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Electrons' Charge Divided by Five
19 May 1999 7:00 pm
Every science student is taught that the indivisible unit of charge is that of the electron. But 2 years ago, scientists found that charge sometimes shatters into "quasi-particles" that have one-third the fundamental charge. And in this week's issue of Nature, researchers announce they have spotted one-fifth-charge quasi-particles--a decisive finding suggesting that its time to change any physics textbooks still claiming that electron charge is indivisible.
The Hall effect, known since 1879, describes how application of a magnetic field perpendicular to a current-carrying wire creates a voltage across the wire, because the field causes all the electrons to bunch up on one side. In the 1980s, physicists discovered the fractional quantum Hall effect: When electrons are trapped in a thin layer between two semiconductors at low temperature and in a high magnetic field, the Hall voltage across the conductor increases in discrete steps rather than continuously. The size of the steps reflects the discrete charge of the electron or multiples of it. But to their surprise, physicists also discovered steps that could only be explained by multiples of fractions of that charge: 1/3, 2/3, 2/5, and 3/7.
To explain this, in 1983 Stanford physicist Robert Laughlin proposed that quasi-particles form in the electron layer when the electrons team up with magnetic vortices, tiny whirlpools of magnetism. In very simple terms, the vortices bound to an electron repel other nearby electrons and in effect "shield" part of its charge. Quasi-particles made up of an electron bound to two vortices would give rise to fractional charges of one-third; and at slightly lower magnetic fields, even smaller fractional charges could appear.
In September 1997, teams from Israel and France announced that they had found direct proof of one-third fractional charges in a quantum Hall setup (Science, 19 September 1997, p. 1766). Now the Israeli team, led by Mordehai Heiblum at the Weizmann Institute of Science in Rehovot, has refined its technique and spotted the more elusive one-fifth-charge quasi-particles. To sieve these quasi-particles from the electron layer, they directed the charges toward a very narrow channel which allowed the one-third-charge quasi-particles through, while reflecting a small number of one-fifth-charge particles back. The researchers then detected the reflected quasi-particles and their charges by the impulses they create in a very low noise amplifier.
Although Laughlin's theory is widely accepted and he shared the 1998 Nobel Prize in physics for it, some physicists were not entirely comfortable until a fractional charge was positively identified. Says Laughlin, "I'm very happy" about the new results, "but I am not surprised."