ondensed-matter physicists are having a feeding frenzy over the properties of magnesium diboride (MgB₂), an ordinary laboratory chemical that turns out to have the highest transition temperature of any Type-II superconductor. The discovery paper from Jun Akimitsu’s group of Aoyama Gakuin University, Tokyo, is rocketing away, jumping from #8 last period to #3 this time, aided and abetted by 55 new citations. However Lisa Randall, MIT, and Raman Sundrum, now at Johns Hopkins, still hold the top two places, with papers published in 1999, both heavily cited by the "new wave" cosmologists dealing in hidden extra dimensions.

The extreme speed with which MgB₂ physics has developed is captured by the date sequence for the "discovery" announcements. Prof. Akimitsu’s first disclosure of the extraordinary properties of MgB₂ was made on January 10, 2001, at a symposium on transition metal oxides held in Sendai, Japan, where he revealed that it becomes superconducting below 39 K. Within 10 days other physicists in Japan confirmed this result. On January 30, Physical Review Letters received paper #5 from Paul Canfield’s group at Iowa State University; this was published on February 26, beating by three days the publication of #3 in Nature. Also in January, theorists posted preprints on the structure of MgB₂, suggesting that B forms a honeycomb lattice with Mg as a space filler.

With MgB₂, solid-state physicists have an intermetallic superconductor with a record high transition temperature (T_c = 39 K ) for a non-oxide and non-C60-based compound. The simplicity of its structure, just three atoms per unit cell, is a far cry from the complexities of the ceramic cuprates. It is also much cheaper than niobium superconductors, thanks to the natural abundances of Mg and B compared to Nb.

In #5 Canfield and his colleagues describe how to prepare high-quality samples. The binary phase diagram for B-Mg has no exposed liquid-solid line, which implies that it will be difficult to make single crystals. Instead, they made powders by combining elemental pure Mg and B in a sealed tantalum tube and heating to 950º C for two hours; the reaction appears to take place via the diffusion of Mg into the B particles. But what #5 is really about is the mechanism of superconductivity, which was explored by looking at differences in the magnetization and specific heat between Mg¹¹B₂ and Mg¹⁰B₂. In simple terms, T_c shifts down by 1.0 K for the heavier isotope, and this tells us that in MgB₂ we have a conventional BCS superconductor of a kind outlined decades ago. The coupling phonons that mediate the superconducting holes and electrons are contributed by boron. The good news here is that MgB₂ is much easier to understand than the cuprates. Consequently we can foresee rapid progress in finding both applications and variations on this compound. Physicists are already suggesting that some nifty doping with other metals could push up T_c to about 50 K.

Elsewhere in the Top Ten, papers #6 and #7, on applying spin to semiconductors, are attracting attention. In traditional electronic devices, the storage and motion of electron charge underlies the operation. However, electrons have spin as well as charge. So what kinds of new device can you make by manipulating electron spin as well? Paper #6 tells us that the new field of "spintronics" would allow the reading and writing of non-volatile information through magnetism. Quantum computing in the solid state may also be realizable. But what’s holding up spintronics is the huge challenge of how to get spin-polarized electrons inside a semiconductor. Both papers report successful injection of electrons or holes with spin into a light-emitting diode. This is a huge leap forward and the race for commercialization is now on.

Finally this period, the universe gets a mention, too. Paper #10 describes measurements of small irregularities in the cosmic microwave background. MAXIMA-1 is a microwave telescope that gets raised to about 40 km by balloon. Its photometers are cooled to just 100 mK and are the most sensitive available in their class. The maps from MAXIMA cover a large range of angular scales, and the lumpiness in the distribution is consistent with an ever-expanding, accelerating, universe. MAXIMA thus confirms the cosmological model now in fashion: a flat universe propelled by mysterious dark energy.