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A Glimpse of Forbidden Charges Leads to Physics Nobel
13 October 1998 7:00 pm
No one has figured out how to chop up an electron--or the apparently indivisible charge it carries. But in the early 1980s, three researchers did manage to make the crowds of electrons in a semiconducting solid dance together as if they carried those forbidden fractional charges. For that discovery, called the fractional quantum Hall effect, the three researchers--Daniel Tsui of Princeton University, Horst Stšrmer of Columbia University and Bell Labs in New Jersey, and Robert Laughlin of Stanford University--were today awarded the 1998 Nobel Prize in Physics.
Scientists have known about the ordinary Hall effect since the 1800s: It occurs when a magnetic field is applied at right angles to a current-carrying wire. Because the electrons try to orbit around the field lines, current is forced across the width of the wire, causing charges to build up on one side and producing a voltage drop across the width of the wire. In most cases the voltage drop is linearly proportional to the magnetic field for a given current.
In 1980, the German physicist Klaus von Klitzing pushed the Hall effect further, discovering a quantum Hall effect in a "gas" of highly mobile electrons trapped between two layers of semiconductor in a strong magnetic field. Von Klitzing found that as he increased the strength of the magnetic field, the voltage drop across the layer increased in steps rather than steadily. The steps reflect the electrons' quantum mechanical nature: The electron orbits around the field lines are limited to specific sizes, just as the electrons in an atom are limited to particular energies. As the field strength increased, the orbit sizes--and thus the inclination of the electrons to drift--stayed constant for a time, then changed suddenly as a new permissible orbit size opened up. Von Klitzing won a Nobel Prize in 1985 for the discovery.
At even greater electron mobilities and stronger magnetic fields, Tsui and Stšrmer discovered in 1982, the resistance steps developed subdivisions--the fractional quantum Hall effect. Shortly afterward, Laughlin realized why: The fixed sizes of the orbits give rise to a kind of incompressible, quantum fluid of electrons and magnetic fields. Like the musical vibrations of a xylophone, excitations of that fluid could produce waves--and the collective dance of those waves could behave as something different from either the particles or fields alone: "quasiparticles" that appear to have fractional charge.
"It's really deep, and it's really quantum mechanical," says Steven Kivelson, a physicist at the University of California, Los Angeles. And since its discovery and explanation, says Kivelson, studies of the fractional effect have "led to surprise after surprise after surprise."