MAGNETIC-FIELD-INDUCED METAL-INSULATOR TRANSITIONS IN MULTIPLE-QUANTUM-WELL STRUCTURES

Far infrared spectroscopy and electrical transport measurements on a s et of modulation-doped GaAs/Al0.3Ga0.7As multiple-quantum-well structu res show metallic and insulating behavior that depends on doping densi ty and magnetic field; impurity band insulator, impurity band metal, a nd quantum Hall conductor states are observed. In the latter case a pl ateau 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.