Browsing by Author "Lai, S. K."

Now showing 1 - 4 of 4

Comparative study of cluster Ag(17)Cu(2) by instantaneous normal mode analysis and by isothermal Brownian-type molecular dynamics simulation
(2011-09-07)
Tang, Ping-Han
;
Wu, Ten-Ming
;
Yen, Tsung-Wen
;
Lai, S. K.
;
Hsu, P. J.
We perform isothermal Brownian-type molecular dynamics simulations to obtain the velocity auto-correlation function and its time Fourier-transformed power spectral density for the metallic cluster Ag(17)Cu(2). The temperature dependences of these dynamical quantities from T = 0 to 1500 K were examined and across this temperature range the cluster melting temperature T(m), which we define to be the principal maximum position of the specific heat is determined. The instantaneous normal mode analysis is then used to dissect the cluster dynamics by calculating the vibrational instantaneous normal mode density of states and hence its frequency integrated value I(j) which is an ensemble average of all vibrational projection operators for the jth atom in the cluster. In addition to comparing the results with simulation data, we look more closely at the entities I(j) of all atoms using the point group symmetry and diagnose their temperature variations. We find that I(j) exhibit features that may be used to deduce T(m), which turns out to agree very well with those inferred from the power spectral density and specific heat. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3628669]
Comparative study of cluster Ag17Cu2 by instantaneous normal mode analysis and by isothermal Brownian-type molecular dynamics simulation
(2011-09-07)
Tang, Ping-Han
;
Wu, Ten-Ming
;
Yen, Tsung-Wen
;
Lai, S. K.
;
Hsu, P. J.
We perform isothermal Brownian-type molecular dynamics simulations to obtain the velocity auto-correlation function and its time Fourier-transformed power spectral density for the metallic cluster Ag17Cu2. The temperature dependences of these dynamical quantities from T = 0 to 1500 K were examined and across this temperature range the cluster melting temperature T-m, which we define to be the principal maximum position of the specific heat is determined. The instantaneous normal mode analysis is then used to dissect the cluster dynamics by calculating the vibrational instantaneous normal mode density of states and hence its frequency integrated value I-j which is an ensemble average of all vibrational projection operators for the jth atom in the cluster. In addition to comparing the results with simulation data, we look more closely at the entities I-j of all atoms using the point group symmetry and diagnose their temperature variations. We find that I-j exhibit features that may be used to deduce T-m, which turns out to agree very well with those inferred from the power spectral density and specific heat. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3628669]
Melting behavior of Ag-14 cluster: An order parameter by instantaneous normal modes
(2012-12-28)
Tang, Ping-Han
;
Wu, Ten-Ming
;
Hsu, P. J.
;
Lai, S. K.
;
物理研究所
This paper studies the melting behavior of Ag-14 cluster employing the instantaneous normal mode (INM) analysis that was previously developed for bimetallic cluster Ag17Cu2. The isothermal Brownian-type molecular dynamics simulation is used to generate atom configurations of Ag-14 at different temperatures up to 1500 K. At each temperature, these atomic configurations are then analyzed by the INM technique. To delve into the melting behavior of Ag-14 cluster which differs from Ag17Cu2 by the occurrence of an anomalous prepeak in the specific heat curve in addition to the typical principal peak, we appeal to examining the order parameter tau(T) defined in the context of the INM method. Two general approaches are proposed to calculate tau(T). In one, tau(T) is defined in terms of the INM vibrational density of states; in another, tau(T) is defined considering the cluster as a rigid body with its rotational motions described by three orthogonal eigenvectors. Our results for Ag-14 by these two methods indicate the mutual agreement of tau(T) calculated and also the consistent interpretation of the melting behavior with the specific heat data. The order parameter tau(T) provides in addition an insightful interpretation between the melting of clusters and the concept of broken symmetry which has been found successful in studies of the melting transition of bulk systems. (C) 2012 American Institute of Physics. [http://dx.doi.org/10.1063/1.4772096]
Order parameter by instantaneous normal mode analysis for melting behaviour of cluster Ag17Cu2
(2013)
Tang, Pin-Han
;
Wu, Ten-Ming
;
Lai, S. K.
;
物理研究所
;
Institute of Physics
We perform isothermal Brownian-type molecular dynamics simulations with the Gupta potential for bimetallic cluster Ag17Cu2 from T=0 to 1500K, across the temperature range associated with the melting behaviour determined by the specific heat of the cluster. We also use the instantaneous normal mode (INM) analysis to dissect dynamics of the cluster. In terms of the projected density of states of the vibrational INMs, we propose a new order parameter that specifically describes the melting behaviour of the cluster. The calculated result of our order parameter is consistent with the information inferred from the specific heat data.