Stimulus dependence of the collective vibration of atoms in an icosahedral cluster.doc

免费查阅精品论文Stimulus dependence of the collective vibration of atoms in an icosahedral clusterLiu H. H. 1，Jiang E. Y. 1*，Bai H. L. 1，Wu P. 1，Li Z. Q1，Sun Chang Qing 21 Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparing Technology，Institute of Advanced Materials Physics，Faculty of Science，Tianjin University，Tianjin（300072）2 School of Electrical and electronic Engineering, Nanyang Technological University, Singapore（639798）E- mail：eyjiang@，ecqsun@ntu.edu.sgAbstractMolecular dynamical calculations of the thermal relaxation of a Lennard-Jones 147-atom cluster revealedthat the collective vibration of the atoms in the cluster at the initial stage of relaxation upon perturbation of dislocation and that the stability of the collective vibration is affected by the strength and extent of the mechanical stimulus. A stronger or larger-scale stimulus will cause the vibration to decay faster compared to the vibration stimulated by a weaker or small-scale stimulus. Understanding may suggest a possible way on how the stimulus-introduced energy is transferred in the cluster: the collective vibration of atoms and its decay.Keywords：molecular dynamics，nanostructures，lattice dynamics1. IntroductionExcitation by means of structure relaxation and electronic oscillation is of great importance in understanding the process of phase transition and energy transfer in materials [1,2,3,4,5]. Among all sorts of vibration modes in material structure, collective vibration, or say collective excitation, stands out itself by consistent movement of a collection of particles [6,7,8,9]. Such collectively excited states were first discovered in nuclei [10,11], and predicted in the free electron gas in metals by Pines et al. [12,13], then confirmed experimentally by Kaplan et al. [14]. Using molecular dynamic simulation, Salian et al [15], investigated the collective excitations in an Ar13 cluster. They moved the twelve atoms on the surface radially away of the center atom by the same distance, causing the cluster expanding to its maximum. They found in the subsequent relaxing process that the cluster gave radial uniform expansion and contraction, which is considered to be collective atomic vibration.In the present work, we introduce dislocation stimulus into the icodahedral cluster containing147-atom with Lennard-Jones (LJ147) potential to study the relaxing process. Different from that conducted by Salian et al.[15], we disturb a certain portion of the atoms randomly, the monopole collective atomic vibration is found to be followed by a decay of the vibration. Results show that stronger stimulus can accelerate the decaying of the collective vibration.2. MethodWe may divide the LJ147 cluster, from the central atom as core-0 to surface, into shell-1, shell-2, shell-3 [16] as shown in Fig. 1. In order to show the relationship between the atoms clearly, we mark the atoms by groups [Fig. 1b]: the atoms on the vertex are marked as [shell no] a, those at the edges are marked as [shell no]b, those on the facets are marked as [shell no]c. For example, atoms on the vertex of shell-3 are marked as 3a, atoms at the edges of shell-2 are marked as 2b. Because there is only one sort of atoms in shell-1, we mark them as 1a.We combined the molecular dynamic simulation with Verlet algorithm [17] in calculation. The atoms are bonded through the Lennard-Jones potential- 1 -ijP(rij) = 4ε[(σ / rij)12 − (σ / r)6 ], (1)whereri j is the distance between atom i and j . ε , σ and the mass of a single atom m areused as the unit of energy, length and mass, respectively. Then the unit of temperature and time wouldbe ε / k Bandmσ 2 / ε , respectively, where kBis the Boltzmann constant. We can obtain thepositions and velocities of atoms each time by Verlet algorithm, the time interval between two timesteps is set to be10−4 mσ 2 / ε .At the beginning of the simulation, we introduced weak stimulus into specific local structures of the LJ147 cluster by moving randomly the specified atoms a short distance dxyz along axis x, y and z. Larger dxyz means heavier stimulus. For convenience, we call dxyz as the scale of stimulus. Since the icosahedral LJ147 cluster is the global optimized configuration for clusters of this size, introducing stimulus will provide excessive energy to the cluster. In the current work, we study how the position and energy of the specific atoms change during the excessive energy is released from the structure.We move randomly the atoms in core-2 (atoms in core-0, shell-1 and shell-2) and core-3 (atoms in core-0, shell-1, shell-2 and shell-3, or say the atoms of the whole cluster) by dxyz along the x, y and z axis, then the positions and energies of atoms in the LJ147 cluster are investigated during the excessive energy is released.。