714 research outputs found
The High Temperature Dynamics of a mean field Potts glass
We use Monte Carlo simulations to investigate the dynamical properties of the
infinite range 10 states Potts glass. By analyzing the spin autocorrelation
function for system sizes up to N=2560, we show that strong finite size effects
are present around the predicted dynamical transition temperature. The
autocorrelation function shows strong self-averaging at high temperatures,
whereas close to the dynamical transition they show the lack of self-averaging.Comment: 7 pages of Latex, 4 figure
High Frequency Dynamics of Amorphous Silica
We present the results of extensive molecular dynamics computer simulations
in which the high frequency dynamics of silica, nu>0.5 THz, is investigated in
the viscous liquid state as well as in the glass state. We characterize the
properties of high frequency sound modes by analyzing J_l(q,nu) and J_t(q,nu),
the longitudinal and transverse current correlation function, respectively. For
wave-vectors q>0.4 Angstrom^{-1} the spectra are sitting on top of a flat
background which is due to multiphonon excitations. In the acoustic frequency
band, i.e. for nu<20 THz, the intensity of J_l(q,nu) and J_t(q,nu) in the
liquid and the glass approximately proportional to temperature, in agreement
with the harmonic approximation. In contrast to this, strong deviations from a
linear scaling are found for nu>20 THz. The dynamic structure factor S(q,nu)
exhibits for q>0.23 Angstrom^{-1} a boson peak which is located nearly
independent of q around 1.7 THz. We show that the low frequency part of the
boson peak is mainly due to the elastic scattering of transverse acoustic modes
with frequencies around 1 THz. The strength of this scattering depends on q and
is largest around q=1.7 Angstrom^{-1}, the location of the first sharp
diffraction peak in the static structure factor. By studying S(q,nu) for
different system sizes we show that strong finite size effects are present in
the low frequency part of the boson peak in that for small systems part of its
intensity is missing. We discuss the consequences of these finite size effects
for the structural relaxation.Comment: 33 pages of Latex, 15 figure
Static and dynamical properties of a supercooled liquid confined in a pore
We present the results of a Molecular Dynamics computer simulation of a
binary Lennard-Jones liquid confined in a narrow pore. The surface of the pore
has an amorphous structure similar to that of the confined liquid. We find that
the static properties of the liquid are not affected by the confinement, while
the dynamics changes dramatically. By investigating the time and temperature
dependence of the intermediate scattering function we show that the dynamics of
the particles close to the center of the tube is similar to the one in the
bulk, whereas the characteristic relaxation time tau_q(T,rho) of the
intermediate scattering function at wavevector q and distance rho from the axis
of the pore increases continuously when approaching the wall, leading to an
apparent divergence in the vicinity of the wall. This effect is seen for
intermediate temperatures down to temperatures close to the glass transition.
The rho-dependence of tau_q(T,rho) can be described by an empirical law of the
form tau_q(T,\rho)=f_q(T) exp [Delta_q/(rho_p-rho)], where Delta_q and \rho_q
are constants, and f_q(T) is the only parameter which shows a significant
temperature dependence.Comment: 4 pages of Latex, 4 figures Pari
The Boson Peak in Amorphous Silica: Results from Molecular Dynamics Computer Simulations
We investigate a prominent vibrational feature in amorphous silica, the
so-called boson peak, by means of molecular dynamics computer simulations. The
dynamic structure factor S(q,nu) in the liquid, as well as in the glass state,
scales roughly with temperature, in agreement with the harmonic approximation.
By varying the size of the system and the masses of silicon and oxygen we show
that the excitations giving rise to the boson peak are due to the coupling to
transverse acoustic modes.Comment: 6 pages of Latex, 3 figures, uses aipproc.sty; to appear in
proceedings of "Neutrons and Numerical Methods" Grenoble, Dec. 1998, Ed. H.G.
Buttner et a
The relaxation dynamics of a supercooled liquid confined by rough walls
We present the results of molecular dynamics computer simulations of a binary
Lennard-Jones liquid confined between two parallel rough walls. These walls are
realized by frozen amorphous configurations of the same liquid and therefore
the structural properties of the confined fluid are identical to the ones of
the bulk system. Hence this setup allows us to study how the relaxation
dynamics is affected by the pure effect of confinement, i.e. if structural
changes are completely avoided. We find that the local relaxation dynamics is a
strong function of z, the distance of the particles from the wall, and that
close to the surface the typical relaxation times are orders of magnitude
larger than the ones in the bulk. Because of the cooperative nature of the
particle dynamics, the slow dynamics also affects the dynamics of the particles
for large values of z. Using various empirical laws, we are able to
parameterize accurately the z-dependence of the generalized incoherent
intermediate scattering function F_s(q,z,t) and also the spatial dependence of
structural relaxation times. These laws allow us to determine various dynamical
length scales and we find that their temperature dependence is compatible with
an Arrhenius law. Furthermore, we find that at low temperatures time and space
dependent correlation function fulfill a generalized factorization property
similar to the one predicted by mode-coupling theory for bulk systems. For thin
films and/or at sufficiently low temperatures, we find that the relaxation
dynamics is influenced by the two walls in a strongly non-linear way in that
the slowing down is much stronger than the one expected from the presence of
only one confining wall. ....Comment: 22 pages of Late
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