EXACT DIFFUSION CONSTANT FOR ONE-DIMENSIONAL ASYMMETRIC EXCLUSION MODELS

The one-dimensional fully asymmetric exclusion model, which describes a system of particles hopping in a preferred direction with hard core interactions, is considered on a ring of size N. The steady state of t his system is known (all configurations have equal weight), which allo ws for easy computation of the average velocity of a particle in the s teady state. Here an exact expression for the diffusion constant of a particle is obtained for arbitrary number of particles and system size , by using a matrix formulation. Two limits of infinite system size N are discussed: firstly, when the number of particles remains finite as N --> infinity the diffusion constant remains dependent on the exact number of particles due to correlations between successive collisions; secondly, when the density rho of particles is non-zero (i.e. when th e number of particles is equal to Nrho as N --> infinity) the diffusio n constant scales as N-1/2 . The exponent - 1 /2 is related to the dyn amic exponent z = 3/2 of the KPZ equation in (1+1) dimensions.