CHARGING EFFECTS IN QUANTUM DOTS AT HIGH MAGNETIC-FIELDS

We have studied charging effects in quantum dots, defined by gates in the two-dimensional electron gas of an AlGaAs-GaAs heterostructure. We have investigated Coulomb oscillations at zero magnetic field, and in the integer and fractional QHE regimes. Coulomb oscillations are found to appear only when both barrier conductances G are below the first well resolved plateau: at zero field G < 2e2/h, in the IQHE regime G < e2/h, and in the FQHE regime G < 1/3e2/h. The period of the oscillations is shown to be field independent. We furthermore studied the interplay between charging effects and discrete energy states in the dot. In the IQHE, we found a regular pattern in the amplitude of the Coulomb oscillations that corresponds to the number of Landau levels in the dot. In a smaller dot containing only 25 electrons, the excitation spectrum of the dot is investigated at finite source-drain voltages.