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Conduction by molecular wires - the proof from DNA

Hélène Bouchiat (March 2001)

The ever increasing miniaturization of electronics has encouraged researchers to study conductors at the nanometric scale, to find the thinnest wire imaginable - single molecules. Until recently, experiments on conduction through DNA molecules have given contradictory results. Recently, however, researchers at the Solid Physics Laboratory in Orsay ("Laboratoire de physique des solides", CNRS-Université Paris 11), and the Moscow Academy of Science have observed conduction through a DNA molecule stretched across a 0.5 micron gap separating two metallic electrodes. The difficulty in attaching the ends of the DNA molecule to the electrodes seems to be at the root of the contradictions between previous reports. Very often, the connections form "tunnel junctions" which have a resistance that varies enormously from one sample to another. The material used to make the connection is therefore crucial. Rather than using gold, researchers in Orsay and Moscow chose a combination of carbon and a superconducting metal, Rhenium.

Under these conditions, DNA was found to be a conductor nearly as efficient as copper. The resistance was approximately 20 kilo Ohms at room temperature - the best conductance so far observed for DNA. Cooling down to one Kelvin (1K) had little effect on conductivity, but below 1K, the critical temperature, the researchers were surprised to observe superconductivity in the DNA wire, induced by the Rhenium electrodes.

Schematic representation of the studied samples, consisting of DNA molecules combed between two superconducting Re/C electrodes.
Temperature dependence of a sample made of about ten DNA molecules, in parallel, for various values of the magnetic field. In zero magnetic field, a significant decrease in resistance is observed below the transition temperature of the contacts (about 1 K). This effect tends to vanish as the magnetic field is increased.

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