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 Halt phases in two-dimensiona l electron systems, as well as the transition to the insulating phase at hi gh magnetic fields. We focus mainly on the resistivity at the transitions. We first present results of a numerical calculation for a non-interacting e lectron model, which shows that the Thouless conductance is universal at in teger quantum Hall transitions, just like the conductivity tensor. We then present experimental results for the measured longitudinal resistivity at t he quantum Hall-insulator transition, which are found to be clustered aroun d the universal value suggested theoretically for the non-interacting mode! , within a range of 15%. Finally, we investigate departure from universalit y due to finite temperature and finite system size near the transition poin t.