136 research outputs found
Molecular dynamics study of cage decay, near constant loss and crossover to cooperative ion hopping in lithium metasilicate
金沢大学理学部Molecular dynamics ~MD! simulations of lithium metasilicate (Li2SiO3) in the glassy and supercooled liquid states have been performed to illustrate the decay with time of the cages that confine individual Li1 ions before they hop out to diffuse cooperatively with each other. The self-part of the van Hove function of Li1 ions, Gs(r,t), is used as an indicator of the cage decay. At 700 K, in the early time regime t,tx1 , when the cage decays very slowly, the mean square displacement ^r2& of Li1 ions also increases very slowly with time approximately as t0.1 and has weak temperature dependence. Such ^r2& can be identified with the near constant loss ~NCL! observed in the dielectric response of ionic conductors. At longer times, when the cage decays more rapidly as indicated by the increasing buildup of the intensity of Gs(r,t) at the distance between Li1 ion sites, ^r2& broadly crosses over from the NCL regime to another power law tb with b\u270.64 and eventually it becomes t1.0, corresponding to long-range diffusion. Both tb and t1.0 terms have strong temperature dependence and they are the analogs of the ac conductivity @s(v)}v12b # and dc conductivity of hopping ions. The MD results in conjunction with the coupling model support the following proposed interpretation for conductivity relaxation of ionic conductors: ~1! the NCL originates from very slow initial decay of the cage with time caused by few independent hops of the ions because tx1!t o , where t o is the independent hop relaxation time; ~2! the broad crossover from the NCL to the cooperative ion hopping conductivity s(v)}v12b occurs when the cage decays more rapidly starting at tx1 ; ~3! s(v)}v12b is fully established at a time tx2 comparable to t o when the cage has decayed to such an extent that thereafter all ions participate in the slowed dynamics of cooperative jump motion; and ~4! finally, at long times s~v! becomes frequency independent, i.e., the dc conductivity. MD simulations show the non-Gaussian parameter peaks at approximately tx2 and the motion of the Li1 ions is dynamically heterogeneous. Roughly divided into two categories of slow ~A! and fast ~B! moving ions, their mean square displacements ^rA 2 & and ^rB 2 & are about the same for t,tx2 , but ^rB 2 & of the fast ions increases much more rapidly for t.tx2 . The self-part of the van Hove function of Li1 reveals that first jumps for some Li1 ions, which are apparently independent free jumps, have taken place before tx2 . While after tx2 the angle between the first jump and the next is affected by the other ions, again indicating cooperative jump motion. The dynamic properties are analogous to those found in supercooled colloidal particle suspension by confocal microscopy
Distinguishing different classes of secondary relaxations from vapour deposited ultrastable glasses
Secondary relaxations persistent in the glassy state after structural arrest are especially relevant for the
properties of the glass. A major thrust in research in dynamics of glass-forming liquids is to identify what
secondary relaxations exhibit a connection to the structural relaxation and are hence more relevant. Via
the Coupling Model, secondary relaxations having such connection have been identified by properties
similar to the primitive relaxation of the Coupling Model and are called the Johari–Goldstein (JG)
b-relaxations. They involve the motion of the entire molecule and act as the precursor of the structural
a-relaxation. The change in dynamics of the secondary relaxation by aging an ordinary glass is one way
to understand the connection between the two relaxations, but the results are often equivocal.
Ultrastable glasses, formed by physical vapour deposition, exhibit density and enthalpy levels comparable
to ordinary glasses aged for thousands of years, as well as some particular molecular arrangement. Thus,
ultrastable glasses enable the monitoring of the evolution of secondary processes in case aging does
not provide any definitive information. Here, we study the secondary relaxation of several ultrastable
glasses to identify different types of secondary relaxations from their different relationship with the
structural relaxation. We show the existence of two clearly differentiated groups of relaxations: those
becoming slower in the ultrastable state and those becoming faster, with respect to the ordinary unaged
glass. We propose ultrastability as a way to distinguish between secondary processes arising from the
particular microstructure of the system and those connected in properties to and acting as the
precursor of the structural relaxation in the sense of the Coupling Model
Concentration Dependence of Superconductivity and Order-Disorder Transition in the Hexagonal Rubidium Tungsten Bronze RbxWO3. Interfacial and bulk properties
We revisited the problem of the stability of the superconducting state in
RbxWO3 and identified the main causes of the contradictory data previously
published. We have shown that the ordering of the Rb vacancies in the
nonstoichiometric compounds have a major detrimental effect on the
superconducting temperature Tc.The order-disorder transition is first order
only near x = 0.25, where it cannot be quenched effectively and Tc is reduced
below 1K. We found that the high Tc's which were sometimes deduced from
resistivity measurements, and attributed to compounds with .25 < x < .30, are
to be ascribed to interfacial superconductivity which generates spectacular
non-linear effects. We also clarified the effect of acid etching and set more
precisely the low-rubidium-content boundary of the hexagonal phase.This work
makes clear that Tc would increase continuously (from 2 K to 5.5 K) as we
approach this boundary (x = 0.20), if no ordering would take place - as its is
approximately the case in CsxWO3. This behaviour is reminiscent of the
tetragonal tungsten bronze NaxWO3 and asks the same question : what mechanism
is responsible for this large increase of Tc despite the considerable
associated reduction of the electron density of state ? By reviewing the other
available data on these bronzes we conclude that the theoretical models which
are able to answer this question are probably those where the instability of
the lattice plays a major role and, particularly, the model which call upon
local structural excitations (LSE), associated with the missing alkali atoms.Comment: To be published in Physical Review
Out-of-equilibrium thermodynamic relations in systems with aging and slow relaxation
The experimental time scale dependence of thermodynamic relations in
out-of-equilibrium systems with aging phenomena is investigated theoretically
by using only aging properties of the two-time correlation functions and the
generalized fluctuation-dissipation theorem (FDT). We show that there are two
experimental time regimes characterized by different thermal properties. In the
first regime where the waiting time is much longer than the measurement time,
the principle of minimum work holds even though a system is out of equilibrium.
In the second regime where both the measurement time and the waiting time are
long, the thermal properties are completely different from properties in
equilibrium. For the single-correlation-scale systems such as -spin
spherical spin-glasses, contrary to a fundamental assumption of thermodynamics,
the work done in an infinitely slow operation depends on the path of change of
the external field even when the waiting time is infinite. On the other hand,
for the multi-correlation-scale systems such as Sherrington-Kirkpatrick model,
the work done in an infinitely slow operation is independent of the path. Our
results imply that in order to describe thermodynamic properties of systems
with aging it is essential to consider the experimental time scales and history
of a system as a state variable is necessary.Comment: 28 pages(REVTeX), 4 figure(EPS). To be published in Phys. Rev.
Non-Arrhenius Behavior of Secondary Relaxation in Supercooled Liquids
Dielectric relaxation spectroscopy (1 Hz - 20 GHz) has been performed on
supercooled glass-formers from the temperature of glass transition (T_g) up to
that of melting. Precise measurements particularly in the frequencies of
MHz-order have revealed that the temperature dependences of secondary
beta-relaxation times deviate from the Arrhenius relation in well above T_g.
Consequently, our results indicate that the beta-process merges into the
primary alpha-mode around the melting temperature, and not at the dynamical
transition point T which is approximately equal to 1.2 T_g.Comment: 4 pages, 4 figures, revtex
Local influence of boundary conditions on a confined supercooled colloidal liquid
We study confined colloidal suspensions as a model system which approximates
the behavior of confined small molecule glass-formers. Dense colloidal
suspensions become glassier when confined between parallel glass plates. We use
confocal microscopy to study the motion of confined colloidal particles. In
particular, we examine the influence particles stuck to the glass plates have
on nearby free particles. Confinement appears to be the primary influence
slowing free particle motion, and proximity to stuck particles causes a
secondary reduction in the mobility of free particles. Overall, particle
mobility is fairly constant across the width of the sample chamber, but a
strong asymmetry in boundary conditions results in a slight gradient of
particle mobility.Comment: For conference proceedings, "Dynamics in Confinement", Grenoble,
March 201
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