Uni Hamburg Fachbereich Physik
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Recent Results

Scannig Tunneling Spectroscopy of a 2DES in magnetic field

Two dimensional electron systems (2DES) in magnetic field exhibit the quantum Hall effect. This effect means that the conductivity of the 2DES vanishes except at half filling. This intriguing property has been explained by the formation of drift states in each Landau level. In simple terms, these drift states are caused by electrons running in skipping orbits along equipotential lines of the 2DES. For topological reasons all these drift states except the one in the center of the Landau level form a closed path and thus the corresponding electron states are localized. The single extended state in the center is responsible for the conductivity at half filling.

Fig. 1: Spatially averaged dI/dV-curve of the 2DES at different magnetic fields; the peaks at 5 T and 6 T are Landau levels of the 2DES. Fig. 3: Histogram of the width of the drift states observed in Fig. 2; the magnetic length at B=6 T is l=10.5 nm in good correspondance with the average width of 10.6 nm.
Fig. 2: dI/dV-Images of the 2DES at B=6 T at different voltages all corresponding to the same Landau level.

With STS we were able to image the drift states. Landau quantization of the adsorbate induced 2DES is shown in Fig. 1. Fig. 2 shows the corresponding LDOS, where stripe like structures running irregularly through the area are visible. The width distribution of the stripes is shown in Fig. 3 and its average is exactly the magnetic length as expected from the radius of the skipping orbit. Drift states adjacent in energy are shown in Fig. 4 demonstrating how the next state in energy is found more uphill in the potential landscape (see here). The detection of drift states offers the unique possibility to study multifractal properties of a quantum critical state, which is the extended one. Clear predictions exist from theory. However, we have to apply lower temperatures to isolate the extended state from localized ones.

Fig. 4: A single drift state observed at different voltage.

Related Papers

M. Morgenstern, J. Klijn, Chr. Meyer, and R. Wiesendanger, Phys. Rev. Lett., in press:
Real-space Observation of Drift States in a Two-Dimensional Electron System at High Magnetic Fields.
M. Morgenstern, D. Haude, J. Klijn, L. Sacharow, S. Heinze, S. Blügel, and R. Wiesendanger, Physica E, in press:
Comparing the Local Density of States of Three- and Two-Dimensional Electron Systems by Low-Temperature Scanning Tunneling Spectroscopy.