Float-up picture of extended levels in the integer quantum Hall effect: A numerical study

The fate of the integer quantum Hall effect (IQHE) at weak magnetic field is studied numerically in the presence of cot-related disorders. We find a systematic float-tip and merging picture for extended levels on the low-energy side which results in direct transitions from higher-plateau IQHE states to the insulator. Such direct transitions are controlled by a quantum critical point with a universal scaling form of conductance. The phase diagram is in good agreement with recent experiments.