SOIREE ON LIQUIDS AND GLASSES

A Soiree on Liquids and Glasses

Understanding heat capacity of liquids and glass transition on the basis of phonons and elastic interactions
Dr Kostya Trachenko

Existing textbook expressions for the energy and heat capacity of gases and solids are widely taught in physics courses. However, no such expression exists for a liquid. The reason for this was summarized by Landau as "liquids have no small parameter", and discussed in some detail in Landau&Lifshitz Statistical Physics textbook. Based on the old idea of J Frenkel, I formulate the problem in the language of phonons, and calculate liquid energy and heat capacity for both classical and quantum cases. The resulting equation relates liquid heat capacity to its relaxation time (or viscosity) with no fitting parameters, and is compared with the experimental data of several liquids, including metallic, noble, molecular and network liquids.

I subsequently address old and very controversial problem of glass transition, the physical origin of the jump of heat capacity at glass transition temperature Tg. I propose a solution that is based on liquid elastic and thermal properties. The jump of heat capacity at Tg takes place as a result of the change of the liquid’s elastic, vibrational, and thermal properties. In this theory, I discuss time-dependent effects of glass transition, including the widely observed logarithmic increase of Tg with the quench rate.

The present understanding of the vibrational dynamics in glasses
Professor Giancarlo Ruocco (Department of Physics, “Sapienza” Universita’ di Roma, Italy)

After almost twenty years of application of the Inelastic X-ray Scattering (IXS) technique, which complements the well established Inelastic Neutron Scattering (INS) one,  the main characteristics of the high frequency (THz) collective modes in topologically disordered systems have been identified.  Despite the contributions of this experimental tool, less is know about the collective excitations of glasses in a momentum-energy region that lie between the IXS (Q>1 nm^-1) and the traditional Brillouin Light Scattering (Q<0.04 nm^-1) domains. Very recently, the first steps were taken to fill this gap using the synchrotron based UV Brillouin spectroscopy and the systems response to ultrafast laser pulse propagation.  

In this talk I will review what is the present understanding on the high frequency atomic dynamics in glasses and liquids,  as obtained by IXS experiments and compared with the most recent molecular dynamic simulations and theories.