Ref HAL: hal-01437143_v1
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Computer simulations give precious insight into the microscopic behavior of amorphous materials, but their typical timescales are orders of magnitude shorter than the experimentally relevant ones. We close this enormous gap for a class of polydisperse models, which we successfully equilibrate beyond experimental timescales by means of the swap Monte Carlo algorithm. This numerical advance allows us to address some crucial outstanding questions, such as the relationship between jamming and glass transitions and the relevance of an entropy crisis for glass formation.

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The dynamics of glass forming liquids shows a tremendous slowing down if temperature is decreased: Relaxation times, transport coefficients such as the diffusion constant or the shear viscosity grow by more than 15 orders of magnitude within narrow temperature range. One of the central goals in the field of glass physics is to understand the origin of this dramatic slowing down of the dynamics.There are several scenarios that seem to be able to explain the slowing down of the dynamics and the low temperature state of the glass former. A very popular one is to rationalize the slow dynamics by invoking the existence of a thermodynamic transition, the so-called ideal glass transition from the liquid to an ideal glass states. This ideal glass transition occurs when the configurational entropy, defined as the logarithm of the number of available states, becomes zero. Due to thissingular behavior of the configurational entropy, the relaxation time andtransport coefficients diverge if the transition point is approached [1].However, confirming the existence of the ideal glass transition is a verydifficult task since in practice the system falls out of equilibriumbefore reaching this putative ideal glass transition temperature,thus making the test of this theory most difficult. Furthermore, otherscenarios are also able to explain the slow dynamics without resortingany thermodynamic transition [2].In the present study, we investigate the existence of the idealglass transition by using computer simulation of a canonical binaryLennard-Jones glass former in three dimensions. Massive computationaleffort and efficient sampling algorithm (parallel tempering) allow us toreach equilibrium states that are very deeply supercooled. By measuringthe configurational entropy directly (obtained via thermodynamicintegration), we find that the configuration entropy does not go to zeroat a finite temperature. This implies that the thermodynamic singularityis indeed avoided before reaching a putative ideal glass transitiontemperature predicted by previous studies. Our results indicates thatcontrary to the previous results the system remains in a liquid statedown to zero temperature without showing an ideal glass transition.Furthermore, we analyze the microscopic structure and the potentialenergy landscape of the system to clarify the mechanism that leads tothe avoidance of the ideal glass transition. We find that locallyfavored structures present in the liquid state hardly change in thetemperature range we consider, which rules out another possibility forthe slow dynamics, e.g. the transformation between two distinct liquids,so-called liquid-liquid transition [3]. Instead, we find the potentialenergy landscape contains extensive number of minima even very lowtemperature, which rationalize the avoided singularity.Our results support a theoretical argument that the ideal glasstransition can not exist at bulk system in finite dimensions [5].[1] Adam G and Gibbs JH, J. Chem. Phys., 43, 139 (1965)[2] Chandler D and Garrahan JP, Annu. Rev. Phys. Chem., 61, 191 (2010)[3] Speck T, Royall CP, and Williams SR, arXiv:1409.0751, (2014)[4] Saksaengwijit A and Heuer A, Phys. Rev. Lett., 93, 235701 (2004)[5] Stillinger FH, J. Chem. Phys., 88, 7818 (1988)

Ref HAL: hal-01390728_v1
DOI: 10.1088/0953-8984/28/41/414015
WoS: WOS:000383988100015
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5 Citations
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Systems with short-range attractive and long-range repulsive interactions are able to form mesophases at sufficiently low temperatures. In two dimensions, such mesophases emerge as clusters, stripes or bubbles. Using extensive Monte Carlo simulations we investigate the static and the dynamic properties of such a cluster-forming system over a broad temperature range and for different densities. Via the static properties we analyse how ordering into close packed configurations sets in both at the level of the particles as well as at the level of the clusters. The dynamic properties provide information on how, at low temperature, the motion of individual particles is influenced by the dynamic slowing down of the clusters. Finally, we discuss the different diffusion mechanisms at play at low and intermediate

Ref HAL: hal-01382770_v1
Ref Arxiv: 1602.07589
DOI: 10.1088/1742-5468/2016/07/074012
WoS: WOS:000381379800041
Ref. & Cit.: NASA ADS
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10 Citations
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We study the structure of inactive states in a prototypical model glass, the Kob-Andersen binary Lennard-Jones mixture. These inactive states are obtained by transition path sampling and are at dynamical phase coexistence with an active equilibrium state. Configurations in the inactive states are kinetically stable and are located in deeper basins of the energy landscape than their active counterparts. By analyzing trajectory-to-trajectory fluctuations within the inactive state, we assess correlations between kinetic stability, energy and other structural properties. We show that measures of local order associated to stable local packings and bond-orientational order are weakly correlated with energy and kinetic stability. We discuss what kinds of structural measurement might capture the relevant dynamical features of the inactive state.

Ref HAL: hal-01348542_v1
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The dynamics of glass-forming liquids is heterogeneous and displays growing spatial correlations upon cooling. Whether such behavior arises from fluctuations in local structure or more complex forms of amorphous order is a highly debated question. To clarify this issue, we studied several model liquids within a coherent simulation framework based on the iso-configurational ensemble [1]. We found that the correlation between the preferred local structure and dynamic heterogeneity is system-dependent: it is pronounced in systems that deviate markedly from the mean-field picture of glassy dynamics and weak or absent in models that adhere to it to a good extent. I will review these results and then assess recent proposals to account for dynamic fluctuations using more genericmeasures of structure, such as overlap distributions and predictability analysis. Finally, I will characterize the structure of ultra-stable glassy samples of hard and soft spheres, which we recently equilibrated at large packing fractions using an optimized swap Monte Carlo algorithm [2]. [1] G. M. Hocky, D. Coslovich, A. Ikeda, D. R. Reichman, Phys. Rev. Lett. 113, 157801 (2014)[2] L. Berthier, D. Coslovich, A. Ninarello, M. Ozawa, arXiv:1511.06182 (2015)

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Ref Arxiv: 1511.06182
DOI: 10.1103/PhysRevLett.116.238002
WoS: 000377509900014
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76 Citations
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We implement and optimize a particle-swap Monte-Carlo algorithm that allows us to thermalize a polydisperse system of hard spheres up to unprecedentedly-large volume fractions, where \revise{previous} algorithms and experiments fail to equilibrate. We show that no glass singularity intervenes before the jamming density, which we independently determine through two distinct non-equilibrium protocols. We demonstrate that equilibrium fluid and non-equilibrium jammed states can have the same density, showing that the jamming transition cannot be the end-point of the fluid branch.

Commentaires: 5 pages, 3 figs; To be published in Phys. Rev. Lett. Réf Journal: Phys. Rev. Lett. 116, 238002 (2016)

Ref HAL: hal-01310191_v1
DOI: 10.1103/PhysRevE.93.042602
WoS: WOS:000373585500004
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7 Citations
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We use molecular dynamics simulations to investigate dynamic heterogeneities and the potential energy landscape of the Gaussian core model (GCM). Despite the nearly Gaussian statistics of particles' displacements, the GCM exhibits giant dynamic heterogeneities close to the dynamic transition temperature. The divergence of the four-point susceptibility is quantitatively well described by the inhomogeneous version of the mode-coupling theory. Furthermore, the potential energy landscape of the GCM is characterized by large energy barriers, as expected from the lack of activated, hopping dynamics, and display features compatible with a geometric transition. These observations demonstrate that all major features of mean-field dynamic criticality can be observed in a physically sound, three-dimensional model.