LOW-TEMPERATURE ELECTRONIC TRANSPORT MEASUREMENTS ON A GATED DELTA-DOPED GAAS SAMPLE - MAGNETORESISTANCE, QUANTUM HALL-EFFECT AND CONDUCTIVITY FLUCTUATIONS

We present magnetotransport measurements (up to 7 T) performed at very low temperatures (down to 20 mK) on a GaAs sample containing two para llel delta-doped layers whose carrier concentration can be varied by m eans of a gate electrode. With increasing negative gate voltage the re sistance becomes,more strongly temperature-dependent, indicating a mor e localized electron system. The magnetoresistance is found to be stro ngly anisotropic. When the field is parallel to the layers we find a l arge positive magnetoresistance which we attribute to orbital shrinkin g of the strongly localized donor wavefunction. In contrast, in the pe rpendicular orientation, we observe a strong negative magnetoresistanc e at low fields whose origin remains unclear, and the quantum Hall eff ect at larger fields. At low gate voltages both delta-layers are in th e quantum Hall state whereas at larger negative voltages the layer adj acent to the gate becomes insulating. In the case of strong depletion the high-ohmic sample shows reproducible conductivity fluctuations as a function of either the gate voltage or the magnetic field. The fluct uations diminish at higher temperatures and larger measuring currents.