Kammerlingh Onnes discovered superconductivity, in mercury, in 1911, winning the Nobel Prize in 1913 for his work with liquid helium. For the next 80 years research on superconductors was held back because the highest transition temperature (T_c) was only 23 K. This meant all research required challenging cryogenics at liquid helium temperatures, and applications were limited to mega-projects, such as the superconducting magnets in particle accelerators.

     All this changed a decade ago with the discovery of high-T_c copper oxide ceramic superconductors. A new kind of physics emerged, with a language of its own: pseudogaps, stripes, and new pairing mechanisms. T_c climbed ever higher, reaching 160 K under pressure. The theorists struggled to understand the mechanisms, and the experimentalists tweaked the doping, composition, and physical structure of the multi-element compounds in their search for a superconducting structure that could be made easily. The superconducting devices currently being made require a complex processing.

     It took until 1957 to explain the mechanism of superconductivity. In that year John Bardeen, Leon Cooper, and Robert Schrieffer showed how an electron-phonon interaction could produce the electron pairs. This theory explained the puzzle of the isotope effect: T_c falls as the mass of the superconducting atoms increases. That’s because the lattice vibrations (the phonons) depend on the masses of the atoms in the lattice.

     The isotope effect suggested that compounds rich in the light elements would be a good hunting ground for high-T_c materials. Akimitsu’s group, for example, was looking at the Ti-B-Mg phase diagram, and other researchers were trawling three-element and four-element compounds. But the binary compound MgB₂ turned out to be the long-sought high-T_c material. The point about a T_c of 40 K is that inexpensive refrigerators can cool to this level.

     MgB₂ cannot be grown into crystals, and that may be why it was missed for so long. It is made as a grey powder from the reaction of B with Mg vapor at a temperature of 900° C. To add to the excitement of the high T_c, here was a superconductor made from elements that are abundant and inexpensive, in contrast to the copper-oxides, with their rare earth elements. Furthermore, it is so easy to make and to handle that superconducting research groups worldwide were able to get into MgB₂ research immediately, and this accessibility accounts for the high citations of #1.

     When MgB₂’s properties were announced, theorists were still struggling with a theory for the cuprates. Imagine the relief then when Sergei Bud’ko and coworkers announced in #3 that they had found the isotope effect. This immediately signalled that MgB₂ is an "old-fashioned" superconductor that everyone could understand. The high citations to this paper are from researchers who gladly acknowledge that the physics of MgB₂ is familiar!

     Hot Paper #10 from Paul Canfield’s group is remarkable for the speed with which the team made MgB₂ wires. Obviously, if MgB₂ is to be fabricated into magnets, then wires and films are a must. The technique outlined in #10 is simplicity itself: they suspended a commercially available B fiber in Mg vapor and turned it into a brittle wire. In a similar manner they have made MgB₂ films from B films. Another attractive feature for MgB₂ in terms of magnet and electronic applications is its low molecular weight and very low mass density. It’s a lightweight material that could have important applications in space.

     The "how does it work?" question is answered in #6, from Jens Kortus and colleagues. They showed that B forms layers of honeycomb lattices with hexagonal planes of Mg as a space filler. The high level of T_c is attributed to the remarkable strength of the electron-phonon interaction.

     The five MgB₂ papers on the list have logged a total of 1,400+ citations, which is startlingly high for the Physics Top Ten. New areas for pure and applied research have been mapped out. Medical applications of this promising material, in MRI scanners, may not be far off. And MgB₂ has taught us a lesson about how to do science. Despite 80 years of searches it was missed. No theorist predicted this behavior. The discovery was made as a result of the patient experimental search for new compounds.