ESPCI web site
Jeudi 20 Mars 2014, 14h
Amphi Holweck, Esc C, 1ème etage
Imaging of integer and fractional quantum Hall edge states
Klaus Ensslin
ETH Zurich
Electron flow through quantum point contacts (QPC) in semiconductor nanostructures is governed by ballistic transport resulting in conductance steps in units of 2e2/h. By using a voltage-biased metallic tip which is scanned at a constant height across a QPC, Topinka et al. have observed patterns which demonstrate enhanced electron backscattering along certain branch-like regions emanating from the QPC.
We have used a similar setup. The conducting tip of a cryogenic scanning probe microscope is used to locally perturb electron transport in a QPC. The conductance is monitored as a function of tip position. For a single QPC at zero magnetic field we observe a fringe pattern in the conductance when the tip is close to the QPC. This pattern arises from local depletion of regions close to the QPC and can be explained by a network of constrictions forming between the tip-depleted region and potential profile created by the gate configuration. A detailed analysis reveals that phase coherence of the electrons is relevant. The evolution of the fringe pattern as a function of magnetic field can be explained by the magnetic depopulation of magneto-electric subbands in the constriction network.
In the quantum Hall regime edge states form which can be selectively backscattered at the QPC as well as at the constrictions forming between tip-induced potential and gate-depleted areas. This way a sequence of even-integer states can be imaged as a function of tip position. Compressible and incompressible regions of edge states can be distinguished in the experiment. For higher magnetic fields spin-split edge states can be imaged and even fractional states show up. While the overall pattern of the (integer and fractional) edge states follows the symmetry of the overall potential landscape, the width of the edge states as well as the appearance of the conductance plateaus depends strongly on tip position and therefore on local potential fluctuations.
This work was done in collaboration with N. Pascher, A. Kozikov, D. Weinmann, C. Rössler, T. Ihn, C. Reichl and W. Wegscheider.