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ABSTRACT: We have measured, as a function of the age t_{a}, the aging of the nonlinear dielectric susceptibility χ_{3} of glycerol below the glass transition. Whereas the linear susceptibility can be accurately accounted for in terms of an age dependent relaxation time τ_{α}(t_{a}), this scaling breaks down for χ_{3}, suggesting an increase of the amplitude of χ_{3}. This is a strong indication that the number N_{corr} of molecules involved in relaxation events increases with t_{a}. For T=0.96×T_{g}, we find that N_{corr} increases by ∼10% when t_{a} varies from 1 to 100 ks. This sheds new light on the relation between length scales and time scales in glasses.
Physical Review Letters 10/2012; 109(17):175702. · 7.37 Impact Factor
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ABSTRACT: We show that water penetrates into the silicate glass matrix during stress corrosion fracture by probing what is stored under the fracture surface using neutron reflection. The concentration profile determined for two different values of the external loading exhibits a region close to the fracture surface where the water content is fairly elevated, suggesting a high amount of damage.
Journal of Physics Conference Series 09/2011; 319(1):012005.
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ABSTRACT: We measure the thickness of the heavy water layer trapped under the stress corrosion fracture surface of silica using neutron reflectivity experiments. We show that the penetration depth is 65-85 Å, suggesting the presence of a damaged zone of ∼100 Å extending ahead of the crack tip during its propagation. This estimate of the size of the damaged zone is compatible with other recent results.
Physical Review Letters 04/2011; 106(16):165504. · 7.37 Impact Factor
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ABSTRACT: We have measured the ac nonlinear dielectric response χ3 (ω, T) of an archetypical glassformer (glycerol) just above its glass transition temperature Tg ∼190 K. Our measurements were performed at temperatures between Tg + 4 K and Tg + 35 K, which corresponds to 5 decades of variation of the relaxation time τ (T) of the liquid. We find that χ3(ω, T) is peaked for ωτ ∼ 0.2 and that the height of the peak grows as one approaches Tg. A powerlaw in frequency beyond the peak of χ3 is observed and χ3 (ω, T) displays scaling as a function of ωτ.These features correspond to the main predictions of some recent theoretical works relating the nonlinear susceptibility of supercooled liquids to the average number Ncorr of dynamically correlated molecules. We thus interpret the experimental increase of the peak of nonlinear susceptibility when T decreases, as an increase of Ncorr when T → Tg. This strongly reinforces the collective picture of the dynamics of supercooled liquids close to the glass transition. Finally, we discuss some heating effects and show that they do not affect significantly our conclusions. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
physica status solidi (c) 03/2011; 8(11‐12):3147 - 3150.
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ABSTRACT: 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).
Physical Review Letters 04/2010; 104(16):165703. · 7.37 Impact Factor
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ABSTRACT: By analyzing the displacement statistics of an assembly of horizontally vibrated bidisperse frictional grains in the vicinity of the jamming transition experimentally studied before, we establish that their superdiffusive motion is a genuine Levy flight, but with `jump' size very small compared to the diameter of the grains. The vibration induces a broad distribution of jumps that are random in time, but correlated in space, and that can be interpreted as micro-crack events at all scales. As the volume fraction departs from the critical jamming density, this distribution is truncated at a smaller and smaller jump size, inducing a crossover towards standard diffusive motion at long times. This interpretation contrasts with the idea of temporally persistent, spatially correlated currents and raises new issues regarding the analysis of the dynamics in terms of vibrational modes. Comment: 7 pages, 6 figures
01/2010;
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ABSTRACT: The aim of this paper is to summarise the basic arguments and the intuition bolstering the RFOT picture for glasses, based on a finite dimensional extension of mean-field models with an exponentially large number of metastable states. We review the pros and cons that support or undermine the theory, and the directions, both theoretical and experimental, where progress is needed to ascertain the status of RFOT. We elaborate in particular on the notions of mosaic state and point-to-set correlations, and insist on the importance of fluctuations in finite dimensions, that significantly blur the expected cross-over between a Mode-Coupling like regime and the mosaic, activated regime. We discuss in detail the fundamental predictions of RFOT, in particular the possibility to force a small enough system into an ideal glass state, and present several new ones, concerning aging properties or non-linear rheology. Finally, we compare RFOT to other recent theories, including elastic models, Frustration Limited Domains or Kinetically Constrained models. Comment: 53 pages, to appear in a book on RFOT edited by V. Lubchenko & P. Wolynes
12/2009;
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ABSTRACT: We derive the Mode Coupling Theory (MCT) of the glass transition as a Landau theory, formulated as an expansion of the exact dynamical equations in the difference between the correlation function and its plateau value. This sheds light on the universality of MCT predictions. While our expansion generates higher order non-local corrections that modify the standard MCT equations, we find that the square root singularity of the order parameter, the scaling function in the \beta regime and the functional relation between the exponents defining the \alpha and \beta timescales are universal and left intact by these corrections.
04/2009;
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ABSTRACT: Supercooled liquids exhibit a pronounced slowdown of their dynamics on cooling1 without showing any obvious structural or thermodynamic changes2. Several theories relate this slowdown to increasing spatial correlations3, 4, 5, 6. However, no sign of this is seen in standard static correlation functions, despite indirect evidence from considering specific heat7 and linear dielectric susceptibility8. Whereas the dynamic correlation function progressively becomes more non-exponential as the temperature is reduced, so far no similar signature has been found in static correlations that can distinguish qualitatively between a high-temperature and a deeply supercooled glass-forming liquid in equilibrium. Here, we show evidence of a qualitative thermodynamic signature that differentiates between the two. We show by numerical simulations with fixed boundary conditions that the influence of the boundary propagates into the bulk over increasing length scales on cooling. With the increase of this static correlation length, the influence of the boundary decays non-exponentially. Such long-range susceptibility to boundary conditions is expected within the random first-order theory4, 9, 10 (RFOT) of the glass transition. However, a quantitative account of our numerical results requires a generalization of RFOT, taking into account surface tension fluctuations between states.
Nature Physics 08/2008; Thermodynamic signature of growing amorphous order in glass-forming liquids(4):771. · 18.97 Impact Factor
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ABSTRACT: Although several theories relate the steep slowdown of glass formers to
increasing spatial correlations of some sort, standard static correlation
functions show no evidence for this. We present results that reveal for the
first time a qualitative thermodynamic difference between the high temperature
and deeply supercooled equilibrium glass-forming liquid: the influence of
boundary conditions propagates into the bulk over larger and larger
lengthscales upon cooling, and, as this static correlation length grows, the
influence decays nonexponentially. Increasingly long-range susceptibility to
boundary conditions is expected within the random firt-order theory (RFOT) of
the glass transition, but a quantitative account of our numerical results
requires a generalization of RFOT where the surface tension between states
fluctuates.
05/2008;
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ABSTRACT: In this paper we investigate the superspin glass behavior of a concentrated assembly of interacting maghemite nanoparticles and compare it to that of canonical atomic spin glass systems. ac versus temperature and frequency measurements show evidence of a superspin glass transition taking place at low temperature. In order to fully characterize the superspin glass phase, the aging behavior of both the thermo-remanent magnetization (TRM) and ac susceptibility has been investigated. It is shown that the scaling laws obeyed by superspin glasses and atomic spin glasses are essentially the same, after subtraction of a superparamagnetic contribution from the superspin glass response functions. Finally, we discuss a possible origin of this superparamagnetic contribution in terms of dilute spin glass models.
03/2008;
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ABSTRACT: We track the motion of a horizontally vibrated amorphous assembly of bidisperse hard disks, for densities ranging across the jamming transition. We derive on very general grounds a bound on the dynamical susceptibility in terms of the response of the dynamics to a change in density. This generalizes a similar bound recently derived for equilibrium liquids. We find that in our experimental system the bound is tight and reproduces the non-monotonic behavior of the dynamical susceptibility both in time and density across the jamming transition. The underlying scaling behavior reveals an intimate connection between anomalous diffusion and dynamical heterogeneity.
01/2008;
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ABSTRACT: Glasses are often described as a genuine state of matter. The aim of this paper is to briefly review several ideas, old and
new, about what makes glasses so special as a state of matter: glasses are liquids that do not flow, characterized by increasingly
cooperative dynamics.
Physics of Condensed Matter 01/2008; 64(3):327-330. · 1.53 Impact Factor
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ABSTRACT: 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.
Physical Review E 11/2007; 76(4 Pt 1):041510. · 2.26 Impact Factor
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ABSTRACT: The dynamical properties of a dense horizontally vibrated bidisperse granular monolayer are experimentally investigated. The quench protocol produces states with a frozen structure of the assembly, but the remaining degrees of freedom associated with contact dynamics control the appearance of macroscopic rigidity. We provide decisive experimental evidence that this transition is a critical phenomenon, with increasingly collective and heterogeneous rearrangements occurring at length scales much smaller than the grains' diameter, presumably reflecting the contact force network fluctuations. Dynamical correlation time and length scales soar on both sides of the transition, as the volume fraction varies over a remarkably tiny range ($\delta \phi/\phi \sim 10^{-3}$). We characterize the motion of individual grains, which becomes super-diffusive at the jamming transition $\phi_J$, signaling long-ranged temporal correlations. Correspondingly, the system exhibits long-ranged four-point dynamical correlations in space that obey critical scaling at the transition density.
07/2007;
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ABSTRACT: We study in detail the predictions of various theoretical approaches, in particular, mode-coupling theory (MCT) and kinetically constrained models (KCMs), concerning the time, temperature, and wave vector dependence of multipoint correlation functions that quantify the strength of both induced and spontaneous dynamical fluctuations. We also discuss the precise predictions of MCT concerning the statistical ensemble and microscopic dynamics dependence of these multipoint correlation functions. These predictions are compared to simulations of model fragile and strong glass-forming liquids. Overall, MCT fares quite well in the fragile case, in particular, explaining the observed crucial role of the statistical ensemble and microscopic dynamics, while MCT predictions do not seem to hold in the strong case. KCMs provide a simplified framework for understanding how these multipoint correlation functions may encode dynamic correlations in glassy materials. However, our analysis highlights important unresolved questions concerning the application of KCMs to supercooled liquids.
The Journal of Chemical Physics 06/2007; 126(18):184504. · 3.33 Impact Factor
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ABSTRACT: We study theoretically and numerically a family of multipoint dynamic susceptibilities that quantify the strength and characteristic length scales of dynamic heterogeneities in glass-forming materials. We use general theoretical arguments (fluctuation-dissipation relations and symmetries of relevant dynamical field theories) to relate the sensitivity of averaged two-time correlators to temperature and density to spontaneous fluctuations of the local dynamics. Our theoretical results are then compared to molecular dynamics simulations of the Newtonian, Brownian, and Monte Carlo dynamics of two representative glass-forming liquids, a fragile binary Lennard-Jones mixture, and a model for the strong glass-former silica. We justify in detail the claim made by Berthier et al. [Science 310, 1797 (2005)] that the temperature dependence of correlation functions allows one to extract useful information on dynamic length scales in glassy systems. We also discuss some subtle issues associated with the choice of microscopic dynamics and of statistical ensemble through conserved quantities, which are found to play an important role in determining dynamic correlations.
The Journal of Chemical Physics 06/2007; 126(18):184503. · 3.33 Impact Factor
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ABSTRACT: We show that the glass transition predicted by the Mode-Coupling Theory (MCT) is a critical phenomenon with a diverging length and time scale associated to the cooperativity of the dynamics. We obtain the scaling exponents ν and z that relate space and time scales to the distance from criticality, as well as the scaling form of the critical four-point correlation function. However, both these predictions and other well-known MCT results are mean field in nature and are thus expected to change below the upper critical dimension dc = 6, as suggested by different forms of the Ginzburg criterion.
EPL (Europhysics Letters) 01/2007; 67(1):21. · 2.17 Impact Factor
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ABSTRACT: 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.
Science 01/2006; 310(5755):1797-800. · 31.20 Impact Factor
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ABSTRACT: In this paper, we review several important features of the out-of-equilibrium dynamics of spin glasses. Starting with the simplest experiments, we discuss the scaling laws used to describe the isothermal aging observed in spin glasses after a quench down to the low temperature phase. We report in particular new results on the sub-aging behaviour of spin glasses. We then discuss the rejuvenation and memory effects observed when a spin glass is submitted to temperature variations during aging, from the point of view of both energy landscape pictures and of real space pictures. We highlight the fact that both approaches point out the necessity of hierarchical processes involved in aging. Finally, we report an investigation of the effect of small temperature variations on aging in spin glass samples with various anisotropies which indicates that this hierarchy depends on the spin anisotropy. Comment: submitted for the Proceedings of Stat Phys 22, Bangalore (India)
06/2004;
