Read more: “Instant Expert: Superconductors“
Within 50 years of the discovery of superconductivity, an elegant theory was in place that explained all of its effects. Superconductivity was essentially a solved problem. Then, in 1986, one discovery upset everything. Its implications are still unravelling and we are yet to find a theory to account for it. What we really want, however, is a superconductor that operates at room temperature. Might one be within our grasp?
Bardeen, Cooper, Schrieffer
The most important breakthrough in understanding superconductivity near absolute zero came from the work of , and . Bardeen (pictured) already had one Nobel prize in physics for his part in the invention of the , and the work on superconductivity would earn him his second, shared with Cooper and Schrieffer.
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The ideas they worked on together are now known as and provide a description of the superconducting state in terms of interactions between pairs of electrons. Because they have the same negative charge, electrons tend to repel each other, but this can change in certain materials that have a crystal lattice. The lattice vibrates with more or less energy depending on the temperature. When it is very cold, the gentle vibrations can push electrons together, producing a net attractive force that drives them to pair up. BCS theory shows how this tendency to pair up can result in superconductivity.
A current is essentially a flow of electrons. This flow can be knocked off course by lattice vibrations and impurities that scatter the electrons, and this is the source of electrical resistance in normal metals.
Scattering occurs all the time in superconductors too. But at superconducting temperatures, a scattered pair of electrons will stay together and keeps the same net momentum even though the momenta of the individual electrons may change. Rather like two people running along holding hands, the pairs keep to the same direction and hence so does the current.
BCS theory explains pretty much all of the properties observed in superconductors up to the mid-1980s, by which time we had learned how to make and deploy technological applications such as superconducting magnets. It also suggested that superconductivity is purely a low-temperature phenomenon. However, all that was about to change.
High-temperature discovery
Fifty years after its discovery, superconductivity had been found in a number of elemental metals and also in alloys. No one expected to see the effect in an oxide. Oxidation means rust or tarnishing – something scientists working with metals generally don’t want. Yet in the late 1960s, superconductivity turned up in an oxide called strontium titanate, cooled to well below 1 kelvin. (pictured), working at , Switzerland, was one of very few people to suspect that this discovery heralded an exciting new possibility. Together with colleague , Müller beavered away at preparing various oxides and studying them.
The breakthrough came when the pair spotted a report from a French research group on an oxide compound containing barium, lanthanum and copper. The French team had found their sample conducted like a metal, which was highly unusual for an oxide. But they only studied its properties at high temperatures; they were more interested in its possible use as a catalyst for certain chemical reactions.
Bednorz and Müller suspected there might be more to the unusual oxide. They immediately began preparing samples containing the same elements, though with different compositions, and cooled them to much lower temperatures. Bednorz and Müller’s suspicions were correct. By January 1986, they had found superconductivity and with some further tuning saw it at 30 K – a dizzy new height for a superconductor.
The oxide broke all records, but once the idea was out it was just a matter of optimising the chemical composition. The basic formula seemed to be to keep a structure with planes of copper and oxygen and vary the other atoms. By following this, new superconductors were discovered that worked at higher temperatures. Within a year, yttrium barium copper oxide – also known as YBCO and pronounced ibb-ko – was found to superconduct at 93 K. The temperature record hit 135 K by 1993 and even 150 K when the compound was squeezed to high pressure.
The era of high-temperature superconductivity had begun.
New avenues
With the discovery of superconductivity in oxides, of all things, a concerted attempt ensued to find other materials exhibiting the phenomenon. This field of research is a bit like prospecting during the great US gold rush of the 19th century: there is a lot of hacking through various regions of unpromising rock, but once somebody in an isolated valley stumbles on signs of a rich seam, everyone else quickly arrives with their hammers and starts to hunt nearby.
In this way, researchers recently found a new family of superconducting compounds containing iron. The initial discovery seems unexciting: the compound containing lanthanum, oxygen, iron and phosphorus (LaOFeP) transformed into a superconductor at around 3 kelvin, which is hardly dramatic in terms of temperature. However, the presence of iron raised a few eyebrows. Iron atoms are magnetic and not the sort of constituent you would expect to see in a superconductor. That’s because magnetic fields usually rip apart the electron pairs needed for superconductivity.
So researchers were keen to find out more about this most unusual superconductor. As they replaced lanthanum for samarium, arsenic for phosphorus, and switched some of the oxygen atoms for fluorine, they succeeded in finding a compound that superconducted at 55 K.
What makes it so exciting is that it sets a record for a superconductor that does not contain copper, hitherto a crucial constituent of high-temperature superconductors. Like other high-temperature superconductors, the new family have a layered structure; in this case layers of iron and arsenic are interleaved with samarium-oxygen layers.
Sometimes the discovery is completely accidental, as at Aoyama Gakuin University in Tokyo, Japan, found. In 2000, he was trying to isolate a complicated compound when he found that his sample had an impurity in it that seemed to be superconducting up to 39 K. He isolated the impurity and found it to be magnesium diboride (MgB2).
This incredibly simple compound has been known since the 1950s and sits in a chemical jar in pretty much every chemistry lab in the world. Nobody had ever thought to measure its electrical conductivity at low temperature and so this extremely good superconductor had remained undiscovered. It seems like MgB2 is going to be very useful. It superconducts at the relatively balmy temperature of 39 K, it can be made into wires that carry large currents and is relatively inexpensive.
One uncomfortably large sticking point remains, though: how do these superconductors work? Earlier theories based on electron pairs only explain the phenomenon at very low temperatures, and finding the right conceptual framework for understanding these “unconventional” superconductors has proved to be very tough – although incidentally MgB2 appears to superconduct along the lines that Bardeen, Cooper and Schrieffer prescribed. Solid-state chemists are therefore left to carry on hacking away in the mines looking for the elusive chemical composition that will herald the next gold rush.
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