Physicists rewarded for ‘fractional electrons’

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Pople: work has ‘led to an industry’. Credit: NORTHWESTERN UNIVERSITY

This year's Nobel prize for physics has been awarded to the researchers who first observed and explained the fractional quantum Hall effect. This is the effect in which an electric current within a two-dimensional conducting material appears to be made up of charge carriers bearing a fraction of the charge on an electron.

Horst Störmer of Columbia University, New York, and Bell Laboratories, and Daniel Tsui of Princeton University, who both saw the effect experimentally in 1982, share the prize with Robert Laughlin of Stanford University, California, who provided a theoretical explanation shortly afterwards.

The standard Hall effect is the lateral deflection of moving charge carriers — an electric current — in a magnetic field. It was discovered in 1879 by Edwin Hall, and today provides the basis for determining the charge and density of charge carriers in a semiconductor (electrons and holes are deflected in different directions).

In 1980 Klaus von Klitzing found that, when the charge carriers are confined within a very thin conducting film (that is, in two dimensions), the magnitude of the Hall current (or, equivalently, the conductance of the material) no longer varies smoothly with magnetic-field strength at very low temperatures.

Instead, the conductance varies with field strength in a series of abrupt steps. In other words, the conductance is quantized: it changes in integral multiples of the fundamental quantum unit of conductance, e²/h (where e is the charge on the electron and h is Planck's constant).

The fractional quantum Hall effect represents a deeper puzzle, since it seems to reveal a change in the nature of the fundamental particles. Much the same can be said of superconductivity, in which electrons appear to attract one another (or, more properly, to show bosonic instead of fermionic behaviour), and of superfluidity, in which the atoms of the superfluid no longer generate viscosity.

The FQHE was seen by Tsui and Störmer for the transport of a two-dimensional electron ‘gas’ in a semiconductor heterostructure fabricated by Art Gossard, now at the University of California at Santa Barbara. On applying magnetic fields of up to 30 tesla to a sample cooled to about a tenth of a degree kelvin, they observed jumps in conductance with a value of e²/3h, implying that the charge carriers had a fractional charge of e/3. Subsequent studies revealed charges of 2e/5, 3e/7 and other (odd-denominator) fractions.

Laughlin proposed that the magnetic flux lines penetrating the sample encourage the charge carriers to condense into quasiparticles. He demonstrated that such quasiparticles act as though they have fractional charges with the values seen in the experiments.

The crucial insight, says Moty Heiblum of the Weizmann Institute in Israel, was the recognition of the role of electron correlations. In semiconductor physics, says Heiblum, “all of us managed to work with a single-electron picture for many years”. But the study of strongly correlated electrons in solid-state physics has now become an important field of research.