One of the great puzzles of science celebrates its centenary

 

We still don’t fully understand superconductors, or their role in atom smashing and other applications, writes CORMAIC O'RAIFEARTAIGH

THIS WEEK marks the centenary of the discovery of superconductivity, one of the great puzzles of modern science. A superconductor is a material that, under certain circumstances, behaves like a perfect conductor of electricity, ie offers no resistance whatever to the passage of electric current through it.

Such materials are of immense interest to society as they hold out the promise of extremely cheap electricity (a material with zero resistance can conduct electricity indefinitely, even after its power supply is removed). However, progress in superconductor science has been frustratingly slow, despite breakthroughs during the 20th century.

The phenomenon of superconductivity was first observed in 1911, by Dutch physicist Heike Kamerlingh Onnes. On investigating how different materials behave when cooled to extremely low temperatures, Onnes observed that the electrical resistance of the element mercury disappeared completely at a temperature of four degrees above absolute zero (four Kelvin or -269 degrees Celcius). This was a great surprise, but the discovery did not seem to be of immediate practical value, as working at such temperatures presented many technical challenges. Indeed, Onnes was awarded the 1913 Nobel Prize in physics for his work in low-temperature physics.

It soon transpired that some other elements could become superconducting at very low temperatures, but nobody had an explanation for the effect. The puzzle deepened in 1933 when German researchers Walther Meissner and Robert Ochsenfeld discovered that superconductors had other bizarre properties; a material in superconducting state could expel any magnetic field within the material, and even repulse an external magnetic field brought into the vicinity. This effect, now known as the Meissner effect, is so strong that it can cause a magnet to levitate above a superconductor.

By the end of the 1950s, it had been discovered that many metal alloys become superconducting if cooled to a low temperature. More importantly, a theoretical explanation had finally emerged. US physicists John Bardeen, Leon Cooper and John Schrieffer provided a beautiful explanation for the effect in terms of quantum physics (now known as BCS theory); they were later awarded a Nobel Prize in physics for this work. By the 1980s, numerous applications of superconductivity existed, from sophisticated devices for measuring minute magnetic fields to giant electromagnets that consume almost no power. (The latter proved to be ideal for the huge magnetic fields required by high-energy particle accelerators).

And then, along came superconductivity mark II. In 1986, Alex Müller and Georg Bednorz, at an IBM research laboratory in Switzerland, created a ceramic compound that became superconducting at the relatively high temperature of 30 Kelvin. This was a completely unexpected breakthrough, and researchers around the world began cooking up ceramics of every imaginable combination in a quest for materials that would super- conduct at even higher temper- atures. By January 1987, a research team at the University of Alabama had synthesised a material that exhibited zero electrical resistance at 92 K. This was an important milestone as it was a temperature that was easily achievable in the laboratory, and the prospect of widespread application of superconductor technology seemed within reach. By the year 2000, materials that become superconducting at temperatures above 100K had been discovered.

However, these exciting advances in experiments were not matched by advances in theory. It soon became apparent that the good old BCS theory could not account for the new class of supercon- ductors. Indeed, a problem of fundamental importance had emerged; because supercon- ductivity involved the co- operative behaviour of a vast number of atoms, it constituted a complex system that could not be modelled simply from a knowledge of the behaviour of individual atoms. This discovery, that certain phenomena in nature are too complex to be successfully explained from the bottom up, was a new challenge to science; such phenomena are now known as “emergent”.

What is the state of play with superconductivity today? In the absence of an overarching theory, empirical work has continued in a hit-and-miss manner quite unusual in modern science. The current world record is a com- pound that becomes supercon- ducting when cooled to a temperature of 138 K.

Superconductor technology has found important applications in society, from the supercon- ducting magnets of MRI scanners in hospitals to the famous “Maglev” levitating trains in Japan. However, the holy grail of this field, a material that exhibits superconductivity at room temperature – and hence can deliver electricity in a manner that is both cheap and convenient – remains as tantalisingly elusive as ever.

Dr Cormac ORaifeartaigh lectures in physics at Waterford Institute of Technology and authors the science blog Antimatter. He is currently a research fellow at the Kennedy School of Government of Harvard University.

The technology has found important applications, from magnets in MRI scanners to levitating trains in Japan