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Thursday, January 26, 2012

New electron interactions observed


Scientists have spotted electrons interacting in new ways at the quantum level.
Image: alengo/iStockphoto
Physicists at the University of New South Wales have observed a new kind of interaction that can arise between electrons in a single-atom silicon transistor.

The findings, published this week in the journalPhysical Review Letters, offer a more complete understanding of the mechanisms for electron transport in nanostructures at the atomic level. The study is already available on arXiv

“We have been able to study some of the most complicated transport mechanisms that can arise up to the single atom level,” says lead author Dr Giuseppe C. Tettamanzi, from the School of Physics at UNSW.

The results indicate that quantum electronics could be driven by the orbital nature of electrons, and not just the spin or the charge as was previously thought, he says, which opens the door for a new type of electronics to be explored.

The study, in collaboration with scientists from the ICMM in Madrid and the Kavli Institute in The Netherlands, describes how a single electron bound to a dopant atom in a silicon matrix can interact with many electrons throughout the transistor.

In these geometries, electron-electron interactions can be dominated by something called the Kondo effect. Conventionally, this arises from the spin degree of freedom, which represents an angular momentum intrinsic to each electron and is always in the up or in the down state.

However, researchers also observed that similar interactions could arise through the orbital degree of freedom of the electron. This describes the wave-like function of an electron and can be used to help determine an electrons’ probable location around the atom’s nucleus.

Importantly, by applying a strong magnetic field, the researchers were able to tune this effect to eliminate the spin-spin interactions while preserving the orbital-orbital interactions.

The Kondo effect is considered one of the most complex phenomena found in solid-state physics, says Tettamanzi. In bulk material it causes an increase in electrical resistivity – the ease of current flow – at certain temperatures.

But in nanostructures, like the system studied here, it allows for the precise quantifications of electron interactions up to the single elements. 

“By tuning the effect in two different symmetries of the fundamental state of the system…we have observed a symmetry crossover identical to those seen in high-energy physics,” says Tettamanzi.

“In our case this crossover was observed simply by using a semiconductor device which is not too different from the transistor you use daily to send your emails.”

Tettamanzi, who was recently awarded a prestigious ARC Discovery Early Career Researcher Award, will now investigate another transport mechanism that can arise in quantum dots and single atom transistors called “quantised charge pumping”.

The idea here is to create a current flowing through a nanostructure without applying a voltage between the leads, but by applying varying potentials at one or more gates of the transistor, in an apparent violation of Ohm’s law.
Editor's Note: Original news release can be found here.

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