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Research Area Summary:
We study the nature of vibrons in disordered systems of various types
using modeling and supercell methods. The characterization of normal
modes in disordered crystals, glasses, amorphous materials, and
materials with crystalline amorphous interfaces gives insight into
thermal conduction and vibrational spectroscopic properties.
 | FIG. 1. Dynamical structure factor
functions corresponding to lowest allowed magnitudes of Q that are consistent
with the periodic boundary conditions of a 4096-atom model of a-Si.
Averages of Fourier contents of modes are taken over all allowed
directions of Q indicated. Both raw numerical results and Lorentzian
fits are shown.
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Vibrational Modes & Energy Transport: A Kubo theory
of thermal conductivity that is partially within the harmonic
approximation involves matrix elements of a heat current operator and,
in turn, the exact knowledge of the normal mode eigenvectors. More
than twelve years ago, Professor Philip B. Allen of SUNY at Stony
Brook and I initiated a calculation of thermal conductivity of
amorphous silicon utilizing literature models of the structure and of
the interatomic potential. Originally only a 216-atom model, developed
by Wooten, Winer, and Weaire, and also by Broughton and Li, was
used. However, the size of the model determines the low frequency
cutoff above which the theory is rigorously implemented. Therefore
larger models have been examined and a phonon diffusivity (which
enters the formula for the thermal conductivity) has been investigated
as a function of frequency. For many years it was presumed that
scattering of phonons by structural disorder should be Rayleigh-like
at the lowest frequencies, but that notion is presently being
seriously questioned. A recent important experimental development in
the area of dynamics of glasses is inelastic x-ray scattering, and
results of such experiments on several glasses (though not yet on
amorphous Si) suggest that there may be a Q2 broadening of
phonon peaks at low Q as opposed to the Rayleigh Q4 law.
Interestingly, our analyses of modes in 1000- and 4096-atom models
give qualitative agreement with these inelastic scattering
experiments.
Further Reading:
Point of contact:
Joseph.Feldman@nrl.navy.mil (Privacy Advisory)
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