Current at Just a Fraction of the Charge

Electric charges normally come as multiples of an indivisible unit: the charge of an electron. But two groups of physicists have demonstrated an exception--under special conditions, electric charges can be just a fraction of an electron's. The results, published by an Israeli team in tomorrow's issue of Nature and by a French team in the 29 September Physical Review Letters, are the first direct measurements of fractional charges, which were predicted 14 years ago as part of a theory to explain the fractional quantum Hall (FQH) effect.

Tinkering with wires and magnets, Edwin H. Hall in 1879 first described a curious phenomenon, now called the Hall effect: If he applied a magnetic field perpendicular to a current-carrying wire, electrons would migrate to one edge of the wire and create a voltage across the wire's width. In the 1980s, physicists found that under extreme conditions--when electrons were restricted to an ultrathin layer of a solid at very low temperatures and high magnetic fields--increasing the magnetic field caused the Hall voltage to increase in discrete steps, rather than continuously, as it does under normal conditions. The effect was known as the integer quantum Hall effect or the fractional quantum Hall effect, depending on the values of the steps.

In 1983 Robert Laughlin, now at Stanford University, proposed an explanation for the FQH effect, and although the theory was widely accepted, it included a strange concept: fractional charges. To look for these fractional charges, the two research teams--one led by physicist Rafi de-Picciotto and his colleagues from the Weizmann Institute of Science in Rehovot, Israel, and the other by D. Christian Glattli and his colleagues at the Commission of Atomic Energy in Saclay, France--measured the fluctuations in the current through a quantum Hall setup, chilled to within a tenth of a degree of absolute zero.

The method is like finding the size of hailstones by listening to them hit a tin roof: "Small hail stones would generate a pitter-patter almost indistinguishable from continuous noise, whereas larger stones would result in less frequent yet louder crashes," say physicists Charles Kane, of the University of Pennsylvania, and Matthew Fisher of the University of California, Santa Barbara, in a "News and Views" article accompanying the Nature paper. By reducing the current until the individual "hailstones" could be detected, the researchers found that they were caused by units with a charge just one-third that of a normal electron.