700 research outputs found
Minimal model for beta relaxation in viscous liquids
Contrasts between beta relaxation in equilibrium viscous liquids and glasses
are rationalized in terms of a double-well potential model with
structure-dependent asymmetry, assuming structure is described by a single
order parameter. The model is tested for tripropylene glycol where it accounts
for the hysteresis of the dielectric beta loss peak frequency and magnitude
during cooling and reheating through the glass transition.Comment: Phys. Rev. Lett. (in press
Charge and momentum transfer in supercooled melts: Why should their relaxation times differ?
The steady state values of the viscosity and the intrinsic ionic-conductivity
of quenched melts are computed, in terms of independently measurable
quantities. The frequency dependence of the ac dielectric response is
estimated. The discrepancy between the corresponding characteristic relaxation
times is only apparent; it does not imply distinct mechanisms, but stems from
the intrinsic barrier distribution for -relaxation in supercooled
fluids and glasses. This type of intrinsic ``decoupling'' is argued not to
exceed four orders in magnitude, for known glassformers. We explain the origin
of the discrepancy between the stretching exponent , as extracted from
and the dielectric modulus data. The actual width of the
barrier distribution always grows with lowering the temperature. The contrary
is an artifact of the large contribution of the dc-conductivity component to
the modulus data. The methodology allows one to single out other contributions
to the conductivity, as in ``superionic'' liquids or when charge carriers are
delocalized, implying that in those systems, charge transfer does not require
structural reconfiguration.Comment: submitted to J Chem Phy
Relation between the High Density Phase and the Very-High Density Phase of Amorphous Solid Water
It has been suggested that high-density amorphous (HDA) ice is a structurally
arrested form of high-density liquid (HDL) water, while low-density amorphous
(LDA) ice is a structurally arrested form of low-density liquid (LDL) water.
Recent experiments and simulations have been interpreted to support the
possibility of a second "distinct" high-density structural state, named very
high-density amorphous (VHDA) ice, questioning the LDL-HDL hypothesis. We test
this interpretation using extensive computer simulations, and find that VHDA is
a more stable form of HDA and that in fact VHDA should be considered as the
amorphous ice of the quenched HDL.Comment: 5 pages, 4 fig
Time-temperature superposition in viscous liquids
Dielectric relaxation measurements on supercooled triphenyl phosphite show
that at low temperatures time-temperature superposition (TTS) is accurately
obeyed for the primary (alpha) relaxation process. Measurements on 6 other
molecular liquids close to the calorimetric glass transition indicate that TTS
is linked to an high-frequency decay of the alpha loss, while
the loss peak width is nonuniversal.Comment: 4 page
Anisotropic Local Stress and Particle Hopping in a Deeply Supercooled Liquid
The origin of the microscopic motions that lead to stress relaxation in
deeply supercooled liquid remains unclear. We show that in such a liquid the
stress relaxation is locally anisotropic which can serve as the driving force
for the hopping of the system on its free energy surface. However, not all
hopping are equally effective in relaxing the local stress, suggesting that
diffusion can decouple from viscosity even at local level. On the other hand,
orientational relaxation is found to be always coupled to stress relaxation.Comment: 4 pages, 3 figure
Energy landscape - a key concept for the dynamics of glasses and liquids
There is a growing belief that the mode coupling theory is the proper
microscopic theory for the dynamics of the undercooled liquid above a critical
temperature T_c. In addition, there is some evidence that the system leaves the
saddlepoints of the energy landscape to settle in the valleys at this critical
temperature. Finally, there is a microscopic theory for the entropy at the
calorimetric glass transition T_g by Mezard and Parisi, which allows to
calculate the Kauzmann temperature from the atomic pair potentials.
The dynamics of the frozen glass phase is at present limited to
phenomenological models. In the spirit of the energy landscape concept, one
considers an ensemble of independent asymmetric double-well potentials with a
wide distribution of barrier heights and asymmetries (ADWP or Gilroy-Phillips
model). The model gives an excellent description of the relaxation of glasses
up to about T_g/4. Above this temperature, the interaction between different
relaxation centers begins to play a role. One can show that the interaction
reduces the number of relaxation centers needed to bring the shear modulus down
to zero by a factor of three.Comment: Contribution to the III Workshop on Nonequilibrium Phenomena in
Supercooled Fluids, Glasses and Amorphous Materials, 22-27 September 2002,
Pisa; 14 pages, 3 figures; Version 3 takes criticque at Pisa into account;
final version 4 will be published in J.Phys.: Condens.Matte
Scaling behavior in the -relaxation regime of a supercooled Lennard-Jones mixture
We report the results of a molecular dynamics simulation of a supercooled
binary Lennard-Jones mixture. By plotting the self intermediate scattering
functions vs. rescaled time, we find a master curve in the -relaxation
regime. This master curve can be fitted well by a power-law for almost three
decades in rescaled time and the scaling time, or relaxation time, has a
power-law dependence on temperature. Thus the predictions of
mode-coupling-theory on the existence of a von Schweidler law are found to hold
for this system; moreover, the exponents in these two power-laws are very close
to satisfying the exponent relationship predicted by the mode-coupling-theory.
At low temperatures, the diffusion constants also show a power-law behavior
with the same critical temperature. However, the exponent for diffusion differs
from that of the relaxation time, a result that is in disagreement with the
theory.Comment: 8 pages, RevTex, four postscript figures available on request,
MZ-Physics-10
Mechanical Relaxation in Glasses and at the Glass Transition
The Gilroy-Phillips model of relaxational jumps in asymmetric double-well
potentials, developed for the Arrhenius-type secondary relaxations of the glass
phase, is extended to a formal description of the breakdown of the shear
modulus at the glass transition, the flow process.Comment: 13 pages, 11 figures, 49 ref
Crossover behavior and multi-step relaxation in a schematic model of the cut-off glass transition
We study a schematic mode-coupling model in which the ideal glass transition
is cut off by a decay of the quadratic coupling constant in the memory
function. (Such a decay, on a time scale tau_I, has been suggested as the
likely consequence of activated processes.) If this decay is complete, so that
only a linear coupling remains at late times, then the alpha relaxation shows a
temporal crossover from a relaxation typical of the unmodified schematic model
to a final strongly slower-than-exponential relaxation. This crossover, which
differs somewhat in form from previous schematic models of the cut-off glass
transition, resembles light-scattering experiments on colloidal systems, and
can exhibit a `slower-than-alpha' relaxation feature hinted at there. We also
consider what happens when a similar but incomplete decay occurs, so that a
significant level of quadratic coupling remains for t>>tau_I. In this case the
correlator acquires a third, weaker relaxation mode at intermediate times. This
empirically resembles the beta process seen in many molecular glass formers. It
disappears when the initial as well as the final quadratic coupling lies on the
liquid side of the glass transition, but remains present even when the final
coupling is only just inside the liquid (so that the alpha relaxation time is
finite, but too long to measure). Our results are suggestive of how, in a
cut-off glass, the underlying `ideal' glass transition predicted by
mode-coupling theory can remain detectable through qualitative features in
dynamics.Comment: 14 pages revtex inc 10 figs; submitted to pr
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