D. L’Hôte

Spatial correlations in the dynamics of glassforming liquids : Experimental determination of their temperature dependence

We use recently introduced three-point dynamic susceptibilities to obtain an experimental determination of the temperature evolution of the number of molecules Ncorr that are dynamically correlated during the structural relaxation of supercooled liquids. We first discuss in detail the physical content of three-point functions that relate the sensitivity of the averaged two-time dynamics to external control parameters (such as temperature or density), as well as their connection to the more standard four-point dynamic susceptibility associated with dynamical heterogeneities. We then demonstrate that these functions can be experimentally determined with good precision. We gather available data to obtain the temperature dependence of Ncorr for a large number of supercooled liquids over a wide range of relaxation time scales from the glass transition up to the onset of slow dynamics. We find that Ncorr systematically grows when approaching the glass transition. It does so in a modest manner close to the glass transition, which is consistent with an activation-based picture of the dynamics in glassforming materials. For higher temperatures, there appears to be a regime where Ncorr behaves as a power-law of the relaxation time. Finally, we find that the dynamic response to density, while being smaller than the dynamic response to temperature, behaves similarly, in agreement with theoretical expectations.

Evidence of growing spatial correlations at the glass transition from nonlinear response experiments.

The ac nonlinear dielectric response chi3(omega,T) of glycerol was measured close to its glass transition temperature T(g) to investigate the prediction that supercooled liquids respond in an increasingly nonlinear way as the dynamics slows down (as spin glasses do). We find that chi3(omega,T) indeed displays several nontrivial features. It is peaked as a function of the frequency omega and obeys scaling as a function of omega tau(T), with tau(T) the relaxation time of the liquid. The height of the peak, proportional to the number of dynamically correlated molecules N(corr)(T), increases as the system becomes glassy, and chi3 decays as a power law of omega over several decades beyond the peak. These findings confirm the collective nature of the glassy dynamics and provide the first direct estimate of the T dependence of N(corr).

Observation of superspin glass state in magnetically textured ferrofluid (γ-Fe2O3)

Magnetic properties in a magnetically textured ferrofluid made out of interacting maghemite (γ-Fe2O3) nanoparticles suspended in glycerin have been investigated. Despite the loss of uniform distribution of anisotropy axes, a superspin glass state exists at low temperature in a concentrated textured ferrofluid as in the case of its nontextured counterpart. The onset of superspin glass state was verified from the sample’s ac susceptibility. The influence of the anisotropy axis orientation on the aging behavior in the glassy states is also discussed.

Superspin glass aging behavior in textured and nontextured frozen ferrofluid

The effect of magnetic anisotropy-axis alignment of individual nanoparticles on the collective aging behavior in the superspin glass state of a frozen ferrofluid has been investigated. The ferrofluid studied here consists of maghemite nanoparticles (γ-Fe2O3, mean diameter=8.6 nm) dispersed in glycerin at a volume fraction of ∼15%. The low temperature aging behavior has been explored through “zero-field cooled magnetization” (ZFCM) relaxation measurements using superconducting quantum interference device magnetometry. The ZFCM response functions were found to scale with effective age of the system in both textured and nontextured superspin glass states, but with markedly different scaling exponents, μ. The value of μ was found to shift from ∼0.9 in nontextured case to ∼0.6 in the textured case, despite the identical cooling protocol used in both experiments.

An experimental search for dynamic heterogeneities in molecular glass formers

We have measured the linear dielectric susceptibility of two molecular glass formers close to Tg in order to estimate the size of the dynamically correlated clusters of molecules which are expected to govern the physics of glass formation. This size has been shown to be related to the dynamic dielectric susceptibility de (ω)/dT (e: dielectric susceptibility, T: temperature, ω: frequency). To allow for an accurate determination of the T derivative, we scanned the interval 192 K < T < 232 K every 1 K for glycerol and 159 K < T < 179 K every 0.5 K for propylene carbonate. The resolution on T variations was about 1 mK. The result for glycerol is that the number of correlated molecules increases by a factor of 3 when T goes from 226 to 195 K. It has been shown that the non-linear susceptibility provides a direct measurement of dynamic correlations. To measure it, we used a standard lock-in technique yielding the third harmonic of the current flowing out of a capacitor. We obtained only an upper limit on the ratio of the third to the first harmonic, due to the non-linear response of standard electronics.

Magnetic noise of a frozen ferrofluid

The magnetic noise of a frozen ferrofluid made of maghemite nanoparticles dispersed in glycerin is measured using a two-dimensional electron gas based quantum well Hall sensor (QWHS) with the spinning current technique. The frozen ferrofluid shows a superspin glass transition at 67 K. Below this glass transition temperature, the relation between the imaginary part of the ac susceptibility of a bulk ferrofluid sample and the magnetic noise measured with the QWHS gives an indication of a violation of the fluctuation dissipation theorem.

A local noise measurement device for magnetic physical systems

We present an experimental setup developed to measure locally the fluctuations of the magnetization of physical systems such as spin and superspin glasses. It is based on micronic and submicronic Hall probes. We present the noise reduction at ambient temperature owing to the use of the spinning current technique. Finally, we show why, with such probes, the noise measured on a macroscopic sample probes only a microscopic volume of the sample.

A simple device for dielectric spectroscopy of polymers with temperature regulation close to 300 K based on a Peltier junction

We present a simple thermostat device for performing dielectric spectroscopy measurements on polymers close to their glass transition temperature. By using a vacuum chamber containing a Peltier junction with its regulator, we show that a very simple setup yields a temperature accuracy which is good enough for accurate studies of polymer dielectric properties. This technique is also more cost effective than standard setups using cryogenic fluids.