Study of universality at integer quantum Hall transitions

We report in this paper results of experimental and theoretical studies of transitions between different integer quantum Hall phases in two-dimensional electron systems, as well as the transition to the insulating phase at high magnetic fields. We focus mainly on the resistivity at the transitions. We first present results of a numerical calculation for a non-interacting electron model, which shows that the Thouless conductance is universal at integer quantum Hall transitions, just like the conductivity tensor. We then present experimental results for the measured longitudinal resistivity at the quantum Hall-insulator transition, which are found to be clustered around the universal value suggested theoretically for the non-interacting model, within a range of 15%. Finally, we investigate departure from universality due to finite temperature and finite system size near the transition point.