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Quantum physics

Electrons jostling in a traffic jam

Munich, 08/30/2013

Quantum point contacts in electrical circuits are narrow constrictions that can impede the passage of electrons in unexpected ways. LMU physicists have now achieved a detailed microscopic understanding of this transport anomaly.

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The miniaturization of the electronic building blocks in modern integrated circuits continues apace – indeed it remains a major driver of technological progress. But component size has now been reduced to dimensions at which quantum mechanical effects become significant. A better understanding of the relevant quantum phenomena at this scale could lead to the development of a new generation of electrical microcomponents, still smaller and even more efficient than those currently in use. One of the most prominent of these phenomena concerns the transport of electrons through a short, one-dimensional constriction known as a quantum point contact. In contrast to the situation at macroscopic scales, the electrical conductance through such a contact is quantized. When the effective width of the contact is decreased by tuning a gate voltage, its conductance falls in a stepwise fashion, and the size of each step is equal to the fundamental quantum of conductance.

However, quantum point contacts also display a subtle many-body effect within the last conductance step, which results in what is called the 0.7 anomaly. When the conductance reaches a value of 0.7 of the quantum of conductance, it shows an unexpected and significant reduction - here the electrons apparently encounter an extra hurdle which makes it more difficult for them to traverse the contact. “Because its impact is so significant, this effect has been investigated extensively for the past 15 years or so, but its underlying cause at the microscopic level has remained unknown,” says LMU physicist PD Dr. Stefan Ludwig.

Ludwig and his LMU colleague, Jan von Delft have now solved this long-standing riddle. Together with their coworkers, the two physicists, who are also affiliated with the Nanosystems Initiative Munich (NIM, a Cluster of Excellence), have succeeded in developing a detailed microscopic understanding of the 0.7 anomaly, thanks to a clever combination of experimental measurements and numerical modeling. “The basis for the effect is actually fairly obvious. As the constriction gets progressively narrower, the electrons within the bottleneck move more slowly,” explain Florian Bauer and Jan Heyder, who performed the numerical calculations. “Shortly before the constriction becomes so narrow that electrons cannot pass at all, they become congested, and thus inhibit the mobility of other electrons in the vicinity.” - Anybody who has been jostled in a crowd trying to pass through a narrow entrance will have experienced an analogous effect. Achieving a detailed theoretical description of the resulting electronic traffic jam was a great challenge, however, which required close cooperation between theorists and experimentalists.

The team now plans to extend their study of interactions in quantum point contacts to more complex hybrid systems. “We are particularly interested in such contacts formed between superconducting and semiconducting materials, as these manifest a number of exotic effects that remain unexplained,” says von Delft. (Nature 2013)göd

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