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Superconductivity

Superconductivity

New frontiers in superconductivity

07 Jan 2002

Basic research into the electrical and thermal properties of metallic, magnetic and organic materials is leading to the discovery of novel superconductors.

Research into superconductivity is enjoying a renaissance. Over the past two years physicists have discovered a wide variety of materials – including iron, single crystals of carbon-60 and even DNA – that lose their electrical resistance at low temperatures. Meanwhile, power cables made from ribbons of high-temperature superconductors have been installed in Detroit and there are similar plans for Los Angeles.

Superconductivity researchers last felt the same surge of excitement back in 1986, when Georg Bednorz and Alex Müller discovered that barium-doped lanthanum copper oxide became a superconductor at 36 K – some 12 K above the previous record temperature. A flood of new materials was discovered soon afterwards. And as records crumbled, the superconducting transition temperature, Tc, climbed above the temperature of liquid nitrogen (77 K), opening up the possibility of new applications. Currently the highest known transition temperature is 130 K for a mercury-based cuprate, yet there is still no agreed theory to explain superconductivity in these materials (see Explaining high-Tc superconductors Physics World December 1999 p55).

Breakthrough for metal compounds

While transition temperatures in the cuprates were climbing to dizzy heights, however, superconductivity in metal alloys and compounds was stuck in a rut at about 20 K. But as Paul Canfield and Sergey Bud’ko describe on page 29 (print version, see summary), the situation changed dramatically last January with the discovery that magnesium diboride superconducts at 40 K. The news sparked a worldwide race to uncover the basic properties of this humble black powder, which had been sitting on the shelf for decades. After all, a superconductor that is both easy to process and can be cooled using electrical refrigerators, rather than messy cryogens, would find many applications.

But how could physicists have failed to spot superconductivity in magnesium diboride for so long? The search for intermetallic superconductors has largely been guided by a theory developed over 40 years ago by John Bardeen, Leon Cooper and Robert Schrieffer. The predictions of BCS theory – as well as physicists’ own prejudices – has largely limited the search for intermetallic superconductors to compounds that contain light elements and transition metals.

Many physicists assumed that a more exotic underlying mechanism was responsible for superconductivity in magnesium diboride. However, the wealth of experiments that followed has proved those physicists wrong – magnesium diboride is an extreme example of a conventional superconductor.

Several groups have made magnesium-diboride wires and it could soon be possible to build lightweight superconducting magnets suitable for magnetic separation and for magnetic-resonance-imaging systems in hospitals. Not bad for a material that has only been in the physics spotlight for a year.

Theoretical challenge

Magnetism and superconductivity are usually thought of as incompatible – superconductors expel any residual internal magnetic field, while a sufficiently high magnetic field can destroy superconductivity. According to the BCS approach, the internal magnetic field in ferromagnets is expected to break apart the electron pairs that are responsible for superconductivity. However, in their article Jacques Flouquet and Alexandre Buzdin describe the recent discovery of three ferromagnetic superconductors.

With a transition temperature of less than 1K, these materials are unlikely to lead to immediate applications. However, they have revealed problems with existing theories of ferromagnetism and superconductivity that are likely to keep researchers busy for some time to come.

Crystalline organic materials are also a rich playground for exotic forms of superconductivity as John Singleton and Charles Mielke explain on page 35 (print version, see summary). These chemically complex materials can, in fact, provide more information about superconductivity and magnetism than supposedly simple materials. Indeed, the number of papers on crystalline organic metals overtook those on high-temperature cuprates three years ago, and the gap has continued to widen.

The rapid pace of recent developments has breathed new life into an already active field. As our understanding of superconductivity grows, we can expect superconductors to make further inroads into industry, and we might even find an explanation for high-Tc superconductivity.

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