Research Highlight

Subject Category: Electronic, magnetic and superconducting materials

NPG Asia Materials research highlight; doi:10.1038/asiamat.2009.170
Published online 8 April 2009

Quantum Hall effect: Defective story

Scanning tunneling microscopy provides details on the electronic states of an electron gas in the quantum Hall regime.

The quantum Hall effect (QHE) is one of the most spectacular phenomena in condensed mater physics. The quantization of conductivity in a so-called two-dimensional electron gas (2DEG) —a layer of electrons free to move only within a plane—has been extensively studied. However, there are still unclear aspects regarding the microscopic origin of the effect.

When a magnetic field is applied perpendicularly to a 2DEG plane, the transverse, the so-called Hall conductivity shows a series of plateaus, while the longitudinal conductivity shows a series of oscillations with the same periodicity as the plateaus in the Hall conductivity. It has been long believed that at the Hall plateaus the electrons are localized around defects, while in between the plateaus the electrons are delocalized over the whole plane. But only recently did scanning tunneling microscopy experiments allow verifying this assumption.

Hiroshi Fukuyama, Yasuhiro Niimi and colleagues1 at the University of Tokyo have gone a step further. They used scanning tunneling microscopy to study the localization of electrons around defects—purposely introduced by argon ion sputtering—in a 2DEG formed at the surface of highly oriented pyrolytic graphite.

The researchers unexpectedly found two types of defects. In one, the electronic states were distributed in the form of a ring around the defects (Fig. 1).

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Fig. 1: The scanning tunneling spectrograph on the right reveals two types of localized electronic states: ring-shaped, indicated by the blue arrows and corresponding to a parabolic potential (top left) and peaked, indicated by the red arrows and corresponding to a hyperbolic potential (bottom left).

In the second type, the electrons were located mainly on top of the defects, and started delocalizing into a ring for higher energies. Calculations showed these observations to be consistent with a parabolic potential (first case) introduced by the defects and with a hyperbolic potential (second case).

For the researchers, the results revealed unambiguously that the QHE is a single electron problem in a complicated potential landscape with defects. “This is probably the first direct and real-space experimental verification of electron localization and extension in the QHE which has been widely accepted by researchers but only theoretically,” says Niimi.

Team leader Hiroshi Fukuyama says, “The same technique should be applicable to the more exotic QHE in graphene, a monolayer of graphite, or to the fractional QHE that appears in much higher magnetic fields associated with many-body effects.”

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References

  1. Niimi, Y., Kambara, H. & Fukuyama, H. Localized Distributions of Quasi-Two-Dimensional Electronic States near Defects Artificially Created at Graphite Surfaces in Magnetic Fields. Phys. Rev. Lett. 102, 026803 (2009). | Article | PubMed | ChemPort |

This research highlight has been approved by the author of the original article and all empirical data contained within has been provided by said author.

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