Résumé
Phys. Rev. E 64, 041503 (2001) We use molecular dynamics computer simulations to study the relaxation
dynamics of a viscous melt of silica. The coherent and incoherent intermediate
scattering functions, F_d(q,t) and F_s(q,t), show a crossover from a nearly
exponential decay at high temperatures to a two-step relaxation at low
temperatures. Close to the critical temperature of mode-coupling theory (MCT)
the correlators obey in the alpha-regime the time temperature superposition
principle (TTSP) and show a weak stretching. We determine the wave-vector
dependence of the stretching parameter and find that for F_d(q,t) it shows
oscillations which are in phase with the static structure factor. The
temperature dependence of the alpha- relaxation times tau shows a crossover
from an Arrhenius law at low temperatures to a weaker T-dependence at
intermediate and high temperatures. At the latter temperatures the T-dependence
is described well by a power law. We find that the exponent gamma of the power
law for tau are significantly larger than the one for the diffusion constant.
The q-dependence of the alpha-relaxation times for F_d(q,t) oscillates around
tau(q) for F_s(q,t) and is in phase with the structure factor. Due to the
strong vibrational component of the dynamics at short times the TTSP is not
valid in the beta- relaxation regime. We show, however, that in this time
window the shape of the curves is independent of the correlator and is given by
a functional form proposed by MCT. We find that the value of the von Schweidler
exponent and the value of gamma for finite q are compatible with the expression
proposed by MCT. We conclude that, in the temperature regime where the
relaxation times are mesoscopic, many aspects of the dynamics of this strong
glass former can be rationalized very well by MCT.