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 0.5e2/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.