'Warm' superconductor crystal structure identified

Researchers in Japan have made a huge step towards room-temperature superconductors. The research team has clarified the crystal structure of hydrogen sulfide in its superconducting phase at -70°C — a relatively high temperature for superconductors.

While the possible scenarios for superconductor use are manifold, such as using them as energy transmission lines without energy loss, widespread use is difficult as costs for cooling are high. In 2015, hydrogen sulfide set a new record for the highest superconducting transition temperature under high pressure. However, its crystal structure, necessary for understanding its superconductivity mechanism, was not understood.

A research group led by Professor Katsuya Shimizu and Dr Mari Einaga at the Center for Science and Technology Under Extreme Conditions, Graduate School of Engineering Science, Osaka University, together with Dr Mikhail Eremets at the Max Planck Institute for Chemistry and Dr Yasuo Ohishi at the Japan Synchrotron Radiation Research Institute, has now succeeded in clarifying this structure by simultaneously conducting measurements of high-pressure electrical resistance and X-ray diffraction.

Since hydrogen sulfide consists of light elements, measurements required a special set-up. Conducted at one of the world’s largest synchrotron radiation facilities, SPring-8, these measurements consisted of a diamond anvil cell to conduct measurement under high pressure and low temperature, and the high-pressure beamline BL10XU with which high-intensity, high-energy and microdiameter X-ray beams for X-ray diffraction can be used, in order to examine the material’s crystal structure. The researchers clarified that under high pressure, H₂S molecules underwent a structural change to H₃S and that this H₃S structure exhibited superconductivity.

Furthermore, from simultaneously measuring changes in pressure of superconducting transition temperature, they discovered that H₃S displayed two superconducting phases: one with a cubic structure, the other with a hexagonal structure. They thereby managed to prove previous predictions from theoretical calculations.

The results of this study will contribute to clarifying the mechanisms of the high-temperature superconductivity observed in hydrogen sulfide. They also mark a considerable step in developing room-temperature superconductors and provide new insights that will be useful in the development of new materials that spread under high pressure.