Hopping Conductivity in the Quantum Hall Effect: Revival of Universal Scaling

We have measured the temperature dependence of the conductivity ${\sigma }_{\mathrm{xx}}$ of a two-dimensional electron system deep into the localized regime of the quantum Hall plateau transition. Using variable-range hopping theory we extract directly the localization length $\xi$ from this experiment. We use our results to study the scaling behavior of $\xi$ as a function of the filling factor distance $|\delta \nu |$ to the critical point of the transition. We find for all samples a power-law behavior $\xi \propto |\delta \nu {|}^{-\gamma }$ in agreement with the theoretically proposed universal exponent $\gamma =2.35\mathrm{}$.