MAGNETIC-FIELD-INDUCED INSULATOR-QUANTUM HALL-INSULATOR TRANSITION INA DISORDERED 2-DIMENSIONAL ELECTRON-GAS

We present low-temperature transport measurements on the two-dimension al electron gas in delta-doped GaAs which undergoes an insulator-quant um Hall-insulator transition as the magnetic field is increased. Both low- and high-field transitions are marked by peaks in sigma(xx) and t he temperature-independent critical value of sigma(xy) of 0.5e(2)/h pe r spin. We map out the phase diagram versus disorder and magnetic fiel d and study the temperature dependence of sigma(xx) throughout. In the quantum Hall region we observe Mott variable range hopping and, aroun d the high-field transitions, scaling via a single parameter: z = (B - B)T--0.45. The functional dependence on z above this transition is f itted by recent network percolation calculations.