42 research outputs found
Structure and relaxations in liquid and amorphous Selenium
We report a molecular dynamics simulation of selenium, described by a
three-body interaction. The temperatures T_g and T_c and the structural
properties are in agreement with experiment. The mean nearest neighbor
coordination number is 2.1. A small pre-peak at about 1 AA^-1 can be explained
in terms of void correlations. In the intermediate self-scattering function,
i.e. the density fluctuation correlation, classical behavior, alpha- and
beta-regimes, is found. We also observe the plateau in the beta-regime below
T_g. In a second step, we investigated the heterogeneous and/or homogeneous
behavior of the relaxations. At both short and long times the relaxations are
homogeneous (or weakly heterogeneous). In the intermediate time scale, lowering
the temperature increases the heterogeneity. We connect these different domains
to the vibrational (ballistic), beta- and alpha-regimes. We have also shown
that the increase in heterogeneity can be understood in terms of relaxations
Properties of the energy landscape of network models for covalent glasses
We investigate the energy landscape of two dimensional network models for
covalent glasses by means of the lid algorithm. For three different particle
densities and for a range of network sizes, we exhaustively analyse many
configuration space regions enclosing deep-lying energy minima. We extract the
local densities of states and of minima, and the number of states and minima
accessible below a certain energy barrier, the 'lid'. These quantities show on
average a close to exponential growth as a function of their respective
arguments. We calculate the configurational entropy for these pockets of states
and find that the excess specific heat exhibits a peak at a critical
temperature associated with the exponential growth in the local density of
states, a feature of the specific heat also observed in real glasses at the
glass transition.Comment: RevTeX, 19 pages, 7 figure
Simulation of thermal conductivity and heat transport in solids
Using molecular dynamics (MD) with classical interaction potentials we
present calculations of thermal conductivity and heat transport in crystals and
glasses. Inducing shock waves and heat pulses into the systems we study the
spreading of energy and temperature over the configurations. Phonon decay is
investigated by exciting single modes in the structures and monitoring the time
evolution of the amplitude using MD in a microcanonical ensemble. As examples,
crystalline and amorphous modifications of Selenium and are
considered.Comment: Revtex, 8 pages, 11 postscript figures, accepted for publication in
PR
Observation of Single Transits in Supercooled Monatomic Liquids
A transit is the motion of a system from one many-particle potential energy
valley to another. We report the observation of transits in molecular dynamics
(MD) calculations of supercooled liquid argon and sodium. Each transit is a
correlated simultaneous shift in the equilibrium positions of a small local
group of particles, as revealed in the fluctuating graphs of the particle
coordinates versus time. This is the first reported direct observation of
transit motion in a monatomic liquid in thermal equilibrium. We found transits
involving 2 to 11 particles, having mean shift in equilibrium position on the
order of 0.4 R_1 in argon and 0.25 R_1 in sodium, where R_1 is the nearest
neighbor distance. The time it takes for a transit to occur is approximately
one mean vibrational period, confirming that transits are fast.Comment: 19 pages, 8 figure
Pressure dependence of diffusion in simple glasses and supercooled liquids
Using molecular dynamics simulation, we have calculated the pressure
dependence of the diffusion constant in a binary Lennard-Jones Glass. We
observe four temperature regimes. The apparent activation volume drops from
high values in the hot liquid to a plateau value. Near the critical temperature
of the mode coupling theory it rises steeply, but in the glassy state we find
again small values, similar to the ones in the liquid. The peak of the
activation volume at the critical temperature is in agreement with the
prediction of mode coupling theory
Voronoi-Delaunay analysis of normal modes in a simple model glass
We combine a conventional harmonic analysis of vibrations in a one-atomic
model glass of soft spheres with a Voronoi-Delaunay geometrical analysis of the
structure. ``Structure potentials'' (tetragonality, sphericity or perfectness)
are introduced to describe the shape of the local atomic configurations
(Delaunay simplices) as function of the atomic coordinates. Apart from the
highest and lowest frequencies the amplitude weighted ``structure potential''
varies only little with frequency. The movement of atoms in soft modes causes
transitions between different ``perfect'' realizations of local structure. As
for the potential energy a dynamic matrix can be defined for the ``structure
potential''. Its expectation value with respect to the vibrational modes
increases nearly linearly with frequency and shows a clear indication of the
boson peak. The structure eigenvectors of this dynamical matrix are strongly
correlated to the vibrational ones. Four subgroups of modes can be
distinguished
Anharmonicity, vibrational instability and Boson peak in glasses
We show that a {\em vibrational instability} of the spectrum of weakly
interacting quasi-local harmonic modes creates the maximum in the inelastic
scattering intensity in glasses, the Boson peak. The instability, limited by
anharmonicity, causes a complete reconstruction of the vibrational density of
states (DOS) below some frequency , proportional to the strength of
interaction. The DOS of the new {\em harmonic modes} is independent of the
actual value of the anharmonicity. It is a universal function of frequency
depending on a single parameter -- the Boson peak frequency, which
is a function of interaction strength. The excess of the DOS over the Debye
value is at low frequencies and linear in in the
interval . Our results are in an excellent
agreement with recent experimental studies.Comment: LaTeX, 8 pages, 6 figure
Interaction of quasilocal harmonic modes and boson peak in glasses
The direct proportionality relation between the boson peak maximum in
glasses, , and the Ioffe-Regel crossover frequency for phonons,
, is established. For several investigated materials . At the frequency the mean free path of the
phonons becomes equal to their wavelength because of strong resonant
scattering on quasilocal harmonic oscillators. Above this frequency phonons
cease to exist. We prove that the established correlation between
and holds in the general case and is a direct consequence of
bilinear coupling of quasilocal oscillators with the strain field.Comment: RevTex, 4 pages, 1 figur
Diffusion and viscosity in a supercooled polydisperse system
We have carried out extensive molecular dynamics simulations of a supercooled
polydisperse Lennard-Jones liquid with large variations in temperature at a
fixed pressure. The particles in the system are considered to be polydisperse
both in size and mass. The temperature dependence of the dynamical properties
such as the viscosity () and the self-diffusion coefficients () of
different size particles is studied. Both viscosity and diffusion coefficients
show super-Arrhenius temperature dependence and fit well to the well-known
Vogel-Fulcher-Tammann (VFT) equation. Within the temperature range
investigated, the value of the Angell's fragility parameter (D )
classifies the present system into a strongly fragile liquid. The critical
temperature for diffusion () increases with the size of the
particles. The critical temperature for viscosity () is larger than
that for the diffusion and a sizeable deviations appear for the smaller size
particles implying a decoupling of translational diffusion from viscosity in
deeply supercooled liquid. Indeed, the diffusion shows markedly non-Stokesian
behavior at low temperatures where a highly nonlinear dependence on size is
observed. An inspection of the trajectories of the particles shows that at low
temperatures the motions of both the smallest and largest size particles are
discontinuous (jump-type). However, the crossover from continuous Brownian to
large length hopping motion takes place at shorter time scales for the smaller
size particles.Comment: Revtex4, 7 pages, 8 figure