SUPERCONDUCTIVITY: ENIGMA OR BREAKTHROUGH?

Introduced by Dr Peter Ford, University of Bath, on 28 November 1997

Superconductivity is the name given to the flow of a current without electrical resistance. It is an
electrical engineer's dream, having the potential for opening up all sorts of exciting technological
possibilities. Superconductivity was discovered in 1911 by Heike Kamerlingh Onnes, the Dutch scientist,
shortly after he first liquified helium, which enabled matter to be studied at temperatures close to absolute
zero (0 K).
In the early materials these very low temperatures were essential and the effect was found to be
suppressed in small magnetic fields. Both these characteristics precluded its use for technical applications.
In the next 75 years, the temperature at which material became superconducting slowly increased
to just over 20 K and the ability to remain superconducting in a large magnetic field also increased
substantially. A small but flourishing industry developed in the production of compact high-field magnets,
for which the ability to pass a high current to produce the magnetic field without at the same time generating
unwanted Joule heating was decisive.
Much had also been discovered about the theory of superconductivity and it was appreciated that
it was a much richer and more complicated phenomenon than merely the absence of electrical resistivity. It
was also found to be remarkably wide-spread and many elements, alloys and compounds were found to
enter the superconducting state.
Magnetic measurements in the 1930s showed that below the transition temperature, the magnetic
flux in many simple metals was suddenly expelled from the bulk of the superconductor. This became known
as the Meissner effect and such materials were called type I superconductors. However, many materials
showed a more complex behaviour in which the flux was only partially excluded over an intermediate state,
whilst the bulk of the material still had zero electrical resistance. These were type II superconductors and
are the materials suitable for technical applications.
A major breakthrough occurred in 1986 when two scientists, Alex M ller and Georg Bednorz,
discovered superconductivity in a complex oxide ceramic material at a temperature of around 35 K, much
higher than previous materials. This fuelled intense research activity and within a few months two groups
collaborating in America had obtained superconductivity in a similar ceramic oxide at a temperature of about
92 K. This was highly significant since it is well above the boiling point of liquid nitrogen (77 K). A
technology based on liquid nitrogen is simpler and cheaper than that based on liquid helium and it opened
up the prospects for wide-spread applications. However, the ceramic oxide, a complex mixture of the
elements yttrium, barium, copper and oxygen, has posed formidable problems in the production of suitable
wires and thin films.
The important point is that these discoveries have removed the widely held belief that
superconductivity is confined to low temperatures and have given rise to intensive theoretical
investigations to try to understand these exotic materials.
Peter Ford