Molecular Circuits and the Kondo Effect

Carbon NanotubeElectronics based on silicon has limits, in that these types of circuit will never be nano-sized due to the physical laws governing the flow of electrons.

These laws force a stop to the process of miniaturization of electronic devices. One possible solution is using molecules as circuits, except that their poor conducting abilities make them improbable candidates.

A recent paper, however, presents a possible way around this.

The Kondo effect was first described in the last century by the Japanese physicist Jun Kondo. It is a phenomenon seen when magnetic impurities, consisting of very few atoms, as little as only 1 in 1000, of magnetic material such as iron are added to metals like gold or copper.

Even molecules such as nitric oxide act like magnetic impurities, and when located between metal electrodes they give rise to a Kondo effect.

Computer Modeling Conductance

This effect, as the study authors show, could be manipulated to change the conductance between two electrodes. Researchers at the International School for Advanced Studies (SISSA) in Trieste created a computer model of the Kondo effect under these conditions and formulated predictions on the behaviour of the molecules. These were then tested in experiments carried out by the experimental physicists in the study.

“Our work demonstrates for the first time that we can predict the Kondo effect quantitatively and it offers a theoretical basis for similar calculations with larger and more complex molecules. In the future it might be helpful when searching for the most appropriate molecules for these purposes”, said study author Ryan Requist.

Kondo Effect

The Kondo effect happens when the presence of a magnetic atom, an impurity, causes the movement of electrons in a material to behave in a peculiar way.

“Every electron has a mechanical or magnetic rotation moment, termed spin”, says co-author Erio Tosatti. “Kondo is a phenomenon related to the spin of metal electrons when they encounter a magnetic impurity. The free metal electrons cluster around the impurity and “screen it out” so that it can no longer be detected, at least so long as the temperature is sufficiently low”.

This results in specific properties of the material, for example an increase in electrical resistance.

“Conversely, in conditions involving very small size scales (the tip of a tunnelling electron microscope) such as those used in this study, the result is instead an increase in conductivity”, adds Requist.

Reference:

Ryan Requist, Silvio Modesti, Pier Paolo Baruselli, Alexander Smogunov, Michele Fabrizio, and Erio Tosatti
Kondo conductance across the smallest spin 1/2 radical molecule
PNAS 2014 111 (1) 69-74; doi:10.1073/pnas.1322239111

photo: ghutchis, Creative Commons License