IS THE PEAK VALUE OF SIGMA(XX) AT THE QUANTUM HALL TRANSITION UNIVERSAL

The question of the universality of the longitudinal peak conductivity at the integer quantum Hall transi tion is considered. For this purpo se, a system of 2D Dirac fermions with random mass characterised by va riance g is proposed as a model which undergoes a quantum Hall transit ion. Whilst for some specific models the longitudinal peak conductivit y sigma(xx) was found to be universal (in agreement with the conjectur e of Lee et al. as well as with some numerical work), we find that sig ma(xx) is reduced by a factor (1 + g/2 pi)(-1), at least for small g. This provides some theoretical evidence for the non-universality of si gma(xx), as observed in a number of experiments.