GIANT MAGNETORESISTANCE AND QUANTUM PHASE-TRANSITIONS IN STRONGLY LOCALIZED MAGNETIC 2-DIMENSIONAL ELECTRON GASES

The application of a small magnetic field (B approximate to 0.1T) eith er parallel or perpendicular to a low-density (approximate to 10(11) c m(-2)) magnetic two-dimensional electron gas (2DEG) creates a striking positive magnetoresistance of up to 700%. This is a spin effect, caus ed by the suppression of spin-dependent hopping paths between localize d states with on-site correlation. At higher fields, a spin-related de localization is observed. In the perpendicular field geometry, orbital effects combine with this delocalization and lead to quantum phase tr ansitions between the spin-polarized insulating state and the nu=1 qua ntum Hall liquid.