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Posted: October 4, 2006

Scientists solve longstanding nanoelectronic puzzle

(Nanowerk News) Ben-Gurion University of the Negev’s theoretical physicist, Professor Yigal Meir has solved one of nanoelectronic’s longstanding puzzles, which has baffled physicists seeking to make smaller, faster computer devices for more than a decade.
Nanoelectronics refers to electronic transport through miniaturized devices. The simplest such device, and the basic building block for more complicated devices, is a “quantum point contact,” a constriction connecting large electron reservoirs.
In a paper published in Nature Magazine, the professor explains the “0.7 anomaly”, a feature in the conductance of quantum point contacts that has so far eluded explanation for almost 20 years.
According to quantum mechanics, and the wavelike nature of electrons, scientists expected the conductance through such a device to increase as the gap grew bigger by integer steps of universal value. While this was indeed found true in early experiments, surprisingly, an additional first step, approximately 0.7 times the expected universal value had also been observed, which scientists first attributed to irregularities in the device (“the 0.7 anomaly” as it became known).
While visiting Princeton University, Meir and a colleague, Ned Wingreen, theorized the existence of a magnetic impurity, a localized electron, in a quantum-point contact to explain the 0.7 anomaly. While their theoretical calculations explained its temperature and magnetic-field dependence, they still needed to identify the proposed impurity to overcome physics community skeptics about how a magnetic moment could form in such a system. “The classical analogy of a quantum point contact is a sea of electrons around a hill,” Meir explains. “The existence of a magnetic impurity on the point of contact is equivalent to the formation of a puddle of water at the top of the hill – a counterintuitive phenomenon.”
In the Nature paper published with his Ben-Gurion University postdoc, Dr. Tomaz Rejec, Meir explains, via extensive numerical calculations, that the existence of a magnetic impurity at the quantum point of contact is possible because a lower density of the electrons near the quantum point attracts the other electrons toward the point. The wavy nature of such electrons then causes the quantum point to form ripples, trapping an electron and causing the 0.7 anomaly.
“This is both good and bad news for quantum computer devices based on quantum dots which require that no outside factors affect the circuits,” Meir concludes. “Magnetic impurities at point contacts would render such computer devices inoperable. However, the magnetic impurity is formed only when conductance through the point of contact is around 0.7, so setting the conductance of each contact below that value should allow a circuit formed by quantum dots to function.”
Source: Ben Gurion University
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