Is the Peak Value of ${\sigma }_{xx}$ at the Quantum Hall Transition Universal?

Abstract

The question of the universality of the longitudinal peak conductivity at the integer quantum Hall transition is considered. For this purpose, a system of 2D Dirac fermions with random mass characterised by variance $g$ is proposed as a model which undergoes a quantum Hall transition. Whilst for some specific models the longitudinal peak conductivity ${\sigma }_{xx}$ was found to be universal (in agreement with the conjecture of Lee et al. as well as with some numerical work), we find that ${\sigma }_{xx}$ 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 ${\sigma }_{xx}$, as observed in a number of experiments.

2 double-column LaTeX pages, no figures, to appear in Z.Phys.B

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