Normally, big discoveries in a given field come at the rate of a few a year, if that. However, the past six weeks have seen not one, but a series of announcements that may change the face of superconductivity research. Starting with a publication in the February 23rd edition of the Journal of the American Chemical Society and ending with three separate announcements from various Chinese research groups, these last few weeks have given us the description of a previously unknown class of high temperature superconductors.

When electrons flow through a conductor, they scatter due to thermal vibrations and material impurities—the amount of scattering is measured as the electrical resistance. For most materials, as the temperature drops towards absolute zero, the electrical resistance asymptotically reaches the material's inherent conductivity. However, once some materials drop below a critical temperature, the resistance drops to zero: these materials are known as superconductors. If one constructs a loop of superconducting material and sends an electric current moving through it, that current will persist for all time, since there is no resistance to stop it. The current explanation for superconductivity is that conducting electrons form coordinated pairs that are inefficiently scattered; since conductance is inversely proportional to scattering, this leads to infinite conductance.

The temperature at which a material begins superconducting—the critical temperature—is very low. Titanium will superconduct below 0.40 K; the figure for lead is all the way up at 7.19 K. Some alloys are known to exhibit superconductivity at higher temperatures than pure compounds—Nb 3 Al has a critical temperature of 18.9 K. According to the canonical explanation for superconductivity, the BCS theory, nothing should be able to exhibit superconductivity at a temperature above 30 K. In 1986 this prediction was shown to be false with the discovery of the cuprate high temperature superconductors. The first, La 1.15 Ba 0.85 CuO, discovered in 1986 by Muller and Bednorz, was shown to have a critical temperature of 35 K. The discovery was so ground breaking, it netted the pair the Nobel Prize in Physics the following year.

In the intervening 20 years, new materials in the cuprate high temperature superconductor family have been discovered. Probably the most well known is the yttrium-barium-copper oxide (YBa 2 Cu 3 O 7 ) which was the first superconducting material shown to superconduct at a temperature of 92 K, well above the boiling point of nitrogen. The current record holder for high temperature superconductor is mercury thallium barium calcium copper oxide (Hg 12 Tl 3 Ba 30 Ca 30 Cu 45 O 125 ) which transitions into superconductive state at a whopping 138 K—with some reports this can be raised to 164 K at high pressures. However, these are still all well short of room temperature, 298 K.





Credit: Kamihara et al., JACS

Up until late February, all known high-temperature superconducting materials were some variation of a copper oxide. In the month's final edition of JACS, a research team from the Tokyo Institute of Technology reported on the groundbreaking discovery of a lanthanum oxygen fluorine iron arsenide (LaO 1-x F x FeAs)* that exhibits superconductivity at 26 K. About a month later, researchers from the University of Science and Technology of China in Hefei announced that they had synthesized a samarium oxygen fluorine iron arsenide (SmO 1-x F x FeAs)** ceramic that exhibited superconductivity at 43 K. Continuing the groundbreaking results, a second Chinese team, this one at the Institute of Physics (IoP) at the Chinese Academy of Sciences in Beijing, reported three days later that they had succeeded in creating praseodymium oxygen fluorine iron arsenide (PrO 1-x F x FeAs)*** which had a critical temperature of 52 K. They made a second announcement a few weeks later, in which they reported that the superconducting temperature of their praseodymium compound could be raised to 55 K by growing it under pressure.

These four materials represent an entire new class of superconducting compounds, and their discovery could provide a big boost to our theoretical knowledge of superconductivity. The field hasn't come to an agreement on to how to account for the behavior of cuprate high-temperature superconductors. It is believed that the layered structure of the cuprates, the ability of electrons to hop from copper ion to copper ion, and the shielding provided by copper-free layers all contribute to the superconductivity. Since these new materials also have a similar-ish layered structure, are bad conductors before they transition, and exhibit antiferromagnetism, it is hoped that they can offer new insights into a general mechanism(s) of high-temperature superconductivity.

According to Steven Kivelson, a theoretical physicist at Stanford, "[there exist] enough similarities that it's a good working hypothesis that they're parts of the same thing." However, not everyone hopes the mechanism is the same. Philip Anderson, a Nobel Laureate and theoretical physicist at Princeton, says that an entirely new mechanism of superconductivity would be far more important than if they mimicked the current understanding of superconductivity. "If it's really a new mechanism, God knows where it will go," says Anderson.

Further Reading:

* x is in the range [0.05-0.12]

** x is one of 0, 0.05, 0.13, 0.3

*** x is 0.11