Room-temperature quantum Hall effect
An international team of scientists from the University of Manchester (UK), Columbia University (USA), the High Field Magnet Laboratory in Nijmegen (The Netherlands) and the National High Magnetic Field Laboratory in Tallahassee (USA) have made the surprising discovery that the quantum Hall effect, hitherto believed only to exist at very low temperatures, can be measured at room temperature. In a paper published in Science Express (February 15, 2007), the team describes how this observation was made possible by using the highest available magnetic fields in Nijmegen (33 Tesla) and Tallahassee (45 Tesla).
The quantum Hall effect, discovered by K. v. Klitzing in 1980, is one of the most important quantum effects occurring in a solid state system. Besides its importance for the fundamental understanding of quantum physics the extremely accurate quantization of the Hall resistance in units of h/e2 (25 812.807 Ohms) had a tremendous impact on quantum metrology by providing a resistance standard with an unprecedented accuracy. Unfortunately, until present it was only possible to measure the quantum Hall effect at very low temperature (a few degrees above absolute zero) and all attempts to extend the quantum Hall effect to considerably higher temperatures were condemned to failure so far.
Recently a new member joined the family of two-dimensional electron systems, graphene, a single layer of graphite arranged in a honeycomb lattice. Electrons in graphene behave relativistically as chiral Dirac fermions, a behaviour which can most effectively be studied by means of high-field quantum-Hall experiments. Due to the peculiar bandstructure in graphene it now became possible to extend the observably of the quantum Hall effect up to ambient temperatures. The scientists are confident that future research may eventually lead to a table-top resistance standard working at elevated temperatures and technologically straightforwardly accessible magnetic fields of a few Tesla.
The quantum Hall physics discovered so far in graphene probably only forms the tip of the iceberg; a lot of phenomena remain to be unravelled. Without any doubt, high magnetic fields will continue to play a decisive role.
Figure: a) Field effect transistor made from graphene on a SIMOX substrate. b) Artistic impression of the honeycomb lattice of carbon atoms in graphene. c) Room temperature quantum Hall effect: The Hall conductivity is quantized at half-inter values of (4e2/h).
This work was published in:
K. S. Novoselov, Z. Jiang, Y. Zhang, S. V. Morozov, H. L. Stormer, U. Zeitler, J. C. Maan, G. S. Boebinger, P. Kim, A. K. Geim,
Room-Temperature Quantum Hall Effect in Graphene,
Science Express, 15 February 2007
For more news, see also
http://www.magnet.fsu.edu/mediacenter/news/pressreleases/2007february15.htmland http://www.fom.nl/live/english/news/artikel.pag?objectnumber=56329(in Dutch).