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"Science": Mechanical Control of Molecular Spin States

IFF-News June 11, 2010

Researchers confirm predictions regarding the "underscreened Kondo effect"

Molecules are the smallest conceivable components of electronic circuits. Nanoscale-size molecules could, in future, help to further improve magnetic storage media for information technologies and to make novel information technologies such as quantum computers a reality. Physicists from Jülich, the USA and Argentina have now demonstrated how the magnetic properties of such individual molecules can be manipulated in a controlled mechanical way. In the current issue of the journal "Science", they report on how their model molecular system can be used to validate physical theories precisely. The researchers used their system to confirm conclusively theoretical predictions for the "underscreened Kondo effect" for the first time.

The underscreened Kondo effect is caused by a partial screening of an atom's magnetic moment - known as spin - by the electrons of a non-magnetic metal surrounding the magnetic atom. The effect is characterized by an unusual electrical conductance at very low temperatures. Compared to the normal Kondo effect with complete screening, the conductance saturates much more slowly when the temperature sinks.

The existence of the underscreened Kondo effect had been predicted since 1980, but as it is always the normal Kondo effect that manifests itself in bulk materials, the underscreened Kondo effect was only demonstrated in 2009 when researchers from Forschungszentrum Jülich and the Institut Néel in Grenoble embedded a C60 molecule in a nanogap between two gold electrodes. In this experiment, only a few samples were stable enough to show the effect. "The idea, however, that molecules in nanojunctions should be ideal systems for observing the underscreened Kondo effect was an important first step for further investigations", says Jülich physicist Dr. Theo Costi, who was part of the team.

Now, scientists from Cornell University in the USA coupled individual molecular complexes from a central cobalt atom – surrounded by organic ligands, primarily for fastening – between the tips of two gold wires of atomic diametre. The researchers stretched the molecules mechanically, thus changing the location and symmetry of the atoms while simultaneously measuring the conductance. The data obtained allowed the physicists from Cornell University, the Argentinian National Commission for Atomic Energy (CNEA) and the Jülich physicist Dr. Theo Costi to understand the characteristic changes in conductance with specific spin configurations over a large temperature range for the first time. This enabled them to provide conclusive evidence of theoretical predictions regarding the underscreened Kondo effect. Costi expects that further experimental groups will soon confirm the effect on magnetic molecules or atoms. The underscreened Kondo effect could be useful for the storage and processing of data in future information technologies.

Kondo-EffektPhysicists from Jülich, the USA and Argentina have demonstrated how the magnetic properties of individual molecules can be controlled in a mechanical way. They hope that such molecules will improve the storage and processing of data in future information technologies. Science 11 June 2010, Vol. 328 (#5984).
Source: Figure: Joshua Parks, Cornell University

Original publications:

Mechanical Control of Spin States in Spin-1 Molecules and the Underscreened Kondo Effect
J. J. Parks, A. R. Champagne, T. A. Costi, W. W. Shum, A. N. Pasupathy, E. Neuscamman, S. Flores-Torres, P. S. Cornaglia, A. A. Aligia, C. A. Balseiro, G. K.-L. Chan, H. D. Abruña, and D. C. Ralph
Science 11 June 2010, Vol. 328 (#5984)

Observation of the Underscreened Kondo Effect in a Molecular Transistor
N. Roch, S. Florens, T. A. Costi, W. Wernsdorfer and F. Balestro
Phys. Rev. Lett. 103, 197202 (2009)
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