84 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]
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
Ground state of the Kagome-like S=1/2 antiferromagnet, Volborthite Cu3V2O7(OH)2.2H2O
Volborthite compound is one of the very few realizations of S=1/2 quantum
spins on a highly frustrated kagome-like lattice. Low-T SQUID measurements
reveal a broad magnetic transition below 2K which is further confirmed by a
peak in the 51V nuclear spin relaxation rate (1/T1) at 1.4K0.2K. Through
51V NMR, the ground state (GS) appears to be a mixture of different spin
configurations, among which 20% correspond to a well defined short range order,
possibly of the type. While the freezing involve all
the Cu spins, only 40% of the copper moment is actually frozen which
suggests that quantum fluctuations strongly renormalize the GS.Comment: 4 pages, 4 figures, to appear in PR
Slow Relaxation Process in Ising like Heisenberg Kagome Antiferromagnets due to Macroscopic Degeneracy in the Ordered State
We study relaxation phenomena in the ferromagnetically ordered state of the
Ising-like Heisenberg kagome antiferromagnets. We introduce the "weathervane
loop" in order to characterize macroscopic degenerate ordered states and study
the microscopic mechanism of the slow relaxation from a view point of the
dynamics of the weathervane loop configuration. This mechanism may give a
possible origin of the slow relaxation reported in recent experiments.Comment: 6pages, 4figures, HFM2006 proceeding
Superconducting diamagnetic fluctuations in ropes of carbon nanotubes
We report low-temperature magnetisation measurements on a large number of
purified ropes of single wall carbon nanotubes. In spite of a large
superparamagnetic contribution due to the small ferromagnetic catalytical
particles still present in the sample, at low temperature () and low
magnetic field (), a diamagnetic signal is detectable. This low
temperature diamagnetism can be interpreted as the Meissner effect in ropes of
carbon nanotubes which have previously been shown to exhibit superconductivity
from transport measurements.Comment: 10 pages 3 figure
Fifth-order susceptibility unveils growth of thermodynamic amorphous order in glass-formers
Glasses are ubiquitous in daily life and technology. However the microscopic
mechanisms generating this state of matter remain subject to debate: Glasses
are considered either as merely hyper-viscous liquids or as resulting from a
genuine thermodynamic phase transition towards a rigid state. We show that
third- and fifth-order susceptibilities provide a definite answer to this
longstanding controversy. Performing the corresponding high-precision nonlinear
dielectric experiments for supercooled glycerol and propylene carbonate, we
find strong support for theories based upon thermodynamic amorphous order.
Moreover, when lowering temperature, we find that the growing transient domains
are compact - that is their fractal dimension d_f = 3. The glass transition may
thus represent a class of critical phenomena different from canonical
second-order phase transitions for which d_f < 3.Comment: 9 pages, 3 figure
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