Regensburg 2000 – wissenschaftliches Programm

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HL: Halbleiterphysik

HL 13: Symposium: Der Quanten-Hall-Effekt wird 20 (I)

HL 13.4: Fachvortrag

Dienstag, 28. März 2000, 12:00–12:30, H15

Single electron contour imaging of the quantum Hall liquid — •Ray Ashoori — MIT, Cambridge, MA 02139-4307, USA

Since the discovery of the integer quantum Hall effect, physicists have suspected that electron localization plays a key role in the phenomenon. In a widely used picture, the short localization length of states with energies between Landau levels results in a vanishing longitudinal conductivity. It has proven difficult to quantify such notions because there existed a little knowledge of the random potential that gives rise to such localization or the response of electrons to it. This talk presents a direct imaging of the random electrostatic potential in the quantum Hall regime, including effects of electron screening, using a scanned Quantum dot inside the two-dimensional electron gas (2DEG). A sensitive cryogenic amplifier attached to a sharp metal scanning tip allows us to detect motion of single electrons inside 2DEG as we scan the tip just above the sample surface. Electrons move in and out the 2DEG in response to a small AC excitation applied to an ohmic contact at the edge of the sample. We apply a positive voltage on the tip with respect to the sample to locally accumulate the 2D electrons underneath it. For magnetic fields such that the bulk Landau level filling factor is slightly less than an integer, we thereby create a quantum dot or "bubble" of electrons in the next higher Landau level. An incompressible strip of integer Landau level filling separates electrons in the bubble from the surrounding 2DEG, and the bubble displays a Coulomb blockade behavior. While raster scanning the tip across the sample we drag the bubble underneath. Changes in the electrostatic potential cause the electrons to leave (or enter) the dot one by one. By counting the number of electrons in the dot as a function of the tip position we map the electrostatic potential directly as sensed by the 2D electrons. We find two striking results. Firstly, that the shape of this potential remains mostly unchanged for different spin-split and integer Landau level fillings. Secondly, to within our detection limits, the self consistent potential does not appear to change at all throughout the entire range of fields of a quantum Hall plateau. The results suggest that electron screening has, at most, a minor impact on transport features in the quantum Hall regime in our samples.