
Structural and dynamical spects of amorphization in a cluster model of a disordered phase are presented. Molecular dynamics simulations were done within a mobile cluster static boundary (MCSB, Fig. 1) model, in which static, disordered particles at the boundary of the simulation cell cause particles within the cell to adjust themselves structurally[1]. With sufficient boundary disorder(d_{0}), both the radial distribution function and the orientational order parameter[2] show onset of amorphization behavior (Fig. 2). For LennardJones particles, which are not good glass formers, the amorphization occurs only within a certain healing distance of the disordered boundary. With increasing boundary disorder, the orientational correlation function and the radial distribution function indeed show orientational phase scrambling, disappearance of longrange order, and the onset of glasslike, amorphized structure. The correlation function shows stretchedexponential relaxation behavior[3], which is typical for glass systems, is observed(Fig. 3). The fundamental processes of relaxation in glass forming systems consists of the system point crossing over the barriers that intervene between basins of attraction on the multidimensional energy landscape[4]. Such rearrangements are local in nature: they do not correspond to the entire material undergoing some transition, but rather to a local modification of the structure, occurring in a reduceddimensional space. Pictorial understanding of the local rearrangements, in the case of atomic glass, has obtained. We see a chain of local structural rearrangements in a small cluster of eight particles (Fig. 4) representing characteristic elementary kinetic events in this very simple model of glass behavior. The possibility to interpret these events in terms of defect motion is discussed[5].
[1] E. Sim, A. Z. Patashinski and M. A. Ratner, J. Chem. Phys.
(in press).
[2] A. Z. Patashinski and M. A. Ratner, J. Chem. Phys. 103,
10779 (1995).
[3] F. H. Stillinger, Phys. Rev. E52, 46685 (1995); M.
F. Shlesinger and J. T. Bendler, Phase Transitions in Soft Condensed
Matter, T. Riste and D. Sherrington, eds. (Plenum, 1989).
[4] M. Goldstein, J. Chem. Phys. 51, 3728 (1969); F.
H. Stillinger and T. A. Weber, Phys. Rev. A25, 978 (1982).
[5] E. Sim, A. Z. Patashinski and M. A. Ratner, J. Chem. Phys.
(submitted).
(a) d_{0 }= 0.0
(b) d_{0 }= 0.5
Figure 1 MCSB model of the LennardJones system with
46 mobile particles and 108 static boundary particles, open circles are
mobile particles and solid brown circles are static boundary particles.
(a) Initial structure without the boundary disorder. (b) One equilibrium
configuration snapshot at temperature T = 0.1 with boundary disorder amplitude
d_{0 }= 0.5.
Figure 2 Bond order parameter probability distribution
function with d_{0 }= 0.5 where x and y axes represent real and
imaginary part of the bond order parameter, respectively. The distribution
becomes quite homogeneous, such that the phase is almost a random variable,
corresponding to complete amorphization of the hexadic structure.
Figure 3 Temperature dependence of the bond order
parameter correlation function with d_{0 }= 0.5. Notice nonmonotonic
structure: at the lowest temperature, velocity overshoot is seen; this
is simply a remnant of the phononlike excitation at short times. With
further increase in T, the correlation function persists to longer times,
but loses any velocity memory. With yet further increase in temperature,
the system effectively melts for T > 0.7, so that correlations vanish very
rapidly in the system at higher T.
(a) t = 680
(b) t = 695
(c) t = 710
Figure 4 Snapshot of configuration at chosen times with T = 0.5. Eight chosen particles are shown in blue solid circles. During the course of number of short lifetime jumps, the inner cluster gets to many other minima in energy landscape. Notice that Figure 4(c) has more ordered structure than the initial structure.