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The box model, originally introduced to account for the nonresonant hole burning (NHB) dielectric experiments in supercooled liquids, is compared to the measurements of the third harmonics P3 of the polarisation, reported recently in glycerol, close to the glass transition temperature Tg [C. Crauste-Thibierge, C. Brun, F. Ladieu, D. L'Hôte, G. Biroli, and J.-P. Bouchaud, Phys. Rev. Lett. 104, 165703 (2010)]. In this model, each box is a distinct dynamical relaxing entity (hereafter called dynamical heterogeneity (DH)) which follows a Debye dynamics with its own relaxation time τdh. When it is submitted to a strong electric field, the model posits that a temperature increase δTdh, depending on τdh, arises due to the dissipation of the electrical power. Each DH has thus its own temperature increase, on top of the temperature increase of the phonon bath δTph. Contrary to the 'fast' hole burning experiments where δTph is usually neglected, the P3 measurements are, from a thermal point of view, fully in a stationary regime, which means that δTph can no longer be neglected a priori. This is why the version of the box model that we study here takes δTph into account, which implies that the δTdh of the DHs are all coupled together. The value of P3, including both the 'intrinsic' contribution of each DH as well as the 'spurious' one coming from δTph, is computed within this box model and compared to the P3 measurements for glycerol, in the same range of frequencies and temperatures T. Qualitatively, we find that this version of the box model shares with experiments some nontrivial features, e.g., the existence of a peak at finite frequency in the modulus of P3 as well as its order of magnitude. Quantitatively, however, some experimental features are not accounted for by this model. We show that these differences between the model and the experiments do not come from δTph but from the 'intrinsic' contribution of the DHs. Finally, we show that the interferences between the 3ω response of the various DHs are the most important issue leading to the discrepancies between the box model prediction and the experiments. We argue that this could explain why the box model is quite successful to account for some kinds of nonlinear experiments (such as NHB) performed close to Tg, even if it does not completely account for all of them (such as the P3 measurements). This conclusion is supported by an analytical argument which helps understanding how a 'space-free' model as the box model is able to account for some of the experimental nonlinear features. [ABSTRACT FROM AUTHOR]

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 contribution to the nonlinear third harmonics susceptibility χ3, 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 Tg≈190 K [Crauste-Thibierge et al., Phys. Rev. Lett. 104, 165703 (2010)]. We show that the heating contribution is totally negligible (i) below 204 K at any frequency; (ii) for any temperature at the frequency where the third harmonics response χ3 is maximum. Besides, this heating contribution does not scale as a function of f/fα, with fα(T) the relaxation frequency of the liquid. In the high frequency range, when f/fα≥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. [ABSTRACT FROM AUTHOR]

The impact of the magnetic anisotropy-axis alignment on the collective aging behavior observed in the superspin glass state of the textured and nontextured frozen ferrofluid is analyzed.

Interacting magnetic nanoparticles display a wide variety of magnetic behaviors that are now being gathered in the emerging field of 'supermagnetism.' We have investigated how the out-of-equilibrium dynamics in the disordered superspin glass (SSG) state of a frozen ferrofluid sample is affected by texturation. Via magnetization relaxation experiments at low temperatures, we were able to estimate superspin correlation lengths for both textured and non-textured samples. The comparison with simulations and experiments on atomic spin glasses shows that the dynamic correlations in SSG's appear to develop in a way reminiscent to those in atomic spin glasses at intermediate time/length scales. [ABSTRACT FROM AUTHOR]

Abstract: The existence of a growing correlation length associated with the strong increase of relaxation times of glass-formers close to the glass transition is still a major open question in glass physics. It has been recently proposed that the ac nonlinear susceptibility of a supercooled liquid close to the glass transition temperature T g would be a probe of dynamical correlations. As for spin glasses, where the nonlinear susceptibility diverges at the transition, this quantity is tailored to reveal the possible “hidden” critical behavior of the glass transition. We have developed a high sensitivity experimental device to measure the nonlinear dielectric susceptibility of an insulating material at finite frequency. It measures the third harmonics of the current flowing out of a capacitor with the supercooled liquid as the dielectric layer. It is based on a bridge with two capacitors, and reaches a sensitivity better than 10−7 (ratio of third to first harmonics). Our first results on supercooled glycerol are presented. They clearly reveal the growing of the correlations close to the glass transition. [ABSTRACT FROM AUTHOR]