Indication of universal behavior of Hall conductivity near the metal-insulator transition in disordered systems

We have measured the Hall conductance in amorphous ${\mathrm{Bi}}_{\mathrm{x}}$ ${\mathrm{Kr}}_{1\mathrm{-}\mathrm{x}}$ mixtures near the metal-insulator transition (MIT). The Hall conductivity ${\sigma }_H$ on the metallic side of the MIT behaves like ${\sigma }_H$∝(x${\mathrm{-}0.55)}^{1.19\pm 0.1}$ or ${\sigma }_H$∝${\sigma }^{1.28\pm 0.05}$, where σ is the longitudinal conductivity. The critical exponents are in excellent agreement with those found in a quite different system, namely highly doped Ge:Sb, but they are in serious disagreement with existing scaling theory.