Physicists create first superconducting polymer
Mar 9, 2001
NB: The publication on which this article is based has been retracted by its authors. The retraction followed an investigation into scientific misconduct by J Hendrik Schön. We are leaving this article here for archival purposes. Further information can be found at http://physicsweb.org/article/news/6/9/15
A superconducting polymer is the latest innovation to emerge from the recent explosion of research into organic superconductors. Bertram Batlogg and colleagues at Bell Laboratories in the US have achieved resistance-free current flow in poly(3-hexylthiophene) at 2.35 kelvin. The advance is a fundamental step towards cheaper mass-produced electronics (J H Schön et al 2001 Nature 410 189).
It has long been known that certain polymers - containing alternating single and double bonds along the polymer 'backbone' - conduct electricity. Electrons move by hopping from one bond to another. Introducing 'foreign' atoms to donate extra electrons can make some polymers conduct as well as metals. Superconductivity, however, has never been observed before.
Batlogg and co-workers allowed a solution of poly(3-hexylthiophene) - or P3HTP - to solidify into thin films. The films were found to consist of tiny crystals of polymer interspersed with amorphous regions. Resistance-free current flows when the films are cooled below 2.35 K, and the relationship between conductivity and temperature is typical of that in polycrystalline superconductors. This is thought to result from the co-existence of the superconducting nanocrystals and the insulating amorphous areas.
The Bell team believes that the 'self-organization' is the key to superconductivity in P3HTP. The polymer chains within the crystals spontaneously align themselves as the films are cooled. Indeed, interfering with this self-imposed order was found to suppress superconductivity. For this reason, Batlogg and co-workers injected charge carriers into the polymer - one for every five polymer chains - using a field-effect transistor, which disrupts the structure less than the traditional chemical doping. "Using our method, many organic materials could potentially become superconductors", said team member Zhenan Bao.
The polymer has a clear metal-to-insulator transition, and its superconductivity disappears when a strong magnetic field is applied - the acid test of a true superconductor. But the mechanism for superconductivity in P3HTP is unclear. The Bardeen-Cooper-Schrieffer theory describes, for conventional superconductors, how thermal vibrations of the crystal lattice - or phonons - help electrons to pair up and flow without resistance. There are strong hints that phonons play a role in the P3HTP effect, but other types of electron-electron interaction have not been ruled out.
Within the last year, the prolific Bell Labs team has chalked up the first electrically powered organic laser, and achieved superconductivity in organic crystals and the fullerene, C60. Elsewhere, the discovery of surprisingly high-temperature superconductivity in the metallic compound magnesium diboride is under intense investigation.