Magnetic-field-induced metal-insulator transitions in multiple-quantum-well structures.

Far infrared spectroscopy and electrical transport measurements on a set of modulation-doped GaAs/A${\mathrm{l}}_{0.3}$G${\mathrm{a}}_{0.7}$As multiple-quantum-well structures show metallic and insulating behavior that depends on doping density and magnetic field; impurity band insulator, impurity band metal, and quantum Hall conductor states are observed. In the latter case a plateau around filling factor 1 shows entrance to an insulating state at higher fields; this result is compared with the global phase diagram for the quantum Hall effect. A general phase diagram that encompasses lower doping densities is suggested.