REHOVOT, Israel - September 11, 1997 - Researchers at the Weizmann Institute of Science have provided the first unambiguous evidence that electrons can behave in an intriguing way that seems to defy common sense.
An electron is a tiny particle that carries the smallest negative charge in nature. Yet a daring theory of physics developed 15 years ago argues that under certain conditions, an electric current behaves as if it were made up of fractions of electronic charges.
In an experiment described in the September 11 issue of Nature, Weizmann Institute physicists measured fractional charges one-third that of an electron.
"Mind-boggling as this may seem, this phenomenon is real," says study author Rafael de-Picciotto. "Of course, electrons don't split into fragments in an electric current, but under certain conditions it is indeed possible to measure a charge smaller than that of an electron."
The research team that conducted this experiment included de-Picciotto, Dr. Michael Rejmikov, Prof. Mordehai Heiblum, Dr. Vladimir Umansky, Gregori Bunin and Dr. Diana Mahalu.
Intuition vs. Reality
Ever since American physicist Robert Millikan first measured the charge of an electron 80 years ago, this value has been widely regarded as a basic unit of electric charge. Scientists have consequently come to view electrons that make up an electric current as a flow of negatively charged, indivisible "balls." A current made up of fractions of an electronic charge, therefore, would seem a counter-intuitive idea, just as it would be absurd to describe a crowd made up of "less-than-whole" people or street traffic made up of "less-than-whole" cars.
However, if electrons are always regarded as "whole," it is extremely difficult to understand and describe their behavior under certain conditions. For example, some particular instances of this behavior, as in a phenomenon known as the fractional quantum Hall effect, observed in a strong magnetic field, remain unexplained.
In 1982, physicist Robert Laughlin of the United States proposed a theory that explained this effect and provided a very simple way of describing highly complex interactions between electrons. However, this explanation came at a "cost": the theory made the bizarre assumption that an electric current can be made up of odd-denominator fractions of electronic charges - one-third, one-fifth, one-seventh, etc. - of an electron.
In the new experiment, Weizmann Institute scientists designed a sophisticated system to measure such fractional electric charges, should they exist.The system makes it possible to measure so-called "shot noise." In day-to-day environment, this noise results from random variations in the number and velocity of electrons and causes popping sounds in radio receivers and snow effects in television pictures. Under special laboratory conditions, "shot noise" can be analyzed to reveal the make-up of the electric current. This is possible because the noise has "ripples" left by the flow of electrons in a conductor. The size of each "ripple" is proportional to the unit of electric charge: the smaller the ripple, the smaller the charge, and vice versa.
The scientists passed an electric current through a semiconductor immersed in a high magnetic field, under conditions in which the fractional quantum Hall phenomenon is observed. They used sophisticated equipment to eliminate all extraneous sources of noise. The "shot noise" made by the current was then amplified and measured. It turned out to be made of charges one-third that of an electron.
"This is a beautiful manifestation of the strength of the theoretical methods used to predict such a counter-intuitive phenomenon," says Prof. Heiblum.
The scientists' next challenge is to create conditions for the emergence of even smaller charges, one-fifth of an electron, and to measure these charges. This will require even greater refinement of the system because these tiny charges make smaller ripples that are consequently more difficult to measure.
This work was partly supported by the Israel Science Foundation and Austria's Ministry of Science, Research and Art.
The scientists are members of the Weizmann Institute's Condensed Matter Physics Department. They conducted the research at the Institute's Joseph H. and Belle Braun Center for Submicron Research.
The Weizmann Institute of Science is a major center of scientific research and graduate study located in Rehovot, Israel.