390 research outputs found
The Origin of the Boson Peak and the Thermal Conductivity Plateau in Low Temperature Glasses
We argue that the intrinsic glassy degrees of freedom in amorphous solids
giving rise to the thermal conductivity plateau and the ``boson peak'' in the
heat capacity at moderately low temperatures are directly connected to those
motions giving rise to the two-level like excitations seen at still lower
temperatures. These degrees of freedom can be thought of as strongly anharmonic
transitions between the local minima of the glassy energy landscape that are
accompanied by ripplon-like domain wall motions of the glassy mosaic structure
predicted to occur at by the random first order transition theory. The
energy spectrum of the vibrations of the mosaic depends on the glass transition
temperature, the Debye frequency and the molecular length scale. The resulting
spectrum reproduces the experimental low temperature Boson peak. The
``non-universality'' of the thermal conductivity plateau depends on and arises from calculable interactions with the phonons.Comment: 4 pages, submitted to PR
Theory of Aging in Structural Glasses
The random first order transition theory of the dynamics of supercooled
liquids is extended to treat aging phenomena in nonequilibrium structural
glasses. A reformulation of the idea of ``entropic droplets'' in terms of
libraries of local energy landscapes is introduced which treats in a uniform
way the supercooled liquid (reproducing earlier results) and glassy regimes.
The resulting microscopic theory of aging makes contact with the
Nayaranaswamy-Moynihan-Tool nonlinear relaxation formalism and the
Hodge-Scherer extrapolation of the Adam-Gibbs formula, but deviations from both
approaches are predicted and shown to be consistent with experiment. The
nonlinearity of glassy relaxation is shown to quantitatively correlate with
liquid fragility. The residual nonArrhenius temperature dependence of
relaxation observed in quenched glasses is explained. The broadening of
relaxation spectra in the nonequilibrium glass with decreasing temperature is
quantitatively predicted. The theory leads to the prediction of spatially
fluctuating fictive temperatures in the long-aged glassy state, which have
non-Gaussian statistics. This can give rise to ``ultra-slow'' relaxations in
systems after deep quenches.Comment: Submitted to J. Chem. Phy
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