号的响应. 关键词： 双稳系统 Duffing 系统 线性时滞反馈 振动共振     

Localized vibrational modes in a quasi-one-dimensional lattice with a macrodefect

The vibrational spectrum of a quasi-one-dimensional lattice with a crack-like macrodefect is studied. The localized vibrational modes of two types are found to exist in a system: (i) low-frequency vibrational modes localized inside the defect; (ii) high-frequency vibrational modes localized outside the defect. The elastic relaxation length is estimated for localized modes of both types.     

Fluctuating particle motion during shear induced granular compaction

Using a refractive index matching method, we investigate the trajectories of particles in three dimensional granular packing submitted to cyclic shear deformation. The particle motion observed during compaction is not diffusive but exhibits a transient cage effect, similar to the one observed in colloidal glasses. We precisely study the statistics of the step size between two successive cycles and observe that it is proportional to the shear amplitude. The link between the microscopic observations and the macroscopic evolution of the volume fraction during compaction is discussed.     

Quantitative structure-plasticity relationship in metallic glass:A machine learning study

The lack of the long-range order in the atomic structure challenges the identification of the structural defects, akin to dislocations in crystals, which are responsible for predicting plastic events and mechanical failure in metallic glasses(MGs). Although vast structural indicators have been proposed to identify the structural defects, quantitatively gauging the correlations between these proposed indicators based on the undeformed configuration and the plasticity of MGs upon external loads is still lacking. Here, we systematically analyze the ability of these indicators to predict plastic events in a representative MG model using machine learning method. Moreover, we evaluate the influences of coarse graining method and medium-range order on the predictive power. We demonstrate that indicators relevant to the low-frequency vibrational modes reveal the intrinsic structural characteristics of plastic rearrangements. Our work makes an important step towards quantitative assessments of given indicators, and thereby an effective identification of the structural defects in MGs.     

Phase diagram of patchy colloids: towards empty liquids

We report theoretical and numerical evaluations of the phase diagram for patchy colloidal particles of new generation. We show that the reduction of the number of bonded nearest neighbors offers the possibility of generating liquid states (i.e., states with temperature T lower than the liquid-gas critical temperature) with a vanishing occupied packing fraction (phi), a case which can not be realized with spherically interacting particles. Theoretical results suggest that such reduction is accompanied by an increase of the region of stability of the liquid phase in the (T-phi) plane, possibly favoring the establishment of homogeneous disordered materials at small phi, i.e., stable equilibrium gels.     

Relaxing local modes and the theory of low-frequency Raman scattering in glasses

Raman scattering in glasses is investigated theoretically. The experimental Raman spectra of glasses exhibit a low-frequency peak (at ～10 cm^?1) that, as a rule, is attributed to vibrational modes of nanometer-sized structural units (nanocrystallites). It is established that the elastic moduli of nanocrystallites must necessarily be dependent on their sizes due to the Laplace pressure effect. A theory of the low-frequency peak is constructed using a realistic size distribution function of nanocrystallites with allowance made for the Laplace pressure effect and the dissipation of vibrational energy. Within this theory, the shape of the low-frequency peak and its evolution with temperature can be analyzed quantitatively. The proposed approach offers a physical interpretation of the experimental data and provides insight into the relation of the characteristic nanocrystallite sizes to the elastic moduli and surface tension coefficients of materials.     

Low-temperature dynamics of amorphous polymers and evolution over time of spectra of single impurity molecules: II. Model calculations and analysis of results

The results of a statistical analysis of the spectra of single molecules of tetra-tert-butylterrylene in amorphous polyisobutylene at temperatures of 2, 4.5, and 7 K are presented. Model calculations of such spectra for this system are performed in the context of the stochastic theory of the spectra of single molecules in lowtemperature glasses. Analysis of the multiplet structure of the experimental and model spectra made it possible to obtain data about the minimal distance between impurity chromophore molecules and two-level systems and about the distribution parameters of their coupling constant. The interaction of a chromophore with quasilocal low-frequency vibrational modes of the matrix was found to influence the structure and parameters of the spectra observed. The model calculations performed showed that the specific structure of the spectra of single molecules at low temperatures is determined by the interactions with a small number of nearby two-level systems.     

Controlling vibrational resonance in a multistable system by time delay

The phenomenon of vibrational resonance in a delayed multistable system that is excited by biharmonic signals is investigated in the present paper. Different from the former theory, the appearance and the disappearance of the vibrational resonance are controlled by adjusting the time delay parameter instead of modulating the amplitude of the high-frequency signal. The motion of the orbit within or between the different potential wells can also be controlled. Furthermore, based on both the methods of numerical simulation and analytical analysis, the behavior of delay-induced multiple vibrational resonance and its mechanism are investigated and discussed. The multiple vibrational resonance, which is quantified by the response amplitude at the low-frequency, is found to be periodic in the delay parameter with two periods, i.e., the periods of the two driven signals. The method used in this paper gives a new way for controlling vibrational resonance in a multistable system.     

Properties of cage rearrangements observed near the colloidal glass transition

We use confocal microscopy to study particle motion in colloidal systems. Near the glass transition, motion is inhibited, as particles spend time trapped in transient "cages" formed by neighboring particles. We measure the cage sizes and lifetimes, which, respectively, shrink and grow as the glass transition approaches. Cage rearrangements are more prevalent in regions with lower concentrations and higher disorder. Neighboring rearranging particles typically move in parallel directions, although a nontrivial fraction moves in antiparallel directions, usually from particle pairs with initial separations corresponding to local maxima and minima of the pair correlation function g(r), respectively.     

Jamming, two-fluid behavior, and "self-filtration" in concentrated particulate suspensions

We study the flow of model hard-sphere colloidal suspensions at high volume fraction Phi driven through a constriction by a pressure gradient. Above a particle-size dependent limit Phi(0), direct microscopic observations demonstrate jamming and unjamming-conversion of fluid to solid and vice versa-during flow. We show that such a jamming flow produces a reduction in colloid concentration Phi(x) downstream of the constriction. We propose that this "self-filtration" effect is due to a combination of jamming of the particulate part of the system and continuing flow of the liquid part, i.e., the solvent, through the pores of the jammed solid. Thus we link jamming in colloidal and granular media with a "two-fluid-like" picture of the flow of concentrated suspensions. Results are also discussed in the light of the original experiments of Reynolds on dilation in granular materials.     

Formation of square-order areas on the colloidosome surface

The self-assembly of the colloidosome, i.e., the system of rather densely packed colloidal particles arranged at the spherical interface of two phases, has been simulated. Particles are retained on the sphere surface by surface tension and are ordered due to the interaction described by the Lennard-Jones potential. The particle coordinates are obtained by conditional minimization of the free energy of the system. The violations of the average hexagonal order of colloidosome surface particles, not reduced to the topologically caused formation of pentagonal point defects, are studied. It is shown that the average area excess per colloidal particle can cause the formation of square-order areas observed in colloidosomes.     

Multiscale structures and phase transitions in metallic glasses:A scattering perspective

Amorphous materials are ubiquitous and widely used in human society, yet their structures are far from being fully understood. Metallic glasses, a new class of amorphous materials, have attracted a great deal of interests due to their exceptional properties. In recent years, our understanding of metallic glasses increases dramatically, thanks to the development of advanced instrumentation, such as in situ x-ray and neutron scattering. In this article, we provide a brief review of recent progress in study of the structure of metallic glasses. In particular, we will emphasize, from the scattering perspective, the multiscale structures of metallic glasses, i.e., short-to-medium range atomic packing, and phase transitions in the supercooled liquid region, e.g., crystallization and liquid-to-liquid phase transition. We will also discuss, based on the understanding of their structures and phase stability, the mechanical and magnetic properties of metallic glasses.     

Hydrodynamic Stress Tensor in Inhomogeneous Colloidal Suspensions: an Irving-Kirkwood Extension *

Based on statistical mechanics for classical fluids, general expressions for hydrodynamic stress in inhomogeneous colloidal suspension are derived on a molecular level. The result is exactly an extension of the Iving-Kirkwood stress for atom fluids to colloidal suspensions where dynamic correlation emerges. It is found that besides the inter-particle distance, the obtained hydrodynamic stress depends closely on the velocity of the colloidal particles in the suspension, which is responsible for the appearance of the solvent-mediated hydrodynamic force. Compared to Brady's stresslets for the bulk stress, our results are applicable to inhomogeneous suspension, where the inhomogeneity and anisotropy of the dynamic correlation should be taken into account. In the near-field regime where the packing fraction of colloidal particles is high, our results can reduce to those of Brady. Therefore, our results are applicable to the suspensions with low, moderate, or even high packing fraction of colloidal particles.     

Radiation-pressure-driven vibrational modes in ultrahigh-Q silica microspheres

Quantitative measurements of the vibrational eigenmodes in ultrahigh-Q silica microspheres are reported. The modes are excited via radiation-pressure-induced dynamical backaction of light confined in the optical whispering-gallery modes of the microspheres (i.e., via the parametric oscillation instability). Two families of modes are studied and their frequency dependence on sphere size investigated. The measured frequencies are in good agreement both with Lamb's theory and numerical finite-element simulation and are found to be proportional to the sphere's inverse diameter. In addition, the quality factors of the vibrational modes are studied.     

二维有摩擦颗粒体系振动态密度与玻色峰的研究

利用颗粒离散元方法,研究了由2048个有摩擦的单分散圆盘颗粒组成的体系在各向同性压缩条件下,颗粒摩擦系数μ对颗粒体系结构与振动特性的影响.结果表明:固定压强下,随μ的增大,区分德拜标度与态密度平台的过渡频率ω*与玻色峰频率ωBP均向低频移动,玻色峰高度D(ωBP)/ωBP逐渐增加.主要原因是μ增大导致颗粒体系无序程度增加(平均配位数减小)而在ωω*处出现了大量额外模式.模式分析表明:低频(ω1.0)模式主要是以平动为主的混合模式,中频(1.0ω4.0)模式主要是以平动为主的混合局域化模式,高频(ω4.0)振动模式几乎为纯转动的局域化模式;并且随μ的增大,低频下平动模式更加局域化,同时低频转动模式的贡献也逐渐增加,暗示在高摩擦系数下低频转动模式产生更重要的影响.     

Surface acoustic phonon modes of Ge nanocrystals embedded in SiO2

Ge nanocrystals (NCs) embedded in SiO₂ are synthesized by ion implantation, and the surface vibrational modes of the Ge NCs are investigated using the low-frequency Raman scattering (LFRS) technique. LFRS studies show distinct low-frequency Raman modes in the range 6.5-21.2 cm⁻¹ for the Ge NCs depending on the implant dose and annealing temperature. These low-frequency Raman modes are attributed to the confined surface acoustic phonon modes of Ge NCs with (0,0) spheroidal mode and (0,3) torsional modes. Our results are in excellent agreement with the recent theoretical predictions of surface vibrational modes in Ge NCs.