Near-room-temperature superconductivity in hydrogen sulfide

Room-temperature superconductivity has been a long-held dream and an area of intensive research, despite the common belief—based on more than a century of work in the field—that the highest temperature at which phonon-mediated superconductors can work is only 40 K. Drozdov et al.¹ of the Max Planck Institute in Mainz, Germany, has reported surpassing this traditional limit, demonstrating that hydrogen sulfide under a pressure of 1.5 million atmospheres exhibits phonon-mediated superconductivity at 203 K. This discovery opens the door to achieving room-temperature superconductivity in compressed hydrogen-rich materials.

In 1911 a Dutch physicist, Heike Kamerlingh Onnes, observed superconductivity (i.e., conduction of electricity without resistance) below 4 K in solid mercury. Since then, considerable attention has been paid to the goal of achieving superconductivity at ever-higher temperatures for practical use.

In the following century, research effort was devoted mainly to unconventional cuprate and iron-based superconductors, whose superconducting mechanism is not yet fully understood. These putative ‘high-temperature’ superconductors were believed to represent the sole hope for achieving room-temperature superconductivity. Conventional superconductors follow the Bardeen–Cooper–Schrieffer theory;² their superconductivity arises from their acting as superfluids via electron pairing mediated by the exchange of phonons. The highest superconducting transition temperature of 39 K was observed in MgB₂, a finding that led physicists to believe that conventional superconductivity cannot occur above 40 K.

The search for superconductivity in pressurized hydrogen-rich materials was initiated by Ashcroft,³ who proposed that hydrogen-rich materials, once metalized, have the potential to become high-temperature superconductors. Hydrogen, having the lightest atomic mass, gives these materials the high Debye temperatures necessary for high-temperature phonon-mediated superconductivity.

Solid hydrogen sulfide (H₂S) has not previously been considered a superconductor because, upon metallization under pressure, it was believed to dissociate into its constituent elements. Recent theoretical work⁴ indicated that such dissociation would not occur and predicted that H₂S pressurized at 1.6 million atmospheres would show superconductivity at temperatures above 80 K. This led to the practical work of Drozdov et al.,¹ who found that H₂S compressed in a diamond anvil cell exhibited two astonishing superconductive states at pressures above 1 million atmospheres: the superconductivity ranging from 30 to 150 K measured in the low-temperature runs (Fig. 1)¹ relates to H₂S, as it is consistent with calculations;⁴ the highest superconductivity of 203 K was achieved in samples annealed at the room temperature (Fig. 2).¹ The transition temperature showed a pronounced isotope effect, indicating phonon-mediated superconductivity. The 203 K superconductivity is probably associated with a stoichiometric change to H₃S that was predicted to be a high-temperature superconductor⁵ due to the decomposition of H₂S under pressure.

The work by Drozdov et al.¹ has disproved the conventional wisdom regarding the 40 K superconduction limit of phonon-mediated superconductors, and, more significantly, it supports the general idea of high-temperature superconductivity in hydrogen-rich materials. A broad range of hydrogen-rich materials is ready for exploration, and they offer the tantalizing prospect of the imminent development of room-temperature superconductors.