Random Pinning Glass Model

Glass transition where viscosity of liquids increases dramatically upon decrease of temperature without any major change in structural properties, remains one of the most challenging problems in condensed matter physics (Cavagna, 2009; Berthier and Biroli, 2011) in spite of tremendous research efforts in last decades. On the other hand disordered freezing of spins in a magnetic materials with decreasing temperature, the so-called spin glass transition, is relatively better understood (Mezard, Parisi and Virasoro, 1987; Castellani and Cavagna, 2005). Previously found similarity between some spin glass models with the structural glasses (Kirkpatrick and Thirumalai, 1987; Kirkpatrick and Wolynes, 1987; Kirkpatrick and Wolynes, 1987; Franz and Parisi, 1999; Moore and Drossel, 2002) inspired development of theories of structural glasses (Kirkpatrick, Thirumalai and Wolynes, 1989; Barrat, Franz and Parisi, 1997; M\'ezard and Parisi, 1999; Lubchenko and Wolynes, 2007; Biroli and Bouchaud, 2012) based on the scenario of spin glass transition. This scenario though looks very appealing is still far from being well established. One of the main differences between standard spin systems to molecular systems is the absence of quenched disorder and the presence of translational invariance: it often assumed that this difference is not relevant, but this conjecture is still far from being established. The quantities, which are well defined and characterized for spin models, are not easily calculable for molecular glasses due to the lack of quenched disorder which breaks the translational invariance in the system and the characterization of the similarity between the spin and the structural glass transition remained an elusive subject still now. In this study we introduced a model structural glass with built in quenched disorder which alleviates this main difference between the spin and molecular glasses thereby helping us to compare these two systems: the possibility of producing a good thermalization at rather low temperatures is one of the advantages of this model.Comment: Submitted to PNAS with 7 pages 5 figures and Supplementary Material

Growing length and time scales in glass forming liquids

We study the growing time scales and length scales associated with dynamical slow down for a realistic glass former, using computer simulations. We perform finite size scaling to evaluate a length scale associated with dynamical heterogeneity which grows as temperature decreases. However, relaxation times which also grow with decreasing temperature, do not show the same kind of scaling behavior with system size as the dynamical heterogeneity, indicating that relaxation times are not solely determined by the length scale of dynamical heterogeneity. We show that relaxation times are instead determined, for all studied system sizes and temperatures, by configurational entropy, in accordance with the Adam-Gibbs relation, but in disagreement with theoretical expectations based on spin-glass models that configurational entropy is not relevant at temperatures substantially above the critical temperature of mode coupling theory. The temperature dependence of the heterogeneity length scale shows significant deviations from theoretical expectations, and the length scale one may extract from the system size dependence of the configurational entropy has much weaker temperature dependence compared to the heterogeneity length scale at all studied temperatures. Our results provide new insights into the dynamics of glass-forming liquids and pose serious challenges to existing theoretical descriptions

Short-time β\beta-relaxation in glass-forming liquids is cooperative in nature

Temporal relaxation of density fluctuations in supercooled liquids near the glass transition occurs in multiple steps. The short-time $\beta$-relaxation is generally attributed to spatially local processes involving the rattling motion of a particle in the transient cage formed by its neighbors. Using molecular dynamics simulations for three model glass-forming liquids, we show that the $\beta$-relaxation is actually cooperative in nature. Using finite-size scaling analysis, we extract a growing length-scale associated with $\beta$-relaxation from the observed dependence of the $\beta$-relaxation time on the system size. Remarkably, the temperature dependence of this length scale is found to be the same as that of the length scale that describes the spatial heterogeneity of local dynamics in the long-time $\alpha$-relaxation regime. These results show that the conventional interpretation of $\beta$-relaxation as a local process is too simplified and provide a clear connection between short-time dynamics and long-time structural relaxation in glass-forming liquids

Dynamics of Glass Forming Liquids with Randomly Pinned Particles

It is frequently assumed that in the limit of vanishing cooling rate, the glass transition phenomenon becomes a thermodynamic transition at a temperature $T_{K}$. However, with any finite cooling rate, the system falls out of equilibrium at temperatures near $T_g(>T_{K})$, implying that the very existence of the putative thermodynamic phase transition at $T_{K}$ can be questioned. Recent studies of systems with randomly pinned particles have hinted that the thermodynamic glass transition may be observed in simulations and experiments carried out for liquids with randomly pinned particles. This expectation is based on the results of approximate calculations that suggest that the temperature of the thermodynamic glass transition increases as the concentration of pinned particles is increased and it may be possible to equilibrate the system at temperatures near the increased transition temperature. We test the validity of this prediction through extensive molecular dynamics simulations of two model glass-forming liquids in the presence of random pinning. We fit the temperature-dependence of the structural relaxation time to the Vogel-Fulcher-Tammann form that predicts a divergence of the relaxation time at a temperature $T_{VFT}$ and identify this temperature with the thermodynamic transition temperature $T_K$. We find that $T_{VFT}$ does not show any sign of increasing with increasing concentration of pinned particles. The main effect of pinning is found to be a rapid decrease in the kinetic fragility of the system with increasing pin concentration. Implications of these observations for current theories of the glass transition are discussed.Comment: submitted to scientific repor

Universality of the Plastic Instability in Strained Amorphous Solids

By comparing the response to external strains in metallic glasses and in Lenard-Jones glasses we find a quantitative universality of the fundamental plastic instabilities in the athermal, quasistatic limit. Microscopically these two types of glasses are as different as one can imagine, the latter being determined by binary interactions, whereas the former by multiple interactions due to the effect of the electron gas that cannot be disregarded. In spite of this enormous difference the plastic instability is the same saddle-node bifurcation. As a result the statistics of stress and energy drops in the elasto-plastic steady state are universal, sharing the same system-size exponents