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Date: April 24, 1998
Daniel Tsui receives Benjamin Franklin Medal in Physics
Princeton, N.J. -- Daniel Tsui, the Arthur LeGrand Doty Professor of Electrical Engineering in the School of Engineering and Applied Science at Princeton University, has been selected to receive the Benjamin Franklin Medal in Physics for his role in discovering and explaining the bizarre liquid-like behavior of electrons at extremely cold temperatures and high magnetic fields.
Tsui, along with Horst L. Stormer at Bell Laboratories in New Jersey, discovered the phenomenon in 1982, dubbing it the fractional quantum Hall effect (FQHE). Robert B. Laughlin, then at Lawrence Livermore National Laboratory but now a professor at Stanford University, developed the theoretical explanation for FQHE, and reduced the phenomenon to an astoundingly simple equation.
Stormer and Tsui made the discovery while searching for a way to produce and study electrons that have crystallized into a solid. To do so they created a unique environment, a trap, in which to restrain electrons in a two-dimensional plane. This was done by sandwiching two dissimilar semiconductor wafers -- gallium arsenide (GaAs) on one side and gallium aluminum arsenide (GaAlAs) on the other. Because electrons accumulate at the interface between the two semiconductors, they are tightly confined there in two-dimensions.
The next step was to cool down the electron trap to just a tenth of a degree above absolute zero, in order to stop the atoms and electrons from vibrating. Then they applied an enormous magnetic field -- up to 30 Teslas, which is about a million times more powerful than the earthís magnetic field.
When Stormer and Tsui measured a particular kind of resistance, called Hall resistance, they observed bizarre behavior: At certain magnetic field strengths, the resistance fell to zero. This meant that the electrons were behaving as if they had fractional charges. Yet electrons cannot have fractional charges.
Enter the theorist, Robert Laughlin, who set to work to develop and explanation. He determined that the magnetic field had created what are essentially holes in the 2-D sheet of electrons. These holes were called vortices, which can be likened to whirlpools in a lake. There is no water in the middle of a whirlpool -- that is, there is an absence of water, and the vortices represent an analogous absence of charge.
Now throw billions of electrons into this mix. Because the electrons carry negative electrical charges, they repel each other and seek the safety of these vortices. Not all the vortices become filled, however. The electrons then grab any remaining unfilled vortices as a buffer against nearby electrons. In so doing, they form a peculiar state of matter -- a quantum liquid with unusual properties, such as fractional quantum numbers and practically resistanceless flow.
As it turns out, the charge deficit of a vortex is a fraction of the charges of an electron. Some of the vortices break loose and wander through the plane of electrons, which accounts for Stormer and Tsuiís discovery of fractionally charged particles.
Since this ground-breaking discovery, numerous other perplexing properties of two-dimensional electrons have been discovered. Among them are the composite fermion, an electron with bits of magnetic field attached to it. This mantel of magnetism shields the electrons, allowing them to travel about, oblivious to the existence of high magnetic fields.
While these strange particles have no current practical purpose, per se, the theory that explains them is now being used by Laughlin as a stepping stone to describe a far more practical phenomenon -- high-temperature superconductivity, for instance.
Tsui earned his Ph.D. in physics in 1967 from the University of Chicago. He worked in the Solid State Electronics Research Laboratory at Bell Laboratories, Murray Hill, N.J., before joining Princeton University in 1982. His current research involves the quantum physics of electronic materials in strong magnetic fields and low temperatures, which includes the integer and fractional quantum Hall effects and the physics and fabrication of novel reduced-dimensional semiconductor heterostructures, and the fabrication of quantum well infrared photodetectors.
He is a member of the National Academy of Science, IEEE, American Association for the Advancement of Science, and the American Physical Society and Materials Research Society. He is a long-time resident of Princeton.
The Benjamin Franklin Medal, supported by The Franklin Institute Medals Endowment Fund, honors pioneering work that not only explains a particular phenomenon, but also opens a new realm of scientific inquiry. The Franklin Institute was founded in 1824 to promote scientific inquiry and recognize scientific achievement, and the following year it initiated an awards program that continues today in the same spirit.
The Instituteís Committee on Science and the Arts now annually honors a number of individuals who have contributed outstanding accomplishment in the realms of science, technology, and business. Through the careful consideration of this committee of scientists and engineers, The Franklin Institute has been among the first to recognize the outstanding contributions of such individuals as Marie Curie, the Wright brothers, and Albert Einstein.