To most people it looks rather unremarkable, a brown powder that wouldn鈥檛 attract as much as a second glance. But to Warren Pickett, this dust is more precious than gold. It has even inspired him to draw up a blueprint for a material that many people think is impossible: a superconductor that works in the heat of summer.
Superconductors carry electricity without resistance and could be the key to many technical marvels, from the fastest computer processors ever seen to fleets of eco-friendly ships propelled by electricity. These breakthroughs will never make it into everyday use, however, as long as so-called 鈥渉igh-temperature鈥� superconductors fail to work above about 138 kelvin (-135 掳C), which makes them useless without expensive and bulky refrigeration. When it comes to increasing this critical temperature researchers have a big problem: they know very little about how high-temperature superconductors actually work.
That doesn鈥檛 matter, according to Pickett, who specialises in superconductors at the University of California, Davis. Paradoxically, he believes the answer to higher critical temperatures lies much closer to absolute zero. By looking at how his brown powder, magnesium diboride, loses its resistance at 40 K, he has sketched out a material that superconducts well above room temperature. That means the conventional low-temperature superconductors could one day far outstrip high-temperature superconductors, which look set to rely on liquid nitrogen for a long time to come.
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Superconductivity was first observed back in 1911 by the Dutch physicist Heike Kamerlingh-Onnes, who showed that mercury loses its resistance when cooled to only a few degrees shy of absolute zero. He quickly followed suit with other pure metals, such as aluminium and tin. Ever since then, physicists have been trying to push up the temperature of superconductors, first to 20 K with metal alloys and then with a brand new class of materials. Discovered in 1986, these high-temperature superconductors are based on copper oxides and get their name from their ability to superconduct above a relatively cosy 77 K. That means they can be cooled with liquid nitrogen rather than the more expensive liquid helium.
To this day, no one understands how superconductivity occurs in these materials, but to some extent that hasn鈥檛 mattered. Short lengths of high-temperature superconducting cable have replaced the conventional copper and aluminium ones that carry electricity from a handful of generating stations around the world. In trials last year in Yamanashi, Japan, the first high-temperature maglev train reached speeds in excess of 500 kilometres per hour. And companies such as American Superconductor, based in Westborough, Massachusetts, are building prototype superconducting motors for US navy ships.
Lost power
With such potential, it is no surprise that room-temperature superconductors are at the top of a lot of wish lists. Around 8 per cent of the electricity generated in power stations never reaches our homes and offices, according to a recent report by Robert Hawsey of Oak Ridge National Laboratory in Tennessee and Satoshi Morozumi of the Mitsubishi Research Institute in Tokyo, Japan. Instead it is wasted as heat produced by the electrical resistance in the transmission cables.
Because of their efficiency, superconductors also have the potential to prevent millions of tonnes of carbon dioxide from being needlessly emitted each year. Unfortunately, cooling superconducting wires to incredibly low temperatures is impractical, and with no working theory to guide researchers, the superconducting record still officially stands at 138 K, a temperature that was achieved using an extremely complex blend of mercury, thallium, barium, calcium and copper oxide. Add to that the fact that superconducting wire costs more than 100 times as much to make as copper wire, and it is easy to see why the superconductor revolution has been slow to start.
Frustrated by these efforts, many physicists have become pessimistic about finding a cheap material that superconducts without being cooled. 鈥淚 think a psychological hang-up remains among most researchers in regard to room-temperature superconductivity,鈥� says William Little of Stanford University in California. 鈥淭he challenge is to beat this.鈥�
None of these problems has deterred Pickett. Although no one has figured out how the copper oxides superconduct, 鈥渘ormal鈥� low-temperature superconductors are well understood. The secret lies in what happens to electrons at temperatures close to absolute zero. While electrons in empty space repel one another, inside a superconductor they bind together in pairs. These pairs tend to synchronise their movements and swim collectively through the material without running into obstacles. As a result, they do not lose any energy, which means the material has zero electrical resistance.
Last year, Pickett decided to go back to basics and re-examine the theory, inspired by an astonishing discovery reported in 2001 by Jun Akimitsu鈥檚 team at Aoyama Gakuin University in Tokyo. Akimitsu and his colleagues were playing around with mixtures of titanium, magnesium and boron in an attempt to find a new superconductor. To their surprise, they stumbled across hints of superconductivity at 40 K. That鈥檚 not very impressive compared with some of the high-temperature superconductors, but it is important because it鈥檚 about twice the critical temperature of any previous metallic superconductor, and could pave the way to others that work at far higher temperatures. A closer look at Akimitsu鈥檚 mixture revealed that the superconductor was, in fact, magnesium diboride.
The finding was scientific dynamite. Within two months of Akimitsu鈥檚 announcement, 50 papers were published online as researchers rushed to study magnesium diboride for themselves. 鈥淭he discovery was stunning because of the unimaginably high temperature for a conventional superconductor,鈥� says Pickett. 鈥淚n the 1960s, people thought they understood these materials, but magnesium diboride showed us we were wrong.鈥�
It didn鈥檛 take researchers long to notice that electrons pair up in magnesium diboride just as they do in other metal superconductors, which means the electron-pairing effect does not fall away as close to absolute zero as thought. This pairing was explained in 1957 by John Bardeen, Leon Cooper and Robert Schrieffer, all at the University of Illinois in Urbana-Champaign. They showed that when an electron passes through a material, it sends vibrations through the atoms that are arranged in a lattice. This disturbance can attract another electron, overcoming the normal repulsive force and causing it to pair up with the first electron. Physicists picture the process in terms of phonons, packets of vibrational energy that electrons exchange with each other. It is this exchange that allows superconductivity, at least in ordinary metals.
However, even the best of the metallic low-temperature superconductors lose their superconductivity when they warm up. The problem is that the atoms in a material vibrate more vigorously at higher temperatures, breaking apart the electron pairs and restoring electrical resistance to the material. Until magnesium diboride came on the scene, that critical temperature was frozen at just 20 K.
Pickett realised that if he could identify what made magnesium diboride so special, other metal alloys might be found with even higher critical temperatures. To do this, he studied what affected the critical temperature in Bardeen, Cooper and Schrieffer鈥檚 studies and then compared these factors to the properties of magnesium diboride.
According to their theory, the critical temperature depends on three things: the number of electrons available, the frequency at which the phonons vibrate, and the strength of the interaction or 鈥渃oupling鈥� between the phonons and electrons. Magnesium diboride鈥檚 high transition temperature is due mostly to strong coupling, which is down to its chemical structure. It consists of layers of boron just one atom thick sandwiched between layers of magnesium atoms (see Diagram). Each magnesium atom feeds two electrons into the boron layers, which means that there are abundant electrons in the structure, ready to pair up. Better still, the electrons flow in the same layer as the boron atoms and set up large disturbances, which enhance the coupling between phonons and electrons as they sweep through the material. The upshot is that the electron-pairing still takes place at higher temperatures than expected.
Despite the strong interaction between phonons and electrons, Pickett found a problem when he looked more closely at magnesium diboride: only 3 per cent of phonons interact at all. 鈥淚mpressive as it is, magnesium diboride is doing a poor job of making use of the available phonons,鈥� says Pickett. 鈥淚f we could use most of the phonons, the critical temperature would increase all the way past room temperature.鈥�
Pickett鈥檚 approach differs from previous attempts to boost the critical temperature. Others have tried to increase the number of electron pairs or the coupling strength by adding small amounts of other elements to the crystal lattice, but these approaches have failed. Pickett鈥檚 own calculations provide some clues as to why: while increasing the coupling strength or the vibration frequency of the phonons can raise the temperature by as much as 20 per cent, the crystal structure itself can eventually become unstable and take on very different properties.
Instead he proposes involving more phonons by trying different combinations of elements. What鈥檚 more, his blueprint gives researchers clues as to which elements would work best, rather than resorting to trial and error as they have done in the past. By doing this, his calculations show that it should be possible to find a material that superconducts at a searing 430 K. 鈥淚t seems reasonable to expect that materials exist, or can be made, that will improve on magnesium diboride.鈥�
鈥淧ickett鈥檚 calculations show it should be possible to find a material that superconducts at a searing 430 K鈥�
One person who welcomes Pickett鈥檚 results is Paul Canfield of the US Department of Energy鈥檚 Ames Laboratory in Iowa. He was one of the first to study magnesium diboride and explain how it works. He believes Pickett鈥檚 paper is a rallying cry to researchers. 鈥淚t shows that there are new possibilities for high temperatures in compounds that until 2001 were thought to be tapped out,鈥� he says.
Even with Pickett鈥檚 clues, finding these compounds won鈥檛 be easy. Pickett has already studied materials whose atomic structures bear a striking similarity to magnesium diboride. First he considered layers of graphite sandwiched between layers of fluorine atoms, but this did not superconduct.
Undeterred, he began looking for other materials and eventually came across work by Reinhard Nesper at the Max Planck Institute for Solid State Research in Stuttgart, Germany. Nesper has recently synthesised an exotic alloy called lithium boron carbide in his lab. On its own, this alloy should not superconduct, but Pickett and Nesper have shown that removing a quarter of the lithium atoms could transform the alloy into a superconductor that works at considerably higher temperatures than magnesium diboride.
Fine tuning
The next task is to make it, and Pickett is already talking with people that might be up for the challenge. No one expects to get it right first time: tuning chemical bonds and phonons is notoriously difficult.
Even if Pickett and others fail to make a room-temperature superconductor, there is a silver lining. Temperature isn鈥檛 the only important issue. High-temperature superconductors are fiendishly difficult and expensive to manufacture because of their complexity: introduce any defects and the superconductivity is lost. So any new superconductor with a structure as simple as that of magnesium diboride is likely to have a big impact technologically. 鈥淎 conventional superconductor with a temperature of just 110 K could be much more useful than a copper-oxide superconductor at room temperature,鈥� says Canfield.
鈥淎ny new superconductor with a structure as simple as magnesium diboride鈥檚 is likely to have a big impact鈥�
This view is slowly winning round the hearts and minds of physicists. 鈥淣ot too long ago, mentioning room-temperature superconductivity was considered damaging to one鈥檚 reputation and career,鈥� says Ivan Bozovic at Brookhaven National Laboratory in Upton, New York. 鈥淭oday the general attitude is more open-minded.鈥� So don鈥檛 take those dull-looking powders sitting in jars for granted. They could spark a superconducting revolution.
