263 research outputs found
Electrodynamics of Amorphous Media at Low Temperatures
Amorphous solids exhibit intrinsic, local structural transitions, that give
rise to the well known quantum-mechanical two-level systems at low
temperatures. We explain the microscopic origin of the electric dipole moment
of these two-level systems: The dipole emerges as a result of polarization
fluctuations between near degenerate local configurations, which have nearly
frozen in at the glass transition. An estimate of the dipole's magnitude, based
on the random first order transition theory, is obtained and is found to be
consistent with experiment. The interaction between the dipoles is estimated
and is shown to contribute significantly to the Gr\"{u}neisen parameter anomaly
in low glasses. In completely amorphous media, the dipole moments are
expected to be modest in size despite their collective origin. In partially
crystalline materials, however, very large dipoles may arise, possibly
explaining the findings of Bauer and Kador, J. Chem. Phys. {\bf 118}, 9069
(2003).Comment: Submitted for publication; April 27, 2005 versio
Microscopic theory of network glasses
A molecular theory of the glass transition of network forming liquids is
developed using a combination of self-consistent phonon and liquid state
approaches. Both the dynamical transition and the entropy crisis characteristic
of random first order transitions are mapped out as a function of the degree of
bonding and the density. Using a scaling relation for a soft-core model to
crudely translate the densities into temperatures, the theory predicts that the
ratio of the dynamical transition temperature to the laboratory transition
temperature rises as the degree of bonding increases, while the Kauzmann
temperature falls relative to the laboratory transition. These results indicate
why highly coordinated liquids should be "strong" while van der Waals liquids
without coordination are "fragile".Comment: slightly revised version that has been accepted for publication in
Phys. Rev. Let
Glassy Dynamics of Protein Folding
A coarse grained model of a random polypeptide chain, with only discrete
torsional degrees of freedom and Hookean springs connecting pairs of
hydrophobic residues is shown to display stretched exponential relaxation under
Metropolis dynamics at low temperatures with the exponent , in
agreement with the best experimental results. The time dependent correlation
functions for fluctuations about the native state, computed in the Gaussian
approximation for real proteins, have also been found to have the same
functional form. Our results indicate that the energy landscape exhibits
universal features over a very large range of energies and is relatively
independent of the specific dynamics.Comment: RevTeX, 4 pages, multicolumn, including 5 figures; larger
computations performed, error bars improve
Fractal Analysis of Protein Potential Energy Landscapes
The fractal properties of the total potential energy V as a function of time
t are studied for a number of systems, including realistic models of proteins
(PPT, BPTI and myoglobin). The fractal dimension of V(t), characterized by the
exponent \gamma, is almost independent of temperature and increases with time,
more slowly the larger the protein. Perhaps the most striking observation of
this study is the apparent universality of the fractal dimension, which depends
only weakly on the type of molecular system. We explain this behavior by
assuming that fractality is caused by a self-generated dynamical noise, a
consequence of intermode coupling due to anharmonicity. Global topological
features of the potential energy landscape are found to have little effect on
the observed fractal behavior.Comment: 17 pages, single spaced, including 12 figure
The Intrinsic Quantum Excitations of Low Temperature Glasses
Several puzzling regularities concerning the low temperature excitations of
glasses are quantitatively explained by quantizing domain wall motions of the
random first order glass transition theory. The density of excitations agrees
with experiment and scales with the size of a dynamically coherent region at
, being about 200 molecules. The phonon coupling depends on the Lindemann
ratio for vitrification yielding the observed universal relation between phonon wavelength and mean free path .
Multilevel behavior is predicted to occur in the temperature range of the
thermal conductivity plateau.Comment: 4 pages, submitted to PR
Microscopic Theory of Heterogeneity and Non-Exponential Relaxations in Supercooled Liquids
Recent experiments and computer simulations show that supercooled liquids
around the glass transition temperature are "dynamically heterogeneous" [1].
Such heterogeneity is expected from the random first order transition theory of
the glass transition. Using a microscopic approach based on this theory, we
derive a relation between the departure from Debye relaxation as characterized
by the value of a stretched exponential response function , and the fragility of the liquid. The
value is also predicted to depend on temperature and to vanish as the ideal
glass transition is approached at the Kauzmann temperature.Comment: 4 pages including 3 eps figure
Solution of the local field equations for self-generated glasses
We present a self-consistent local approach to self generated glassiness
which is based on the concept of the dynamical mean field theory to many body
systems. Using a replica approach to self generated glassiness, we map the
problem onto an effective local problem which can be solved exactly. Applying
the approach to the Brazovskii-model, relevant to a large class of systems with
frustrated micro-phase separation, we are able to solve the self-consistent
local theory without using additional approximations. We demonstrate that a
glassy state found earlier in this model is generic and does not arise from the
use of perturbative approximations. In addition we demonstrate that the glassy
state depends strongly on the strength of the frustrated phase separation in
that model.Comment: 11 pages, 3 figure
Viscosity Dependence of the Folding Rates of Proteins
The viscosity dependence of the folding rates for four sequences (the native
state of three sequences is a beta-sheet, while the fourth forms an
alpha-helix) is calculated for off-lattice models of proteins. Assuming that
the dynamics is given by the Langevin equation we show that the folding rates
increase linearly at low viscosities \eta, decrease as 1/\eta at large \eta and
have a maximum at intermediate values. The Kramers theory of barrier crossing
provides a quantitative fit of the numerical results. By mapping the simulation
results to real proteins we estimate that for optimized sequences the time
scale for forming a four turn \alpha-helix topology is about 500 nanoseconds,
whereas the time scale for forming a beta-sheet topology is about 10
microseconds.Comment: 14 pages, Latex, 3 figures. One figure is also available at
http://www.glue.umd.edu/~klimov/seq_I_H.html, to be published in Physical
Review Letter
Polarons as Nucleation Droplets in Non-Degenerate Polymers
We present a study of the nucleation mechanism that allows the decay of the
metastable phase (trans-cisoid) to the stable phase
(cis-transoid) in quasi one-dimensional non-degenerate polymers within the
continuum electron-phonon model. The electron-phonon configurations that lead
to the decay, i.e. the critical droplets (or transition state), are identified
as polarons of the metastable phase. We obtain an estimate for the decay rate
via thermal activation within a range of parameters consistent with
experimental values for the gap of the cis-configuration. It is pointed out
that, upon doping, the activation barriers of the excited states are quite
smaller and the decay rate is greatly enhanced. Typical activation energies for
electron or hole polarons are eV and the typical size for a
critical droplet (polaron) is about . Decay via quantum nucleation is
also studied and it is found that the crossover temperature between quantum
nucleation and thermal activation is of order . Metastable
configurations of non-degenerate polymers may provide examples for mesoscopic
quantum tunneling.Comment: REVTEX 3.0, 28 PAGES, 3 FIGURES AVAILABLE UPON REQUEST, PITT 94-0
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