TIFR Annual Report 1999-00 THEORETICAL PHYSICS Condensed Matter and Statistical Physics

Ordering processes in nonequilibrium statistical systems have features which are not shared by their equilibrium counterparts. One such example arises in a system of hard-core particles sliding under gravity on an independently fluctuating surface. It was found that the fluctuations of the surface lead to clustering of initially randomly-arranged particles, over a length scale which grows indefinitely with time. The steady state of the system has macroscopic clustering of particles, but the fluctuation of the underlying surface makes it qualitatively different from usual phase ordered states: (a) The particle clusters are distributed according to a power law. (b) The order parameter has a distribution which remains broad even in the thermodynamic limit. (c) The scaled two-point spatial correlation function has a cusp at small values of the argument. Useful insight into the nature of this new kind of ordering phenomenon comes from the analytical study of a simplified model related to the fluctuations of the surface. (D. Das and M. Barma)