Superconductivity was first observed when Onnes used liquid helium to cool mercury. It was soon found that quite a few metals would superconduct when cooled to within a few degrees of absolute zero. However, the dream of superconductivity at higher temperatures—perhaps even room temperature—has kept researchers pursuing superconductivity. Now, new research on a class of chemicals has yielded some interesting results that may point superconductor research in a different direction: hydrogen-based compounds.

Despite the attraction of low-loss superconductors, the cooling demands have limited the application of superconductivity to very high field magnets, such as those used in magnetic resonance imaging devices. In the 1980s, a new form of superconductivity that operated at liquid nitrogen temperatures got everyone pretty excited. Unfortunately, these ceramics are hard to make, harder to handle, and don't carry much current, making them even less useful than their lower-temperature brethren. What we need is a substance that has the more robust superconductivity and handling properties of metallic superconductors while retaining the high transition temperature of the ceramics. In short, a different kind of metal.

The ultimate choice would be hydrogen, which, under sufficient pressure, is thought to become metallic. Calculations suggest that the structure and properties of metallic hydrogen would support superconductivity at quite a high temperature. On the other hand, this is just so much mental masturbation, because hydrogen isn't expected to become metallic until pressures of 400GPa—a bit of a squeeze for current lab equipment. Nevertheless, there are several hydrogen-like alternatives, where a compound with lots of hydrogen in it is put under sufficient pressure to become a metal. This works because the presence of the heavier atomic cores act to compress the electrons surrounding the hydrogen nucleus, meaning that it is, in effect, already under a significant amount of pressure. This brings down the metallic transition pressure, putting it within the reach of lab equipment.

This is exactly why researchers at Max Planck Institute for Chemistry have been putting the squeeze on silane. Silane is a silicon atom surrounded by four hydrogen atoms, making it one of two perfect candidates for hydrogen-based metals (the other is methane). They found that silane became metallic at around 50GPa, which is still a pretty substantial pressure. On cooling, the metallic silane begins to superconduct. However, the temperature at which superconductivity occurs exhibits some interesting behavior. It hangs around 5-10K for most of the pressure range (50-200GPa), but in a small range between 100-125GPa, it increases quite sharply. Although the researchers only have five data points in the range and never observed a critical temperature higher than 20K, the shape of the curve indicates that, for some small range of pressures, a very high critical temperature might be achieved.

A note of caution should be injected at this point: DO NOT TRY THIS AT HOME. Silane is a gas at room temperature and pressure. It is a gas that you will not find naturally occurring because it spontaneously combusts in air. In fact, one can imagine that wires and magnets based on a silane superconductor would also make wonderful pipe bombs—not something that you want in the same room as a million-dollar MRI machine. On a slightly more serious note, the higher the required critical temperature, the narrower the pressure range for which superconductivity can be achieved, meaning that very high quality pressure control would be required to maintain silane in a useful state. All in all, it is hard to tell if this a win for superconductivity. It is, however, certainly a win for materials research.

Science, 2008, DOI: 10.1126/science.1153282