National Physical Laboratory

Quantum Hall Effect Research

Since 1990, the quantum Hall effect (QHE) has been used as a primary resistance standard at NPL. The QHE takes place in special layered semiconductor structures with a 2-dimensional sheet of electrons trapped between two of the layers. Usually, Gallium Arsenide (GaAs) is used as the semiconductor. The GaAs devices must be cooled to temperatures of 0.3 K using a helium-3 refrigerator to observe the QHE.

Quantum Hall effect samples
Two QHE devices: The one on the left is an InSb device
fabricated at QinetiQ labs; the one on the right is the
GaAs device used for routine resistance calibration.
The GaAs device was fabricated at PTB, Germany.

We are investigating QHE devices made from alternative materials, which do not need cooling to such low temperatures. In collaboration with Qinetiq at Malvern, UK, we have studied QHE devices made from the semiconductor Indium Antimonide. The electrons in Indium Antimonide have a lower effective mass than those in GaAs - they behave as if they were lighter. This means that the energy levels that give rise to the QHE are spaced more widely, and are not disturbed as much by thermal fluctuations. We recently performed successful primary resistance calibrations using an InSb QHE device cooled to 4.2 K - the temperature of ordinary liquid helium. This is still very cold compared to room temperature, but much easier to achieve than 0.3 K.

A new material which shows promise in many applications is Graphene, a single layer of carbon atoms. The electrons in Graphene behave as if they have zero effective mass, and the QHE has already been observed in Graphene at room temperature[1]. In collaboration with the semiconductor physics group at the University of Cambridge, we are investigating the metrological possibilities of Graphene QHE devices. One important question we are hoping to answer is whether the devices can be made to carry sufficiently high currents (10 μA) to perform a resistance calibration.

1  K. S. Novoselov et al, Science 315, 1379 (2007)

Last Updated: 12 Dec 2012
Created: 8 Jun 2007


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