LOCALIZATION AND QUANTUM HALL-EFFECT IN 2-DIMENSIONAL SYSTEMS UNDER STRONG MAGNETIC-FIELDS

Experimental researches of quantum transport properties of semiconductor two-dimensional electron systems in Si-MOSFETs and GaAs/AlGaAs heterostructures in high magnetic fields up to 27 T and at low temperatures down to 20 mK arc performed. Analysis of the Hall conductivity of Si-MOSFETs based on a mobility edge model shows that the temperature dependence of the mobility edge can not be explained by existing theory of localization. The fractional quantum Hall effect is observed at the filling factor of 1/7 in heterostructures. Sample size dependence and magnetic field dependence of the breakdown of the integral quantum Hall effect in heterostructures reveal that the Hall current is carried not by the edge states but by the extended states in the localization in the bulk of the two-dimensional systems.