Luminescent analysis of microsecond relaxation dynamics of viscous media

The dynamic properties of isobutanol are studied in a wide (90–350 K) temperature range by recording the relaxation shifts in the steady-state luminescence spectra of eosin and erythrosine and by kinetic phosphorescent spectroscopy invoking the Cole-Davidson model for dielectric relaxation. The characteristic dipole relaxation times and the activation energy of relaxation processes in the matrix in the temperature range of 140–170 K are determined from the correlation function of the dynamic Stokes shift of the phosphorescence spectrum of luminescent probe molecules.     

Time scales and mechanisms of relaxation in the energy landscape of polymer glass under deformation: direct atomistic modeling

Molecular-dynamics simulation is used to explore the influence of thermal and mechanical history of typical glassy polymers on their deformation. Polymer stress-strain and energy-strain developments have been followed for different deformation velocities, also in closed extension-recompression loops. The latter simulate for the first time the experimentally observed mechanical rejuvenation and overaging of polymers, and energy partitioning reveals essential differences between mechanical and thermal rejuvenation. All results can be qualitatively interpreted by considering the ratios of the relevant time scales: for cooling down, for deformation, and for segmental relaxation.     

Analysis of microsecond relaxation dynamics of proteins and viscous media by recording relaxation shifts of phosphorescence spectra

We studied the low-temperature dynamics of the Stokes shift of instantaneous phosphorescence spectra of eosin and eosin maleimide covalently bound to hemoglobin in a 66% v/v aqueous solution of glycerol under conditions of pulsed excitation. The kinetics of the Stokes shifts are described using the Cole-Davidson distribution function. From the experimental data, we obtain the parameters τ₀ and β, which describe the Cole-Davidson distribution. Values of τ₀ and β agree well with data obtained by other methods, and these parameters can be used to describe electron transfer reactions in polar solutions and proteins.     

银纳米杆高温熔化断裂弛豫性能的原子级模拟研究

采用分子动力学模拟方法,研究了不同长度银纳米杆在不同温度弛豫过程中的结构演变过程.结果表明:银纳米杆存在一与杆长相关的临界熔断温度,该临界熔断温度随杆长增加而显著降低.当温度大于熔点而小于临界熔断温度时,体系形成一个高度无序的球形团簇,而温度大于临界熔断温度时,体系则熔断成两个球形团簇.并给出了银纳米杆的产生该熔断现象的机理. 关键词： 纳米杆 分子动力学 弛豫 熔化     

Evidence of two viscous relaxation processes in the collective dynamics of liquid lithium

New inelastic x-ray scattering experiments have been performed on liquid lithium in a wide wave vector range. With respect to the previous measurements, the instrumental resolution, improved up to 1. 5 meV, allows one to accurately investigate the dynamical processes determining the observed shape of the dynamic structure factor S(Q, omega). A detailed analysis of the line shapes shows the coexistence of relaxation processes with both slow and fast characteristic time scales, and therefore shows that pictures of the relaxation mechanisms based on a simple viscoelastic model must be abandoned.     

Cross-correlated relaxation in NMR of macromolecules in the presence of fast and slow internal dynamics

In this paper we present an analysis of correlation and spectral density functions involved in autorelaxation and cross-correlated relaxation in the magnetic resonance of macromolecules. Internal dynamics of the macromolecule are described in terms of two distinct fluctuation processes with different, slow and fast, correlation times. The approach developed in this work takes into account the possible coupling between both fluctuating internal processes. To cite this article: L. Vugmeyster et al., C. R. Physique 5 (2004).     

Laser ablation of gold: Experiment and atomistic simulation

The interaction of radiation of a picosecond X-ray laser (wavelength λ = 13.9 nm) with targets made of a thick gold film has been studied theoretically and experimentally. It has been shown experimentally that the action of individual X-ray laser pulses with a fluence of F ≈ 21 mJ/cm² initiates the nanostructuring of the gold surface. Explicitly taking into account the electron subsystem, we have proposed an atomistic model of ablation that makes it possible to adequately describe the experimental results. The atomistic simulation involves the ion-ion potential depending on the electron temperature T _e . The use of such a potential makes it possible to take into account an increase in the pressure in the system with increasing T _e and to reveal two laser ablation mechanisms.     

石墨烯弛豫性能的分子动力学模拟

分别采用REBO势和AIREBO势对单层和多层石墨烯的弛豫性能进行了分子动力学模拟,模拟了石墨烯在弛豫过程中的动态平衡演化过程.模拟结果表明,理想的自由状态下,单层石墨烯薄膜并非完美的平面结构,表面不完全平整,在薄膜边缘处出现明显的波纹状褶皱,而在薄膜内部褶皱并不明显,多层石墨烯边缘处的起伏幅度要比单层石墨烯的稍小.     

Large scale simulation of block copolymers with cell dynamics

Cell dynamics simulation (CDS) is a fast method to simulate kinetic processes in phase separating systems of large sizes. We review this method applied to block copolymer systems and illustrate it on a set of physical phenomena occurring in these systems. Achievements of CDS demonstrate, that the method can describe such complex phenomena rather well and therefore, is a complementary method to other more elaborate but slow techniques to which it can serve as a precursor.     

Thermal stability of silicon nanowires: atomistic simulation study

Using the Stillinger--Weber (SW) potential model, we investigate the thermal stability of pristine silicon nanowires based on classical molecular dynamics (MD) simulations. We explore the structural evolutions and the Lindemann indices of silicon nanowires at different temperatures in order to unveil atomic-level melting behaviour of silicon nanowires. The simulation results show that silicon nanowires with surface reconstructions have higher thermal stability than those without surface reconstructions, and that silicon nanowires with perpendicular dimmer rows on the two (100) surfaces have somewhat higher thermal stability than nanowires with parallel dimmer rows on the two (100) surfaces. Furthermore, the melting temperature of silicon nanowires increases as their diameter increases and reaches a saturation value close to the melting temperature of bulk silicon. The value of the Lindemann index for melting silicon nanowires is 0.037.     

Structural relaxation and viscous flow in amorphous ZrAlCu

The ternary metallic glass Zr₆₅Al_7.5Cu_27.5 offers a wide temperature range between glass transition temperature and crystallization temperature and is therefore well suited for investigation of the glass transition and the state of the super cooled liquid. The non-linear viscosity change caused by structural relaxation has been measured caused by structural relaxation has been measured using tensile creep experiments on as quenched samples. The increase of viscosity can be described by bimolecular annihilation kinetics of flow defects. The Arrhenius plot of equilibrium viscosity shows a kink at a temperature which seems to be the glass transition temperature. The activation energies of viscous flow below and above that glass transition temperature differ by nearly a factor two. Different microscopic processes responsible for viscous flow in the two regimes of temperature are therefore conceivable. This view is also encouraged by Dynamic-Mechanical-Analysis on relaxed samples, a method to examine the viscoelastic behaviour of glassy materials on different time scales and by recent diffusion measurements on a different system.     

Structural relaxation and viscous flow in amorphous ZrAlCu

The ternary metallic glass Zr₆₅Al_7.5Cu_27.5 offers a wide temperature range between glass transition temperature and crystallization temperature and is therefore well suited for investigation of the glass transition and the state of the super cooled liquid. The non-linear viscosity change caused by structural relaxation has been measured caused by structural relaxation has been measured using tensile creep experiments on as quenched samples. The increase of viscosity can be described by bimolecular annihilation kinetics of flow defects. The Arrhenius plot of equilibrium viscosity shows a kink at a temperature which seems to be the glass transition temperature. The activation energies of viscous flow below and above that glass transition temperature differ by nearly a factor two. Different microscopic processes responsible for viscous flow in the two regimes of temperature are therefore conceivable. This view is also encouraged by Dynamic-Mechanical-Analysis on relaxed samples, a method to examine the viscoelastic behaviour of glassy materials on different time scales and by recent diffusion measurements on a different system.     

Evolution of ion-irradiated point defect concentration by cluster dynamics simulation

The relationship between ions irradiation and the induced microstructures(point defects, dislocations, clusters, etc.)could be better analyzed and explained by simulation. The mean field rate theory and cluster dynamics are used to simulate the effect of implanted Fe on the point defects concentration quantitatively. It is found that the depth distribution of point defect concentration is relatively gentle than that of damage calculated by SRIM software. Specifically, the damage rate and point defect concentration increase by 1.5 times and 0.6 times from depth of 120 nm to 825 nm, respectively. With the consideration of implanted Fe ions, which effectively act as interstitial atoms at the depth of high ion implantation rate, the vacancy concentration C_v decreases significantly after reaching the peak value, while the interstitial atom concentration C_i increases significantly after decline of the previous stage. At the peak depth of ion implantation, C_v dropped by 86%, and C_i increased by 6.2 times. Therefore, the implanted ions should be considered into the point defects concentration under high dose of heavy ion irradiation, which may help predict the concentration distribution of defect clusters, further analyzing the evolution behavior of solute precipitation.     

Relaxation and movement of point defect clusters in copper by molecular dynamics simulation

Abstract

The structures of point defect clusters of both interstitial and vacancy type were examined by computer simulation using molecular dynamics and molecular statics with the DYNAMO code (Daw, Foiles and Baskes [6]). The code implements an isotropic potential of embedded atom method (EAM) developed by Daw and Baskes [5]. Interstitial clusters relax to either the immobile mixture of <100> dumbbell and bcc interstitials or a mobile platelet of parallel <110> interstitials. The latter cluster moves along <110> directions. A tri-vacancy relaxes to an un-collapsed stacking fault tetrahedron (sft) of Damask-Dienes type (3v-sft) containing a central atom that vibrates with a large amplitude. A hexa-vacancy relaxes to a stacking fault tetrahedron the structure of which fluctuates between a sft and void. Larger vacancy clusters are stable as a combination of sft and 3v-sft. In these vacancy clusters, atoms show significant vibration with large amplitude. Voids form only with the inclusion of gas-atoms into vacancy clusters.     

Approximate linear response for slow variables of dynamics with explicit time scale separation

Many real-world numerical models are notorious for the time scale separation of different subsets of variables and the inclusion of random processes. The existing algorithms of linear response to external forcing are vulnerable to the time scale separation due to increased response errors at fast scales. Here we develop the approximate linear response algorithm for slow variables in a two-scale dynamical system with explicit separation of slow and fast variables, which has improved numerical stability and reduced computational expense.     

Hydrogen-bond dynamics near a micellar surface: origin of the universal slow relaxation at complex aqueous interfaces

The dynamics of hydrogen bonds among water molecules themselves and with the polar head groups (PHG) at a micellar surface have been investigated by long molecular dynamics simulations. The lifetime of the hydrogen bond between a PHG and a water molecule is found to be much longer than that between any two water molecules, and is likely to be a general feature of hydrophilic surfaces of organized assemblies. Analyses of individual water trajectories suggest that water molecules can remain bound to the micellar surface for more than 100 ps. The activation energy for such a transition from the bound to a free state for the water molecules is estimated to be about 3.5 kcal/mol.     

Cluster dynamics simulation of point defect clusters in neutron irradiated pure iron

The effect of the size dependence of the production rate of point defect clusters is taken into account in the cluster dynamics (CD) modeling of the simultaneous formation of vacancy clusters and self-interstitial atom clusters in neutron-irradiated pure iron. The calibration of material parameters has been carried out. The correspondence between small angle neutron scattering, transmission electron microscopy and positron annihilation spectroscopy and CD data is studied.