World’s smelliest superconductor

Hydrogen sulfide, the molecule that gives rotten eggs their gag-inducing smell, can perfectly conduct electricity at the highest temperature so far. But exactly why the stinky compound is “superconductive” has baffled physicists.

Now, in a theoretical study published in Nature, a team of researchers at the University of the Basque Country and the University of Cambridge unravelled some of the mystery. Quantum physics coupled with a special ordering of the molecules at high pressure are behind the record-breaking superconductivity.

It’s a step towards understanding why superconductors allow electrons to whiz through without losing any energy. And it’s a boost to the quest for a room temperature superconductor – one of the ultimate goals of modern materials science.

Superconductors are materials that carry electrical current with exactly zero electrical resistance. They’re used to make incredibly strong magnets for magnetic levitation (maglev) trains and to keep particles on track as they race around the Large Hadron Collider.

Currently our best superconductors only work at very low temperatures. Discovering a superconductor that works at room temperature could revolutionise modern technology, and let us to transmit power across continents without any loss.

Raising the bar for higher temperature superconductors has progressed in fits in starts for almost a century. Since the early 1990s the record has hovered somewhere around -133 ºC, meaning practical superconductors have to be cooled with liquid nitrogen to work.

Then in August last year, German researchers made a startling discovery. When hydrogen sulfide is compressed to a pressure of about a million times Earth’s atmosphere, it achieves superconductivity at -70 ºC. This smashed the record by about 70 ºC more than its nearest rival.

The reason is due to quantum tunnelling – a weird effect which allows particles to ‘walk through walls’.

The question was, why?

Delving into classical theory, physicists realised that at extremely high pressures, hydrogen sulfide should switch to a special symmetrical structure, where hydrogen atoms shift from their usual off-centre hold-out to sit exactly halfway between two sulfur atoms. Physicists wondered whether this symmetry was the key to its superconductivity.

But there was a problem. The pressure the shift was predicted to occur was much higher than that actually observed by the German group. Something was amiss.

Now, physicists have explained the discrepancy. The new theory describes how the quantum nature of hydrogen atoms allows the structure to pop into its symmetrical arrangement at much lower pressures than classical physics would dictate – and the pressure matches that used in the German-led experiment.

The reason, according to the physicists, is due to quantum tunnelling – a weird effect which allows particles to “walk through walls” if the wall is thin enough. This seemingly magical feat is made possible because the protons behave as both waves and particles – some of their waviness extends beyond the walls which traps them, which gives them the possibility to “tunnel” right through.

In this case, at low pressures each proton (hydrogen nucleus) is trapped in a crystal in an off-centre position. As the pressure is ramped up, quantum tunnelling allows the proton to pop across to its symmetrical position at much lower pressures than otherwise.

It’s a big theoretical advance in the world of superconductors. But could this lead to a network of stinky hydrogen sulfide powerlines crisscrossing the country?

Well, probably not.

The catch, of course, is that achieving super-high pressures is at least as impractical as super-low temperatures. The researchers behind the study are hoping that a deeper understanding of hydrogen sulfide will lead to other (presumably odourless) materials that become superconductors at room temperature and pressure.

Something for our noses to be thankful for, at least.