Physics Nobel spotlights quantum effect.

The discovery in 1982 that a cold cloud of electrons in a powerful magnetic field can display unusual behavior came as a great surprise to researchers. At temperatures near absolute zero, the electrons apparently condense to form a quantum fluid made up of ensembles of electrons, each seeming to have a fractional electric charge (SN: 6/25/83, p. 405; 8/25/84, p. 116).

This week, the Royal Swedish Academy of Sciences in Stockholm awarded the 1998 Nobel Prize in Physics to Daniel C. Tsui of Princeton University and Horst L. Stormer, now at Columbia University, for discovering this so-called fractional quantum Hall effect, and to Robert B. Laughlin of Stanford University for explaining the experimental findings.

The Hall quantum fluid is related to the quantum fluids that occur in superconductivity and in liquid-helium superfluidity (SN: 10/19/96, p. 247). "Events in a drop of quantum fluid can afford.., profound insights into the general inner structure and dynamics of matter," the academy says.

The previously known quantum Hall effect occurs when electrons are trapped at the interface between two semiconductor crystals. That confinement restricts electron motion to two dimensions. A strong magnetic field at right angles to the interface causes electrons to drift sideways compared with the direction of electric current flow. At very low temperatures, gradually increasing the magnetic field causes the electric resistance in the direction of the drifting electrons to increase in steps rather than continuously. Those steps indicate that the resistance is quantized.

In their detailed probes of the quantum Hall effect at very low temperatures, Tsui and Stomer found, to their surprise, additional steps. They therefore named the new phenomenon the fractional quantum Hall effect.

About a year later, Laughlin proposed that the confined electrons, interacting with the magnetic field, coordinate their movements in specific ways. Such collective behavior can be interpreted as belonging to quasiparticles--entities that behave as if they were particles--each having a precise fraction of the electric charge of an electron.

Recent measurements by other researchers have turned up additional fractionally charged steps. So far, Laughlin's model has explained all the findings.

Last year, two groups working independently detected the noise--tiny fluctuations in electric current--produced by quasiparticles making up the fractional quantum fluid. Those experiments showed directly that the current was carried by objects with one-third the charge of an electron.

The physics of systems exhibiting the fractional quantum Hall effect remains an active field both experimentally and theoretically. In one offshoot, researchers are using nuclear magnetic resonance techniques to probe electrons trapped in semiconducting layers. Those studies enable them to determine the spins of electrons. Such data are useful to researchers developing quantum computers (SN: 1/18/97, p. 37).