Magnetic-field-induced insulator-quantum Hall-insulator transition in a disordered two-dimensional electron gas

We present low-temperature transport measurements on the two-dimensional electron gas in delta -doped GaAs which undergoes an insulator-quantum Hall-insulator transition as the magnetic field is increased. Both low- and high-held transitions are marked by peaks in sigma _xx and the temperature-independent critical value of sigma _xy of 0.5e ²/h per spin. We map out the phase diagram versus disorder and magnetic field and study the temperature dependence of sigma _xx throughout. In the quantum Hall region we observe Mott variable range hopping and, around the high-field transitions, scaling via a single parameter: z=(B-B*)T^-0.45. The functional dependence on z above this transition is fitted by recent network percolation calculations.