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Properties of Structural Glass-Formers

We proposed a model for the thermodynamics and dynamics of glass-forming liquids in terms of excitations from an ideal glass state to a Gaussian manifold of configurationally excited states. The quantitative fit of this three parameter model to the experimental data on excess entropy and heat capacity shows that "fragile" behavior, indicated by a sharply rising excess heat capacity as the glass transition is approached from above, occurs in anticipation of a first-order transition--usually hidden below the glass transition--to a "strong" liquid state of low excess entropy. The distinction between fragile and strong behavior of glass formers is traced back to an order of magnitude difference in the Gaussian width of their excitation energies. Simple relations connect the excess heat capacity to the Gaussian width parameter, and the liquid-liquid transition temperature, and strong, testable, predictions concerning the distinct properties of energy landscape for fragile liquids are made.

The dynamic model relates relaxation to a hierarchical sequence of excitation events each involving the probability of accumulating sufficient kinetic energy on a separate excitable unit. Super-Arrhenius behavior of the relaxation rates, and the known correlation of kinetic with thermodynamic fragility, both follow from the way the rugged landscape induces fluctuations in the partitioning of energy between vibrational and configurational manifolds. A relation is derived in which the configurational heat capacity, rather than the configurational entropy of the Adam-Gibbs equation, controls the temperature dependence of the relaxation times, and this gives a comparable account of the experimental observations without postulating a divergent length scale. The familiar coincidence of zero mobility and Kauzmann temperatures is obtained as an approximate extrapolation of the theoretical equations. The comparison of the fits to excess thermodynamic properties of laboratory glass formers, and to configurational thermodynamics from simulations, reveals that the major portion of the excitation entropy responsible for fragile behavior resides in the low-frequency vibrational density of states. The thermodynamic transition predicted for fragile liquids emerges from beneath the glass transition in case of laboratory water and the unusual heat capacity behavior observed for this much studied liquid can be closely reproduced by the model.

A significant component of the model is the use of the temperature-dependent width of the enumeration function originating in the deviation of the density of basin minima from the Gaussian distribution. This prediction was tested on Monte Carlo simulations of the low-temperature thermodynamics of the fluid of dipolar hard spheres (PRE'07) and for the fluid described by the modified Stillinger-Weber potential (JCP'08). In both cases, an approximately linear scaling of the width with increasing temperature was found. This is illustrated by the temperature progression of the distribution of inherent structures of dipolar fluid which becomes narrower with lowering temperature (Figure).

Orientational Order/Disorder Phase Transitions

The problem of spontaneous order in dipolar systems goes back to Debye who suggested a possibility of spontaneously ordered ferroelectric phase in dipolar liquids. The idea was dismissed by Onsager and Kirkwood, but the effect of spontaneous ordering has been discussed by several mean-field theories of polar liquids and magnetic colloids. Recent

computer simulations have indicated that ferroelectric phase is possible for polar fluids at a non-zero temperature. Most of the discussion of spontaneous order in dipolar systems has been focused on systems with permanent dipoles. Real systems, either of molecular or

nano-scale dimension, are composed of polarizable particles. We have therefore decided to look for ferroelectric phase in fluids composed of polarizable two-state molecules. The ultimate goal is to explore the possibility of cooperativity of charge transfer in systems with dense arrangement of charge-transfer molecules (thin films on the surface, Lamgmuir-Blogett films, etc.).

The model fluid of two-state molecules (Phys. Rev, Lett., submitted) shows a complex and rich phase diagram including the transition from a nonpolar to polar, paraelectric phase ( in the Figure) followed by the spontaneous creation of the ferroelectric phase at The system shows some unique physical properties potentially useful for a range of applications including molecular electronics, molecular switching, and solar energy conversion. The non-polar/paraelectric phase transition is characterized by the change

in the dipolar susceptibility amounting 3 orders of magnitude. The dielectric constant in the ferroelectric phase is several thousands in our simulations and its magnitude is limited only by he size of the ferroelectric domain. We are currently pursuing this avenue by

exploring the interfacial phenomena where charge-transfer cooperativity can be modulated electrochemically or photochemically.