Although lending an almost 'miraculous' property to materials, for many decades superconductivity was bound by what seemed to be an intransigent law. It only occurred at critical temperatures (Tc) lying at the very extremes of the physics of matter, in a zone just a few degrees above the impassable barrier of the famous absolute zero (or zero kelvins – 0 K – equivalent to 273°C).

For more than seven decades following Kammerlingh Onnes' discovery, all observations, including the first Theory of Superconductivity published in 1957 (see box), confirmed this basic principle. What is more, the only way to descend to such extremely low temperatures was through the production of cold (or cryogenics) associated with the liquefaction of helium (at a temperature of 4 K). Finally, there were other constraints which applied to all elements likely to serve as superconductors: superconductivity disappeared almost instantly the moment the surrounding magnetic field or current exceeded certain values.

Remarkable applications based on superconductivity – such as medical imaging – were developed, nonetheless, during the closing decades of the last century (see article The long and winding road). Yet this cold constraint remained a seemingly insurmountable obstacle and, by the early 1980s, all the 'dreams' of a stream of new and revolutionary applications for this unique property of matter seemed to have melted away in the face of this harsh reality.

The 1986 breakthrough

Then suddenly it all changed. In 1986, Alex Müller and Georg Bednorz, two Swiss physicists working at the IBM laboratory near Zurich, announced that they had developed a compound of the cuprate family (copper oxides), containing lanthanum and barium, which acted as a superconductor at 30 K. This sudden temperature rise – all the more surprising since it was obtained in a ceramic material, which shows all the characteristics of an insulator at normal temperatures – was greeted with astonishment in the world of physics.

Researchers all over the globe set about making cuprates and, just a few months later, in February 1987, a team from the University of Alabama-Huntsville announced triumphantly that, by replacing lanthanum with yttrium in Müller and Bednorz's compound, they had increased the critical temperature to an incredible 92 K, almost ten times higher than all the elements classified to date! This giant step forward marked the beginning of the era of the HTS (High Temperature Superconductor) and a whole new ballgame. By raising the temperature it was now possible to replace the costly and difficult technique of helium cryogenics with the much more commonplace industrial liquefaction of nitrogen (obtained at 77 K).

Back down to earth with a bump

This breakthrough in critical temperatures was immediately heralded as a huge advance and led to many research projects all over the world. But, after the initial enthusiasm, the world of research soon came back down to earth with a bump. As promising as it was, the discovery of HTS also marked the beginning of a complex and laborious chapter in the long saga of superconductivity. The reality was that converting these new materials into 'electricital superconductors' that could be used in practical applications posed enormous problems.

Europe saw major multidisciplinary efforts by the very best laboratories, both public and industrial, with Union support, make a significant contribution to scientific co-operation in this field. The long quest for superconductive applications was now well and truly under way, with the promise of radical changes to technology in the not-too-distant future.

	Where theory fears to tread Why do some materials become superconductors when they approach absolute zero? It was 50 years before fundamental physics provided the first 'reasoned' explanation for this phenomenon. In 1957, a trio of US scientists – John Bardeen, Leon Cooper and Jon Schrieffer – proposed their Theory of Superconductivity (later known as the BCS Theory after their initials), which came to be universally recognised and accepted. They showed that, under the extremely cold conditions of very low critical temperatures, changes occur in the vibrations and energy levels of atoms within the crystalline structure. In this state, the usual repellent force between electrons ceases and they pair off, forming a flux that is able to move without encountering any resistance. Bardeen, Cooper and Schrieffer were awarded the Nobel Prize in 1972. Unfortunately, since the discovery of new High Temperature Superconducting (HTS) materials in 1986, the BCS theory is no longer able to explain fully the phenomena with which today's physicists are working. Therefore, we have reached the limits of theory. Although empirical hypotheses have been put forward – for example, the notion that the superconductivity of HTS may be linked to a certain degree of alignment, arranged in the same direction, of the 'grains' of matter that make up the material – they have not been demonstrated.