Study of supercooled silicon liquid-liquid critical point structural and dynamical properties

Abstract

This is a synopsis of the thesis entitled “Study of Supercooled Silicon: Liquid-Liquid Critical Point, Structural and Dynamic Proper- ties”, delivered by Vishwas V Vasisht of the Theoretical Sciences Unit, Jawa- harlal Nehru Centre for Advanced Scientific Research, Bangalore. A first order phase transition was shown by Sastry and Angell (Nature Materials, 2, 739, 2003) in supercooled silicon (Stillinger-Weber potential) using computer simulations, from a high density liquid (HDL) to a low density liquid (LDL). In this thesis a detailed and unambiguous simulation evidence is presented for the existence of a liquid-liquid critical point in Stillinger- Weber supercooled silicon. We have taken the approach of constructing the equation of state by performing molecular dynamics simulation and finding the compressibility from the equation of state and from volume fluctuations. We find a van der Waals-like loop below a critical point at negative pressure. Results from our studies of structural and dynamic properties of the two phases of supercooled silicon are presented. From the radial distribution function calculations the structural properties have been studied as a func- tion of pressure and temperature. We have analysed the local structural ii changes using the fifth neighbour distribution and tetrahedrality order pa- rameter. We have calculated the diffusivity of the two phases at different temperature and pressure values and we have found that the diffusivity in- creases upon compression. This is an anomaly which we observe at all the temperatures we have simulated. We find that there exist a close relation between the coordination number and the diffusivity. We also find a scaling behaviour in temperature for this relation between the coordination number and diffusivity. In the work on liquid-liquid transition in supercooled silicon, by Sastry and Angell, it was shown that the transition from HDL to LDL also has a characteristic change of behaviour from a fragile to a strong liquid. The fragile behaviour was observed in terms of non-Arrhenius temperature de- pendence of diffusivity. But the strong liquid identification was based on the existence of a feature in the short time dynamics called the boson peak, which is not universally accepted. We have attempted to provide a clear evidence of strong liquid behaviour by calculating the heat capacity and the diffusivity. Heat capacity was determined from the potential energy fluctu- ations and has values close to the crystal, leading to very small excess heat capacity, a feature of strong liquids.