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.
References
Drozdov, A. P., Eremets, M. I., Troyan, I. A., Ksenofontov, V. & Shylin, S. I. Conventional superconductivity at 203 Kelvin at high pressures in the sulfur hydride system. Nature 525, 73 (2015).
Bardeen, J., Cooper, L. N. & Schrieffer, J. R. Microscopic theory of superconductivity. Phys. Rev. 106, 162–164 (1957).
Ashcroft, N. W. Hydrogen dominant metallic alloys: high temperature superconductors? Phys. Rev. Lett. 92, 187002 (2004).
Li, Y., Hao, J., Liu, H., Li, Y. & Ma, Y. The metallization and superconductivity of dense hydrogen sulfide. J. Chem. Phys. 140, 174712 (2014).
Duan, D., Liu, Y., Tian, F., Li, D., Huang, X., Zhao, Z., Yu, H., Liu, B., Tian, W. & Cui, T. Pressure-induced metallization of dense (H2S)2H2 with high-Tc superconductivity. Sci. Rep. 4, 6968 (2014).
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Ma, Y. Near-room-temperature superconductivity in hydrogen sulfide. NPG Asia Mater 8, e236 (2016). https://doi.org/10.1038/am.2015.147
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DOI: https://doi.org/10.1038/am.2015.147
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