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Electric currents transported quantally.

Electric currents transported quantally

The discovery of quantum effects at the beginning of this century divided the word of physics into two quite different domains, the microscopic and the macroscopic. In the microscopic domain--that of atoms--changes in physical quantities occur in a jumpy fashion: They go by the quantum, the least change possible in a given situation. The macroscopic domain is the familiar world of large objects, where changes can be, or can seem to be, smooth and continuous. Until a couple of years ago, objects that behaved quantally were all given by nature; things built by human belonged to the macroscopic order. Now, working in the borderland between the two domains, physicists have learned to construct objects that behave in a quantum fashion.

These somewhat macroscopic quantum objects are tiny metal wires and rings and tiny metal-oxide semiconductors (MOSFETs). William J. Skocpol of AT&T Bell Laboratories at Holmdel, N.J., calls the MOSFETs "nanolaboratores for quantum transport effects," implying they are a billionth the size of ordinary laboratores. The rings are typically 8,500 angstroms across (the length of one wave of red light) and made of wire 500 angstroms thick. The quantum transport effects involve electrons that make electric currents in these devices, and those currents behave quite strangely compared with currents in ordinary electric wires. The effects were described last week at the American Physical Society meeting in Las Vegas, Nev.

In these very small pieces of metal, quantum mechanical wave effects become important. Every object in the quantum mechanical domain is both a wave and a particle, and the strange effects come from the behavior of the waves associated with electrons. The source of resistance of concern in these experiments is impurities, foreign atoms. When electrons hit impurities they are knocked out of their paths and they scatter. An electrons hit impurities they are knocked out of their paths and they scatter. An electron encountering a lot of impurities executes a random walk. It may even backscatter, as Skocpol points out, and return to its starting point. If one regards electrons as waves, the scattered waves may be split, and when they recombine, if they are still in phase with each other, they will reinforce each other, but if they are out of phase they will cancel each other. This wave interference effect alters the electrical conductanceof the metal.

In the macroscopic domain, the effects of individual impurities should average out. Physicists thought they had understood conductance: It should depend on the metal, its temperature and the relative proportion of impurities in it. It should not depend on the locations of individual impurities. According to A. Douglas Stone of the State University of New York at Stony Brook, "[T]his plausible and long-standing belief is clearly mistaken." In these small samples the conductance depends strongly on the locations of individual impurities. "Each specific sample has a conductance that goes with that impurity configuration," Skocpol says. Magnetic fields can tune the phases of the electron waves. Using varying magnetic fields, the experimenters study the effects of the impurities. Each sample yields its own "magnetoprint," a unique relation of conductance to changes in magnetic field. Sometimes changing just one impurity can have a greater effect than changing the whole sample.

These are wave-coherence effects, occurring because the electron waves "remember" their phases, rather than losing them in the averaging process as they were expected to. The ring experiments sought a particular coherence effect, the Aharonov-Bohm effect. Experts had said that averaging the effects of impurities would destroy the coherence necessary for this effect also.

In experiments looking for the aharonov-Bohn effect (SN: 3/1/86,p. 135), the ring encloses a magnetic field. The current is split to go around the ring both clockwise and counterclockwise. In this geometry the conductance of the ring should oscillate with quantized changes of the magnetic field. As Sean Washburn of the IBM Thomas J. Watson Research Center in Yorktown Heights, N.Y. describes it, they kept finding an oscillation that was half what they were looking for. Stone repeatedly advised them to make the rings larger so that the magnetic field would not actually penetrate the metal. When they finally did it, they found the Aharonov-Bohm oscillations. The rings, it seems, will produce both kinds, depending on the geometry and the field configuration. In one case, it seems, electrons go around the ring once; in the other case they backscatter and go around twice. At Yale University ring experiments with indium and silver have been done by Daniel Prober and collaborators. They see the non-Aharonov-Bohm effect in all their rings but the Aharonov-Bohm only in silver.

Summing up the significance, Prober calls these experiments " a first bridge between the quantum mechanical world of atoms and molecules and the large-scaled world in which we live. ..." He foresees new classes of devices that "will take their operating principles not from the behavior of semiconductors, but rather from the quantum mechanical world of atoms and molecules."
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Author:Thomsen, Dietrick E.
Publication:Science News
Date:Apr 12, 1986
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