Research Highlight

Subject Category: Electronic, magnetic and superconducting materials

NPG Asia Materials research highlight; doi:10.1038/asiamat.2010.146
Published online 6 September 2010

Condensed-matter physics: Electrons in a new light

An optical experiment leads to novel insights into the high-frequency properties of the quantum Hall effect.

In a semiconductor, applying a magnetic field perpendicular to the direction of electron flow through the material causes the electrons to be deflected slightly. This produces an electrical potential in the perpendicular direction — a phenomenon known as the ‘Hall effect’.

In a thin layer of electrons at low temperature, the transverse voltage associated with the Hall effect increases in a stepwise manner as the applied magnetic field is turned up. This quantized Hall effect is called the quantum Hall effect (QHE), and the magnitude of the jumps at each step can be described in terms of fundamental natural constants related to the two-dimensional nature of the thin layers.

Investigations of the QHE have led to profound insights into the physics of electrons in two-dimensional systems. However, it has been debated as to what happens to the QHE in oscillating electric fields, where dynamic effects could alter the electronic states. Unfortunately, measuring the QHE at high frequencies in the terahertz regime is impossible using electrical devices.

Researchers from the University of Tokyo in Japan1 have now shown, using an all-optical technique, that the QHE is valid not only for continuous electrical currents but also for high-frequency alternating currents. “Our results demonstrate the robustness of the QHE with respect to perturbations that could arise at high frequencies,” says lead researcher Ryo Shimano.

The experiment is based on the interaction of light with a QHE medium (Fig. 1).

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Fig. 1: Schematic diagram showing the principle of all-optical quantum Hall effect measurement.

Light is an electromagnetic wave with orthogonal electrical and magnetic components. When a light wave travels through a sample, it induces oscillations in the moving electrons, just like the oscillations produced by applying an alternating current. Applying an additional magnetic field in the direction perpendicular to electron flow leads to electron oscillations in the transverse direction, or in other words, the Hall effect. These oscillations alter the electric field of the light wave, causing it to rotate slightly. Measurements of the rotation angle using a polarizer clearly showed the step-like increments in rotation expected from the QHE.

The optical experiments reveal the QHE phenomenon with precision and confirm that the effect is valid even at terahertz frequencies. “This is the first time QHE steps have been observed in optical experiments,” comments Shimano. “Our new optical technique could be applied to the study of other types of Hall effects and may bring a deeper understanding into the mechanisms of such exotic phenomena.”

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References

  1. Ikebe, Y., Morimoto, T., Masutomi, R., Okamoto, T., Aoki, H. & Shimano, R. Optical Hall effect in the integer quantum hall regime. Phys. Rev. Lett. 104, 256802 (2010). | Article | PubMed |

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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