Evidence of a magnetic-field-induced insulator-metal-insulator transition

We present measurements of the low-temperature conductivity of $n$-type GaAs doped just on the insulating side of the metal-insulator transition. In zero magnetic field the temperature dependence of the conductivity indicated that the sample was insulating. At moderate fields the conductivity extrapolated to a finite value at $T=0\mathrm{}$, implying that the sample was driven metallic by the field. As the field was further increased, the sample became insulating once more due to wave-function shrinkage. These results support the phase diagram suggested by Shapiro.