125 research outputs found
Onset of the nonlinear dielectric response of glasses in the two-level system model
We have calculated the real part of the nonlinear dielectric
susceptibility of amorphous insulators in the kHz range, by using the two-level
system model and a nonperturbative numerical quantum approach. At low
temperature , it is first shown that the standard two-level model should
lead to a \textit{decrease} of when the measuring field is raised,
since raising increases the population of the upper level and induces Rabi
oscillations canceling the ones induced from the ground level. This predicted
-induced decrease of is at \textit{odds} with experiments. However,
a \textit{good agreement} with low-frequency experimental nonlinear data is
achieved if, in our fully quantum simulations, interactions between defects are
taken into account by a new relaxation rate whose efficiency increases as
, as was proposed recently by Burin \textit{et al.} (Phys. Rev. Lett.
{\bf 86}, 5616 (2001)). In this approach, the behavior of at low is
mainly explained by the efficiency of this new relaxation channel. This new
relaxation rate could be further tested since it is shown that it should lead:
\textit{i)} to a completely new nonlinear behavior for samples whose thickness
is nm; \textit{ii)} to a decrease of nonequilibrium effects when
is increased.Comment: latex Sept02.tex, 5 files, 4 figures, 17 pages, submitted to Eur.
Phys. J. B. Text change
Conductance statistics in small insulating GaAs:Si wires at low temperature. II. Experimental study
We have observed reproducible conductance fluctuations at low temperature in
a small GaAs:Si wire driven across the Anderson transition by the application
of a gate voltage. We analyse quantitatively the log-normal conductance
statistics in terms of truncated quantum fluctuations. Quantum fluctuations due
to small changes of the electron energy (controlled by the gate voltage) cannot
develop fully due to identified geometrical fluctuations of the resistor
network describing the hopping through the sample.
The evolution of the fluctuations versus electron energy and magnetic field
shows that the fluctuations are non-ergodic, except in the critical insulating
region of the Anderson transition, where the localization length is larger than
the distance between Si impurities.
The mean magnetoconductance is in good accordance with simulations based on
the Forward-Directed-Paths analysis, i.e. it saturates to as decreases over orders of
magnitude in the strongly localized regime.Comment: Email contact: [email protected]
Nonlinear dielectric susceptibilities in supercooled liquids: a toy model
The dielectric response of supercooled liquids is phenomenologically modeled
by a set of Asymmetric Double Wells (ADW), where each ADW contains a dynamical
heterogeneity of molecules. We find that the linear macroscopic
susceptibility does not depend on contrary to all higher
order susceptibilities . We show that is
proportional to the moment of , which could pave the way for
new experiments on glass transition. In particular, as predicted by Bouchaud
and Biroli on general grounds [Phys. Rev. B, {\bf 72}, 064204 (2005)], we find
that is proportional to the average value of . We fully
calculate and, with plausible values of few parameters our model
accounts for the salient features of the experimental behavior of of
supercooled glycerol.Comment: 13 pages, 5 figure
A method for measuring the nonlinear response in dielectric spectroscopy through third harmonics detection
We present a high sensitivity method allowing the measurement of the non
linear dielectric susceptibility of an insulating material at finite frequency.
It has been developped for the study of dynamic heterogeneities in supercooled
liquids using dielectric spectroscopy at frequencies 0.05 Hz < f < 30000 Hz .
It relies on the measurement of the third harmonics component of the current
flowing out of a capacitor. We first show that standard laboratory electronics
(amplifiers and voltage sources) nonlinearities lead to limits on the third
harmonics measurements that preclude reaching the level needed by our physical
goal, a ratio of the third harmonics to the fundamental signal about 7 orders
of magnitude lower than 1. We show that reaching such a sensitivity needs a
method able to get rid of the nonlinear contributions both of the measuring
device (lock-in amplifier) and of the excitation voltage source. A bridge using
two sources fulfills only the first of these two requirements, but allows to
measure the nonlinearities of the sources. Our final method is based on a
bridge with two plane capacitors characterized by different dielectric layer
thicknesses. It gets rid of the source and amplifier nonlinearities because in
spite of a strong frequency dependence of the capacitors impedance, it is
equilibrated at any frequency. We present the first measurements of the
physical nonlinear response using our method. Two extensions of the method are
suggested.Comment: 25 pages, 8 figure
Study of the heating effect contribution to the nonlinear dielectric response of a supercooled liquid
We present a detailed study of the heating effects in dielectric measurements
carried out on a liquid. Such effects come from the dissipation of the electric
power in the liquid and give a contribution to the nonlinear third harmonics
susceptibility chi_3 which depends on the frequency and temperature. This study
is used to evaluate a possible `spurious' contribution to the recently measured
nonlinear susceptibility of an archetypical glassforming liquid (Glycerol).
Those measurements have been shown to give a direct evaluation of the number of
dynamically correlated molecules temperature dependence close to the glass
transition temperature T_g~190K (Crauste-Thibierge et al., Phys. Rev. Lett
104,165703(2010)). We show that the heating contribution is totally negligible
(i) below 204K at any frequency; (ii) for any temperature at the frequency
where the third harmonics response chi_3 is maximum. Besides, this heating
contribution does not scale as a function of f/f_{\alpha}, with f_{\alpha}(T)
the relaxation frequency of the liquid. In the high frequency range, when
f/f_{\alpha} >= 1, we find that the heating contribution is damped because the
dipoles cannot follow instantaneously the temperature modulation due to the
heating phenomenon. An estimate of the magnitude of this damping is given.Comment: 25 pages, 10 figures, Accepted for publication in Journal of Chemical
Physic
Evidence of growing spatial correlations at the glass transition from nonlinear response experiments
The ac nonlinear dielectric response of glycerol was
measured close to its glass transition temperature to investigate the
prediction that supercooled liquids respond in an increasingly non-linear way
as the dynamics slows down (as spin-glasses do). We find that
indeed displays several non trivial features. It is peaked
as a function of the frequency and obeys scaling as a function of
, with the relaxation time of the liquid. The height
of the peak, proportional to the number of dynamically correlated molecules
, increases as the system becomes glassy, and decays as a
power-law of over several decades beyond the peak. These findings
confirm the collective nature of the glassy dynamics and provide the first
direct estimate of the dependence of .Comment: 22 pages, 6 figures. With respect to v1, a few new sentences were
added in the introduction and conclusion, references were updated, some typos
corrected
Direct experimental evidence of a growing length scale accompanying the glass transition
Understanding glass formation is a challenge because the existence of a true
glass state, distinct from liquid and solid, remains elusive: Glasses are
liquids that have become too viscous to flow. An old idea, as yet unproven
experimentally, is that the dynamics becomes sluggish as the glass transition
approaches because increasingly larger regions of the material have to move
simultaneously to allow flow. We introduce new multipoint dynamical
susceptibilities to estimate quantitatively the size of these regions and
provide direct experimental evidence that the glass formation of molecular
liquids and colloidal suspensions is accompanied by growing dynamic correlation
length scales.Comment: 5 pages, 2 figure
The glass transition in molecules, colloids and grains: universality and specificity
We highlight certain key achievements in experimental work on molecular,
colloidal and granular glassformers. This short review considers these three
classes of experimental systems and focusses largely on the work of the authors
and their coworkers and thus is far from exhaustive. Our goal is rather to
discuss particular experimental results from these classes and to explore
universality and specificity across the broad range of length- and time-scales
they span. We emphasize that a variety of phenomena, not least dynamical
heterogeneity, growing lengthscales and a change in structure, albeit subtle,
are now well established in these three classes of glassformer. We then review
some experimental measurements which depend more specifically on the class of
glassformer, such as the Gardner transition and some which have been
investigated more in one or two classes than in all, such as configurational
entropy and evidence for a dynamical phase transition. We finally put forward
some open questions and consider what could be done to fill some of the gaps
between theoretical approaches and experiments.Comment: 32 pages, 13 figure
Orientational dynamics in supercooled glycerol computed from MD simulations: self and cross contributions
The orientational dynamics of supercooled glycerol using molecular dynamics
simulations for temperatures ranging from 323 K to 253 K, is probed through
correlation functions of first and second ranks of Legendre polynomials,
pertaining respectively to dielectric spectroscopy (DS) and depolarized dynamic
light scattering (DDLS). The self, cross, and total correlation functions are
compared with relevant experimental data. The computations reveal the low
sensitivity of DDLS to cross-correlations, in agreement with what is found in
experimental work, and strengthen the idea of directly comparing DS and DDLS
data to evaluate the effect of cross-correlations in polar liquids. The
analysis of the net static cross-correlations and their spatial decomposition
shows that, although cross-correlations extend over nanometric distances, their
net magnitude originates, in the case of glycerol, from the first shell of
neighbouring molecules. Accessing the angular dependence of the static
correlation allows us to get a microscopic understanding of why the rank-1
correlation function is more sensitive to cross-correlation than its rank-2
counterpart.Comment: 9 pages, 6 figure
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