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Molecular 'wires' show hypersensitivity to magnetic fields

06 July 2013

Researchers create perfect one-dimensional molecular wires whose electrical conductivity is almost entirely suppressed by a weak magnetic field at room temperature.

The geometrical constraints of the zeolite host structure allow for the formation of one-dimensional chains of highly uni-axially orientated molecules (image: MESA+)

Researchers at MESA+, a co-operative nanotechnology research institute comprising the University of Twente, the University of Strasbourg and Eindhoven University of Technology, claim to be the first to successfully create perfect one-dimensional molecular wires, the electrical conductivity of which can almost entirely be suppressed by a weak magnetic field at room temperature.

The underlying mechanism is possibly closely related to the biological compass used by some migratory birds to find their bearings in the geomagnetic field. The discovery may lead to the development of magnetic field sensors for devices such as smartphones. 

In their experiments, the researchers made use of DXP, the organic molecule which is a red dye of the same type as once used by Ferrari for their famous Testarossa. In order to thread the molecules so that they form one-dimensional chains of 30 to 100nm in length, they locked the molecules in zeolite crystals.

Zeolites are porous minerals composed of silicon, aluminium and oxygen atoms with narrow channels, like the lift shafts in a block of flats. The diameter of the channels in the zeolites is only 1nm, just a little wider than the molecule's diameter. This enabled the researchers to create chains of aligned molecules inside the zeolite channel, which are only 1 molecule wide.

Molecular electrically conducting wires
The zeolite crystals containing the molecular wires were then placed on an electrically conductive substrate. By placing the probe of an atomic force microscope on the surface of the zeolite crystals, the researchers were able to measure the electrical conductivity in the molecule chains.

Professor Wilfred van der Wiel, who developed and led the experiment, says that measuring the electrical conductivity in these molecular electrically conducting wires was a unique result in itself. "But the behaviour of these wires is simply spectacular when applying a magnetic field," he adds.

The electrical conductivity nearly completely breaks down in a magnetic field of just a few milli-teslas, a field can easily be generated using nothing more than a refrigerator magnet.

The change in electrical resistance through a magnetic field is called magnetoresistance. Usually, magnetic materials are indispensable for creating magnetoresistance. However, the ultra-high magnetoresistance which has been measured in Twente was achieved without any magnetic materials.

The researchers ascribe this effect to the interaction between the electrons and the magnetic field which is generated by the surrounding atomic nuclei in the organic molecules.

Current suppression in a small magnetic field can ultimately be traced back to the Pauli exclusion principle, which states that no two electrons (fermions) may have identical quantum numbers. Since these molecular wires are essentially one-dimensional, the effect of the Pauli exclusion principle is dramatic.

Migratory birds
The mechanism that is responsible for ultra-high magnetoresistance in molecular wires is possibly closely related to the biological compass used by some migratory birds to find their bearings in the geomagnetic field. Researchers of the University of Twente are conducting follow-up experiments in the hope to be able to shed more light on this analogy.

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