NEGATIVE ELECTRICAL RESISTANCE
Date: Friday, June 09, 2006 @ 22:05:30 UTC
Topic: Science
A HINT OF NEGATIVE ELECTRICAL RESISTANCE emerges from a new experiment in which microwaves of two different frequencies are directed at a 2-dimensional electron gas.
The electrons, moving at the interface between two semiconductor crystals, are subjected to an electric field in the forward (longitudinal) direction and a faint magnetic field in the direction perpendicular to the plane.
In such conditions the electrons execute closed-loop trajectories which will, in addition, drift forward depending on the strength of the applied voltage.
A few years ago, two experimental groups observed that when, furthermore, the electrons were exposed to microwaves, the overall longitudinal resistance could vary widely---for example, by increasing by an order of magnitude or extending down to zero, forming a zero-resistance state, depending on the relation between microwave frequency and the strength of the applied magnetic field (for background, see Physics Today, April 2003).
Some theorists proposed that in such zero-resistance state, the resistance would actually have been less than zero: the swirling electrons would have drifted backwards against the applied voltage. However, this rearwards motion would be difficult to observe because of an instability in the current flow---that is, the current distribution becomes inhomogeneous so as to yield a vanishing voltage drop.
A Utah/Minnesota/Rice/Bell Labs group has by now tested this hypothesis in a clever bichromatic experiment using microwaves at the two frequencies.
Michael Zudov (now at the University of Minnesota, zudov@physics.umn.edu, 612-626-0364) and Rui-Rui Du (now at Rice University) sent microwaves of two different frequencies at the electrons, observing that for nonzero-resistance states the resultant resistance was the average of the values corresponding to the two frequencies separately. On the other hand, when the measurements included frequencies that had yielded a zero resistance, the researchers observed a dramatic reduction of the signal. Judging from the average resistance observed for non-zero measurements, they deduce that whenever zero resistance was detected, the true microscopic resistance had actually been less than zero. In other words, an observed zero resistance was masking what was in fact an unstable negative- resistance state. (Zudov et al., Physical Review Letters, 16 June 2006)
PHYSICS NEWS UPDATE
The American Institute of Physics Bulletin of Physics News
Number 780 June 9, 2006
by Phillip F. Schewe, Ben Stein, and Davide Castelvecchi
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