First direct microscopic evidence of superconductivity in hydrogen-rich materials

Superconductivity, where electricity flows with zero resistance, promises a future of perfectly efficient power grids, magnetic levitation trains, and ultra-sensitive medical imaging. Yet, the mystery of how superconductors really work—especially the ones that perform at high temperatures—remains unsolved.

Hydrogen sulfide drew worldwide attention in 2015 when researchers discovered it could become superconducting at –70 °C when subjected to pressures over 150 gigapascals (GPa). Since then, it has remained one of the most promising leads in the pursuit of room-temperature superconductors.

The key to understanding superconductivity lies in the superconducting gap – a fundamental property that reveals how electrons pair up to form the superconducting state. It is the identification of superconducting state distinguishable from other metallic states.

Still, measuring the superconducting gap in hydrogen-rich materials like H₃S has remained extremely difficult. These compounds must be synthesized in situ under extraordinary pressures – more than a million times atmospheric pressure – making conventional techniques to measure the gap, such as scanning tunneling spectroscopy and angle-resolved photoemission spectroscopy, inapplicable.

Now a team led by the Max Planck Institute in Mainz used high-pressure tunneling spectroscopy to overcome this barrier. The researchers discovered that H₃S exhibits a fully open superconducting gap with a value of approximately 60 millielectronvolt (meV), while its deuterium analogue, D₃S, shows a gap of about 44 meV. Deuterium is a hydrogen isotope and has one more neutron. The fact that the gap in D₃S is smaller than in H₃S confirms that the interaction of electrons with phonons – quantized vibrations of the atomic lattice of a material – causes the superconducting mechanism of H₃S, supporting long-standing theoretical predictions.

Then they came to the ESRF’s ID27 beamline where they carried out X-ray powder diffraction on microcrystalline samples to confirm the presence of the Im-3m cubic phase—the arrangement of atoms thought to be essential for superconductivity in H₃S, which had been observed via tunneling spectroscopy. Therefore, the ESRF provided critical crystallographic validation under extreme conditions.  

Reference:
Du, F. et al. Superconducting gap of H₃S measured by tunnelling spectroscopy, Nature (2025). DOI: 10.1038/s41586-025-08895-2

Top image: More than one million bars can be created between two diamond tips, and some materials can be synthesized to become superconducting at relatively high temperatures. Credits: Feng Du, MPIC