Year in review: Big stride for superconductivity

After a two-decade hiatus, superconductors are again heating up.

A compound of hydrogen and sulfur, when crushed at more than a million times Earth’s standard atmospheric pressure, appears to whisk electrical current along without resistance at temperatures up to 203 kelvins. That’s not exactly balmy — it’s −70° Celsius — but the current record holder performs its magic at temperatures no higher than 164 kelvins.

A room-temperature superconductor would enable robust energy storage devices, MRI machines that don’t require liquid helium coolant and a new generation of levitating trains. But after more than a century of intense research, physicists still aren’t sure exactly which compounds are capable of reaching that goal. Any new superconductor, even one that requires high-pressure equipment installed in only a handful of labs worldwide, could lead physicists to more practical materials.

Mikhail Eremets of the Max Planck Institute for Chemistry in Mainz, Germany, and colleagues first reported superconductivity in hydrogen sulfide in December 2014. They took a small sample of the noxious, flammable gas, also known as sulfur hydride, and crushed it between two diamonds at extremely low temperatures. Measurements indicated that the electrical resistance dropped to zero and remained there even when the compound was heated to 190 kelvins ( SN: 4/4/15, p. 11 ).

Though the finding was initially controversial, Eremets’ team bolstered the claim six months later by showing that the pressurized hydrogen sulfide expelled magnetic fields, a gold standard sign of superconductivity called the Meissner effect (SN: 8/8/15, p. 12). This time the team saw evidence for superconductivity at temperatures up to 203 kelvins, nearly 40 kelvins higher than a copper-based compound reported in 1994.

“The story is evolving in the right direction,” says Ivan Božović, a condensed matter physicist at Brookhaven National Laboratory in Upton, N.Y. Other labs have yet to confirm the Meissner effect, but a team in Japan has reported measuring zero resistance at high temperatures in hydrogen sulfide.

Eremets is already moving on to similar compounds. In August, his team reported that compressed phosphine is a superconductor at temperatures up to 103 kelvins. After decades studying relatively complex materials, physicists hope that these simpler compounds of hydrogen provide the ticket to unraveling the mysteries of superconductivity.