292 research outputs found
Euclidean random matrix theory: low-frequency non-analyticities and Rayleigh scattering
By calculating all terms of the high-density expansion of the euclidean
random matrix theory (up to second-order in the inverse density) for the
vibrational spectrum of a topologically disordered system we show that the
low-frequency behavior of the self energy is given by and not , as claimed previously.
This implies the presence of Rayleigh scattering and long-time tails of the
velocity autocorrelation function of the analogous diffusion problem of the
form .Comment: 27 page
Harmonic Vibrational Excitations in Disordered Solids and the "Boson Peak"
We consider a system of coupled classical harmonic oscillators with spatially
fluctuating nearest-neighbor force constants on a simple cubic lattice. The
model is solved both by numerically diagonalizing the Hamiltonian and by
applying the single-bond coherent potential approximation. The results for the
density of states are in excellent agreement with each other. As
the degree of disorder is increased the system becomes unstable due to the
presence of negative force constants. If the system is near the borderline of
stability a low-frequency peak appears in the reduced density of states
as a precursor of the instability. We argue that this peak
is the analogon of the "boson peak", observed in structural glasses. By means
of the level distance statistics we show that the peak is not associated with
localized states
Physical Origin of the Boson Peak Deduced from a Two-Order-Parameter Model of Liquid
We propose that the boson peak originates from the (quasi-) localized
vibrational modes associated with long-lived locally favored structures, which
are intrinsic to a liquid state and are randomly distributed in a sea of
normal-liquid structures. This tells us that the number density of locally
favored structures is an important physical factor determining the intensity of
the boson peak. In our two-order-parameter model of the liquid-glass
transition, the locally favored structures act as impurities disturbing
crystallization and thus lead to vitrification. This naturally explains the
dependence of the intensity of the boson peak on temperature, pressure, and
fragility, and also the close correlation between the boson peak and the first
sharp diffraction peak (or prepeak).Comment: 5 pages, 1 figure, An error in the reference (Ref. 7) was correcte
Continuum limit of amorphous elastic bodies: A finite-size study of low frequency harmonic vibrations
The approach of the elastic continuum limit in small amorphous bodies formed
by weakly polydisperse Lennard-Jones beads is investigated in a systematic
finite-size study. We show that classical continuum elasticity breaks down when
the wavelength of the sollicitation is smaller than a characteristic length of
approximately 30 molecular sizes. Due to this surprisingly large effect
ensembles containing up to N=40,000 particles have been required in two
dimensions to yield a convincing match with the classical continuum predictions
for the eigenfrequency spectrum of disk-shaped aggregates and periodic bulk
systems. The existence of an effective length scale \xi is confirmed by the
analysis of the (non-gaussian) noisy part of the low frequency vibrational
eigenmodes. Moreover, we relate it to the {\em non-affine} part of the
displacement fields under imposed elongation and shear. Similar correlations
(vortices) are indeed observed on distances up to \xi~30 particle sizes.Comment: 28 pages, 13 figures, 3 table
Nature of vibrational eigenmodes in topologically disordered solids
We use a local projectional analysis method to investigate the effect of
topological disorder on the vibrational dynamics in a model glass simulated by
molecular dynamics. Evidence is presented that the vibrational eigenmodes in
the glass are generically related to the corresponding eigenmodes of its
crystalline counterpart via disorder-induced level-repelling and hybridization
effects. It is argued that the effect of topological disorder in the glass on
the dynamical matrix can be simulated by introducing positional disorder in a
crystalline counterpart.Comment: 7 pages, 6 figures, PRB, to be publishe
Localized and Delocalized Charge Transport in Single-Wall Carbon-Nanotube Mats
We measured the complex dielectric constant in mats of single-wall
carbon-nanotubes between 2.7 K and 300 K up to 0.5 THz. The data are well
understood in a Drude approach with a negligible temperature dependence of the
plasma frequency (omega_p) and scattering time (tau) with an additional
contribution of localized charges. The dielectric properties resemble those of
the best ''metallic'' polypyrroles and polyanilines. The absence of metallic
islands makes the mats a relevant piece in the puzzle of the interpretation of
tau and omega_p in these polymers.Comment: 4 pages including 4 figure
Hopping Transport in the Presence of Site Energy Disorder: Temperature and Concentration Scaling of Conductivity Spectra
Recent measurements on ion conducting glasses have revealed that conductivity
spectra for various temperatures and ionic concentrations can be superimposed
onto a common master curve by an appropriate rescaling of the conductivity and
frequency. In order to understand the origin of the observed scaling behavior,
we investigate by Monte Carlo simulations the diffusion of particles in a
lattice with site energy disorder for a wide range of both temperatures and
concentrations. While the model can account for the changes in ionic activation
energies upon changing the concentration, it in general yields conductivity
spectra that exhibit no scaling behavior. However, for typical concentrations
and sufficiently low temperatures, a fairly good data collapse is obtained
analogous to that found in experiment.Comment: 6 pages, 4 figure
Density of states in random lattices with translational invariance
We propose a random matrix approach to describe vibrational excitations in
disordered systems. The dynamical matrix M is taken in the form M=AA^T where A
is some real (not generally symmetric) random matrix. It guaranties that M is a
positive definite matrix which is necessary for mechanical stability of the
system. We built matrix A on a simple cubic lattice with translational
invariance and interaction between nearest neighbors. We found that for certain
type of disorder phonons cannot propagate through the lattice and the density
of states g(w) is a constant at small w. The reason is a breakdown of affine
assumptions and inapplicability of the elasticity theory. Young modulus goes to
zero in the thermodynamic limit. It strongly reminds of the properties of a
granular matter at the jamming transition point. Most of the vibrations are
delocalized and similar to diffusons introduced by Allen, Feldman et al., Phil.
Mag. B v.79, 1715 (1999).Comment: 4 pages, 5 figure
Elastic constant dishomogeneity and dependence of the broadening of the dynamical structure factor in disordered systems
We propose an explanation for the quadratic dependence on the momentum ,
of the broadening of the acoustic excitation peak recently found in the study
of the dynamic structure factor of many real and simulated glasses. We ascribe
the observed law to the spatial fluctuations of the local wavelength of
the collective vibrational modes, in turn produced by the dishomegeneity of the
inter-particle elastic constants. This explanation is analitically shown to
hold for 1-dimensional disordered chains and satisfatorily numerically tested
in both 1 and 3 dimensions.Comment: 4 pages, RevTeX, 5 postscript figure
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