Conventional superconductivity at 203 kelvin at high pressures in the sulfur hydride system

Abstract

Conventional superconductivity is observed at 203 kelvin in the sulfur hydride system, well above the highest superconducting transition temperature obtained in the copper oxides, raising hopes that even higher transition temperatures will be discovered in other hydrogen-rich systems. The discovery of high-temperature superconductivity in the copper oxides nearly thirty years ago raised hopes for the imminent realization of room-temperature superconductivity. But after initial successes, progress towards this goal stalled. For more than two decades the 'record' has stood at 133 K at ambient pressure and 164 K under high pressures. The quest is now renewed with the discovery of superconductivity at 203 K in the sulfur hydride system. By subjecting hydrogen sulfide (H2S) to extreme pressures, Alexander Drozdov et al. have produced an enigmatic phase — which might be H3S — that shows the clear signatures of superconductivity at 203 K or minus 70°C. The presence of hydrogen is key to this finding, raising the prospect that even higher transition temperatures — possibly even approaching room temperature — will be discovered in other hydrogen-rich systems. A superconductor is a material that can conduct electricity without resistance below a superconducting transition temperature, Tc. The highest Tc that has been achieved to date is in the copper oxide system1: 133 kelvin at ambient pressure2 and 164 kelvin at high pressures3. As the nature of superconductivity in these materials is still not fully understood (they are not conventional superconductors), the prospects for achieving still higher transition temperatures by this route are not clear. In contrast, the Bardeen–Cooper–Schrieffer theory of conventional superconductivity gives a guide for achieving high Tc with no theoretical upper bound—all that is needed is a favourable combination of high-frequency phonons, strong electron–phonon coupling, and a high density of states4. These conditions can in principle be fulfilled for metallic hydrogen and covalent compounds dominated by hydrogen5,6, as hydrogen atoms provide the necessary high-frequency phonon modes as well as the strong electron–phonon coupling. Numerous calculations support this idea and have predicted transition temperatures in the range 50–235 kelvin for many hydrides7, but only a moderate Tc of 17 kelvin has been observed experimentally8. Here we investigate sulfur hydride9, where a Tc of 80 kelvin has been predicted10. We find that this system transforms to a metal at a pressure of approximately 90 gigapascals. On cooling, we see signatures of superconductivity: a sharp drop of the resistivity to zero and a decrease of the transition temperature with magnetic field, with magnetic susceptibility measurements confirming a Tc of 203 kelvin. Moreover, a pronounced isotope shift of Tc in sulfur deuteride is suggestive of an electron–phonon mechanism of superconductivity that is consistent with the Bardeen–Cooper–Schrieffer scenario. We argue that the phase responsible for high-Tc superconductivity in this system is likely to be H3S, formed from H2S by decomposition under pressure. These findings raise hope for the prospects for achieving room-temperature superconductivity in other hydrogen-based materials.