Print this page 15 February 2007 2007/07

Quantum Hall-effect at room temperature

The quantum Hall-effect, discovered in 1980, and since then the unprecedented basis for an electrical resistance standard, can not only be measured close to absolute zero, but also at room temperature. This surprising finding was made by scientists from the University of Manchester (UK), Columbia University in New York (USA), the National High Magnetic Field Laboratory in Tallahassee (USA), the High Field Magnet Laboratory in Nijmegen (Netherlands) and the FOM Foundation (Netherlands). Using very high magnetic fields, the effect was measured in graphene, a recently discovered new form of carbon (consisting of a single-atomic sheet). The scientists believe that these findings open the way towards a compact resistance standard working at elevated temperatures and technologically easily attainable magnetic fields. Their article appears on February 15, 2007 in Science Express, the advanced on-line publication of Science Magazine.

The quantum Hall-effect (QHE) was discovered in 1980 by the German physicist Klaus von Klitzing, who was awarded the Nobel price in Physics in 1985. The QHE is the quantum mechanical version of an effect discovered in 1879 by Edwin Hall. He found out that when placing a thin sheet of a conducting or semiconducting material in a magnetic field a potential difference (the so-called Hall voltage) builds up along its sides. The size of the Hall voltage is a characteristic material property. When such a sheet is thinned down extremely (physicists then talk of a two-dimensional electron system) plateaus appear in the Hall voltage. The Hall resistance is then exactly 25,812.807 Ohms, given by Planck’s constant divided by the square of an electron’s charge. This value can be measured with a precision of one in a billion which led to a new standard for the electrical resistance (called von Klitzing’s constant since 1990).

Until present the QHE could only be measured at temperatures a few degrees above absolute zero; all attempts to extend it to considerably higher temperatures were condemned to failure so far. The new material graphene now seems to bring a change. Graphene is in fact nothing more that a tiny pencil trace. However, this material exhibits some unique properties, including the conservation of the quantum Hall effect at high temperatures. With this observation a metrologically relevant quantum effect was seen in a solid at room temperature for the first time.

In Science Express the scientists describe how their observations were made possible by using the strongest magnetic fields available: a 33-Tesla magnet at the High Field Magnet Laboratory (HFML) in Nijmegen and the 45-Tesla hybrid magnet at the National High Magnetic Field Laboratory in Tallahassee. The experiments in Nijmegen were performed by Kostya Novoselov (from Andre Geim’s group in Manchester) and Uli Zeitler from the HFML. In Tallahassee the Columbia group of Philip Kim en Horst Stormer (Nobel laureate in 1998) conducted comparable experiments. The result of both groups now appears as a common publication in Science Express.

Uli Zeiter, senior scientist at the HFML says: "This is a really astonishing development for a quantum-Hall physicist. In the past years our research was mainly directed towards very low temperatures and more and more sophisticated materials and now we can just see such an elementary quantum effect in a pencil trace at room temperature."
And an enthusiastic Jan Kees Maan, director of the HFML adds: "Large magnetlabs are extremely expensive and it is always a pleasure when expectations for scientific breakthroughs become true."

More information: Dr. Uli Zeitler and Prof.dr.ir. Jan Kees Maan.