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

We have measured the Hall conductance in amorphous Biₓ{Kr}_{1{{-}}x}$ mixtures near the metal-insulator transition (MIT). The Hall conductivity ${{σ}}H$ on the metallic side of the MIT behaves like ${{σ}}H∝(x{{{-}}0.55)}^{1.19±{}0.1}$ or ${{σ}}H∝{{σ}}^{1.28±{}0.05}$, 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.