utting-edge physics makes a step change this period, as hot papers on the physics of extra dimensions move off the radar screen to be replaced by the new superconductor magnesium diboride. Papers #8, 9, and 10 are all on the properties of this remarkable material, and they deal with its physical properties and likely applications. The high level of citations is set to accelerate as more condensed matter physicists jump on the MgB₂ bandwagon.

Superconductivity research enjoyed a renaissance some 15 years ago when it was found that doped copper oxides had a critical temperature (T_c) for the onset of superconductivity at 36 K, then 12 K higher than the record for metallic superconductors. Records soon tumbled as more cuprates were investigated, and a T_c above 77 K (the temperature of liquid nitrogen) hinted at "room temperature" applications. The record T_c stands at 130 K, for a mercury-based cuprate. There is still no consensus on the mechanism of superconductivity in these oxides.

So far as metals and their compounds are concerned, superconductivity seemed to peak at T_c=20 K. In these materials the mechanism is well understood, with electron pairs mediating the resistance-free passage of electric current. But for more than a decade all of the action has been centered on the mysterious oxide-based compounds. This changed abruptly at the beginning of 2001 when it was announced that the binary compound MgB₂ is a superconductor with T_c=40 K. Overnight, an unnoticed gray powder became a superstar, a superconductor made from easy-to-obtain materials with potential to run at temperatures that do not require messy cryogenic refrigeration. Papers #8, 9, and 10 are part of the new revolution in superconductivity research.

Paper #9 from Paul Canfield’s group at Iowa State University looks at the physics that makes MgB₂ tick. This paper is all about looking at how useful MgB₂ could be in superconductor applications. The great news is that MgB₂ can conduct without resistance in relatively high magnetic fields and data are given for fields up to 9 T. The critical current density is of order 10⁵ A cm^-2. These parameters immediately suggest that MgB₂ could be fabricated into superconducting magnets. However, #9 describes the properties of a pellet, but to make a magnet you need wires. And that’s where Canfield’s paper #8 is compelling.

Paper #8 begins by pointing out that MgB₂ is a Type-II superconductor that is similar to niobium tin (Nb₃Sn), apart from its higher T_c. Nb₃Sn is the material used in high-field superconducting magnets, but its low value of T_c requires cryogenic refrigeration. Canfield and his colleagues report a nifty way of making MgB₂ wire. Take a boron fiber (a material used in the garment industry) and surround it with mercury vapor in a furnace running at 950 C for two hours. The result is a brittle wire. In #8 Canfield’s group demonstrates that the wire has even better superconducting properties than the pellet they used in #9. To be fair, although #8 is the first paper on wires to get into the Top Ten, several other groups are now reporting on the fabrication of wires, tapes, and films as the race to commercial applications gets underway in earnest.

Magnesium diboride really does show all the signs of being a dream material for carrying high electrical currents. In its normal (room temperature) state it conducts as well as copper, which is 20 times better than niobium tin. Superconducting magnets have to be designed to cope with the resistive heating that takes place if T_c or the upper limit to the magnetic field is accidentally breached. MgB₂ would require much less protective shielding than Nb₃Sn. In normal superconducting operations MgB₂ can run above 20 K, whereas Nb₃Sn cannot. The significance of this is that MgB₂ could function with electrical closed-cycle refrigeration whereas Nb₃Sn needs cumbersome liquid-helium plumbing. This is a huge advantage that could lead to major cost savings in the next generation of particle accelerators. It is also lightweight, a factor of importance for applications in medicine such as magnetic resonance imaging (MRI), and it is able to carry much higher currents than the ceramic superconductors. The news on the street is that MgB₂ is set to displace niobium compounds as the carrier of first choice for superconducting devices.

The third paper, #10, is a contribution on the solid state physics of MgB₂, particularly the electronic structure. J.M. An and W.E. Pickett (University of California, Davis) draw an analogy with graphite, which has the same structure for the C atoms as MgB₂ has for B. The authors identify the specific features that produce the remarkably high T_c, which is partly due to the precise nature of electron-phonon coupling in this compound.