We propose that there exists a generic class of glass forming systems that have competing states (of crystalline order or not) which are locally close in energy to the ground state (which is typically unique). Upon cooling, such systems exhibit patches (or clusters) of these competing states which become locally stable in the sense of having a relatively high local shear modulus. It is in between these clusters where ageing, relaxation and plasticity under strain can take place. We demonstrate explicitly that relaxation events that lead to ageing occur where the local shear modulus is low (even negative), and result in an increase in the size of local patches of relative order. We examine the ageing events closely from two points of view. On the one hand we show that they are very localized in real space, taking place outside the patches of relative order, and from the other point of view we show that they represent transitions from one local minimum in the potential surface to another. This picture offers a direct relation between structure and dynamics, ascribing the slowing down in glass forming systems to the reduction in relative volume of the amorphous material which is liquid-like. While we agree with the well known Adam-Gibbs proposition that the slowing down is due to an entropic squeeze (a dramatic decrease in the number of available configurations), we do not agree with the Adam-Gibbs (or the Volger-Fulcher) formulae that predict an infinite relaxation time at a finite temperature. Rather, we propose that generically there should be no singular crisis at any finite temperature: the relaxation time and the associated correlation length (average cluster size) increase at most super-exponentially when the temperature is lowered.

We propose that there exists a generic class of glass-forming systems that have competing states (of crystalline order or not) which are locally close in energy to the ground state (which is typically unique). Upon cooling, such systems exhibit patches (or clusters) of these competing states which become locally stable in the sense of having a relatively high local shear modulus. It is in between these clusters where aging, relaxation, and plasticity under strain can take place. We demonstrate explicitly that relaxation events that lead to aging occur where the local shear modulus is low (even negative) and result in an increase in the size of local patches of relative order. We examine the aging events closely from two points of view. On the one hand we show that they are very localized in real space, taking place outside the patches of relative order, and from the other point of view we show that they represent transitions from one local minimum in the potential surface to another. This picture offers a direct relation between structure and dynamics, ascribing the slowing down in glass-forming systems to the reduction in relative volume of the amorphous material which is liquidlike. While we agree with the well-known Adam-Gibbs proposition that the slowing down is due to an entropic squeeze (a dramatic decrease in the number of available configurations), we do not agree with the Adam-Gibbs (or the Volger-Fulcher) formulas that predict an infinite relaxation time at a finite temperature. Rather, we propose that generically there should be no singular crisis at any finite temperature: the relaxation time and the associated correlation length (average cluster size) increase at most superexponentially when the temperature is lowered.

We use multispeckle dynamic light scattering to study the dynamics of physical gelation of methylcellulose in water. Following a temperature ramp to above 55 degrees C, a polymer network is formed which can be destroyed upon cooling the gel. We monitor this process by detecting the total scattered light intensity which shows large hysteresis during the heating-cooling cycle. Following the temperature ramp, there is a fast initial buildup of intensity, followed by a very slow and relatively small increase. The speckle pattern does not equilibrate after very long aging times (of the order of days), even after the total scattered intensity has almost stabilized, a consequence of very slow microscopic reorganization taking place in the gel. The correlation between a speckle pattern taken at a specific aging time and subsequent patterns decays with a characteristic time that increases dramatically with the age of the gel. We find that the decay of the con-elation function cannot be fitted by a single functional form in the different stages of aging; instead, four distinct stages are observed, each of which can be fitted by a different decay function. Such multiple stage relaxation is not observed in other soft condensed matter systems. On the basis of our results and those of previous studies, we argue that gelation proceeds via slow coalescence and aggregation of hydrophobic chain segments, followed by optimization of hydrophobic contacts (effective cross-links) which is opposed by network stresses due to chain stretching.

Experimental measurements of the specific heat in glass-forming systems are obtained from the linear response to either slow cooling (or heating) or to oscillatory perturbations with a given frequency about a constant temperature. The latter method gives rise to a complex specific heat with the constraint that the zero frequency limit of the real part should be identified with thermodynamic measurements. Such measurements reveal anomalies in the temperature dependence of the specific heat, including the so called "specific heat peak" in the vicinity of the glass transition. The aim of this paper is to provide theoretical explanations of these anomalies in general and a quantitative theory in the case of a simple model of glass formation. We first present interesting simulation results for the specific heat in a classical model of a binary mixture glass former. We show that in addition to the formerly observed specific heat peak there is a second peak at lower temperatures which was not observable in earlier simulations. Second, we present a general relation between the specific heat, a caloric quantity, and the bulk modulus of the material, a mechanical quantity, and thus offer a smooth connection between the liquid and amorphous solid states. The central result of this paper is a connection between the micromelting of clusters in the system and the appearance of specific heat peaks; we explain the appearance of two peaks by the micromelting of two types of clusters. We relate the two peaks to changes in the bulk and shear moduli. We propose that the phenomenon of glass formation is accompanied by a fast change in the bulk and the shear moduli, but these fast changes occur in different ranges of the temperature. Last, we demonstrate how to construct a theory of the frequency dependent complex specific heat, expected from heterogeneous clustering in the liquid state of glass formers. A specific example is provided in the context of our model for the dynamics of glycerol. We show that the frequency dependence is determined by the same alpha -relaxation mechanism that operates when measuring the viscosity or the dielectric relaxation spectrum. The theoretical frequency dependent specific heat agrees well with experimental measurements on glycerol. We conclude the paper by stating that there is nothing universal about the temperature dependence of the specific heat in glass formers-unfortunately, one needs to understand each case by itself.

Understanding the mechanical properties of glasses remains elusive since the glass transition itself is not fully understood, even in well-studied examples of glass formers in two dimensions. In this context we demonstrate here: i) a direct evidence for a diverging length scale at the glass transition ii) an identification of the glass transition with the disappearance of fluid-like regions and iii) the appearance in the glass state of fluid-like regions when mechanical strain is applied. These fluid-like regions are associated with the onset of plasticity in the amorphous solid. The relaxation times which diverge upon the approach to the glass transition are related quantitatively to the diverging length scale. Copyright (c) EPLA, 2007.

We address the interesting temperature range of a glass forming system where the mechanical properties are intermediate between those of a liquid and a solid. We employ an efficient Monte Carlo method to calculate the elastic moduli, and show that in this range of temperatures the moduli are finite for short times and vanish for long times, where short and long depend on the temperature. By invoking some exact results from statistical mechanics we offer an alternative method to compute shear moduli using molecular dynamics simulations, and compare those to the Monte Carlo method. The final conclusion is that these systems are not "viscous fluids" in the usual sense, as their actual time-dependence concatenates solidlike materials with varying local shear moduli.

The statistical mechanics of simple glass forming systems in two dimensions is worked out. The glass disorder is encoded via a Voronoi tesselation, and the statistical mechanics is performed directly in this encoding. The theory provides, without free parameters, an explanation of the glass transition phenomenology, including the identification of two different temperatures, T(g) and T(c) , the first associated with jamming and the second associated with crystallization at very low temperatures.

The statistical mechanics of simple glass forming systems in 2 dimensions is worked out. The glass disorder is encoded via a Voronoi tessellation, and the statistical mechanics is performed directly in this encoding. The theory provides, without free parameters, an explanation of the glass transition phenomenology, including the identification of two different temperatures, $T_g$ and $T_k$, the first associated with jamming and the second associated with the appearance of a quasi-crystal at very low temperatures.

Systems of highly degenerate ordered or frozen state may exhibit inverse melting (reversible crystallization upon heating) or inverse freezing (reversible glass transition upon heating). This phenomenon is reviewed, and a list of experimental demonstrations and theoretical models is presented. A simple spin model for inverse melting is introduced and solved analytically for infinite range, constant paramagnetic exchange interaction. The random exchange analogue of this model yields inverse freezing, as implied by the analytic solution based on the replica trick. The qualitative features of this system (generalized Blume-Capel spin model) are shown to resemble a large class of inverse melting phenomena. The appearance of inverse melting is related to an exact rescaling of one of the interaction parameters that measures the entropy of the system. For the case of almost degenerate spin states, perturbative expansion is presented, and the first three terms correspond to the empiric formula for the Flory-Huggins chi parameter in the theory of polymer melts. The possible microscopic origin of this chi parameter and the limitations of the Flory-Huggins theory where the state degeneracy is associated with the different conformations of a single polymer or with the spatial structures of two interacting molecules are discussed.

A spin model that displays inverse melting and inverse glass transition is presented and analyzed. Strong degeneracy of the interacting states of an individual spin leads to entropic preference of the "ferromagnetic" phase, while lower energy associated with the noninteracting states yields a "paramagnetic" phase as temperature decreases. An infinite range model is solved analytically for constant paramagnetic exchange interaction, while for its random exchange analogous results based on the replica symmetric solution are presented. The qualitative features of this model are shown to resemble a large class of inverse melting phenomena. First and second order transition regimes are identified.

We study the variation in the back-scattered light from a slab of silicate glass on a change of the glass temperature. Coherent illumination scattered from a ground surface of the glass creates a speckle pattern resulting from the sum of two different scattering events: one originating from the backward scattering from the rough surface, and the second from the forward-scattered light, which is Fresnel reflected from the flat back surface of the slab and rescattered (in the forward direction) by the rough surface. Experimental and theoretical studies show that on heating the glasses, the correlation between the initial and the final speckle patterns oscillates as a function of temperature with a frequency proportional to the thermally induced changes in the optical path in the glass. A measurement of the thermal expansion coefficient α is obtained from the oscillations with 0.01% accuracy, provided that the change in refractive index with temperature is known. The observed slow decrease in the amplitude of the temperature-induced oscillations is in agreement with the theoretical analysis of the effect of uniform thermal expansion of the scattering surface. The surface expansion also results in a damped, slow oscillation of the correlation due to the nonrandom motion of the expanding scatterers, which result in the partial rephasing of the scattered waves when the scatterers move a distance on the order of a wavelength. If the refractive index change with temperature is unknown, the decrease in the amplitude of oscillations can be used to determine α, the thermal expansion coefficient while the oscillations can be used to determine the refractive index change with temperature. The method presented is thus an alternative for measuring the thermal expansion coefficient and the change of index with temperature, and can be applied when dealing with strongly scattering surfaces, where one cannot use the normal fringe method. Different glass thicknesses and sample tilts with respect to the incident laser beam are shown to modify the correlation function.

We investigate the areas of extremely short, intense (area greater than or equal to pi) resonant optical pulses on propagation through an inhomogeneously broadened atomic medium. Experimentally, there is no apparent change in the shape of such pulses on propagation, in contradiction to the area theorem. Here we show that, although the main part of the pulse is unchanged for short propagation distances (alpha l similar to 1), it is followed by a long weak tail formed by free induction decay from the excited atoms. The tail lengthens and oscillates on propagation through the medium and permits the area theorem to be obeyed. These oscillations, which depend critically on the Doppler broadening, are reflected in the spectral analysis of the propagated pulse. We also introduce another mechanism for pulse reshaping, which is a generalization of the McCall-Hahn pulse breakup and operates at long propagation lengths (alpha l much greater than 1), where the number of absorbing atoms encountered by the pulse is comparable with the number of photons in the pulse. (C) 1999 Optical Society of America [S0740-3224(99)01207-2].