Uncovering a structural mechanism for superconductivity on high pressure hydrides

Researchers have experimentally identified a hydride that exhibits superconductivity below 1 Mbar with a critical temperature above liquid nitrogen.

Superconductors carry electricity with no resistance, but only below a certain temperature known as the critical temperature. This means that for superconductors to function effectively, they must be cooled to temperatures close to absolute zero. Despite over a century of investigating this phenomenon, superconductors are still confined to applications that can justify the high cost of cryogenic cooling, such as MRI scanners, particle accelerators, and the magnets that confine plasma in fusion reactors.

Hydrogen-rich compounds, known as hydrides, show the highest superconducting temperatures achieved to date. Several hydrides can superconduct at temperatures near room temperature, but can only do so under extreme pressures. In order to reach pressures of millions of atmospheres, samples need to be placed between the tips of miniature diamond anvils, making these experiments challenging due to the need for extreme precision and alignment, as well as other factors.

Researchers at the Centre for Science at Extreme Conditions, including members of the School's Institute for Condensed Matter and Complex Systems (ICMCS), have identified a compound that becomes superconducting at 94 GPa (gigapascals) - roughly half the pressure required by most hydride superconductors - and at temperatures of up to 98 K (above the boiling point of liquid nitrogen). This compound is A15 lanthanum hydride, LaH_5.75 – where A15 refers to the specific cubic crystal structure of the compound.

This result marks a new capability for the Centre. Measuring electrical transport in a diamond anvil cell at these pressures requires micron-scale electrodes patterned onto the diamond culet and insulated from the metal gasket, all of which must withstand pressures above one megabar.

Combining synchrotron X-ray diffraction, electrical transport measurements and first-principles calculations, the work shows that superconductivity survives only while the hydrogen sites are almost fully occupied. As the pressure is reduced, the material loses hydrogen, although the A15 crystal structure remains exceptionally stable, down to near-ambient pressure. When the hydrogen content falls below LaH₅, the compound stops being a superconductor and becomes an electrical insulator. This is the first reported example of a superconductor-to-insulator transition caused purely by a change in hydrogen content.

The results help separate two effects that have been difficult to distinguish in hydride superconductors: the role of the crystal structure, and the amount of hydrogen present. They show that, within the A15 crystal structure, it is the amount of hydrogen that determines how the material behaves electrically. This is consistent with the Bardeen–Cooper–Schrieffer theory that explains how superconductivity occurs.

Dr Israel Osmond, first author of the study, said:

A15 lattices show real promise for hosting high-temperature superconductivity, the question is now how we keep the hydrogen in.

Dr Miriam Peña-Alvarez, who led the study, said:

Pressure is currently the tool that lets us make these materials at all. The goal is to reach the same hydrogen networks by chemical means instead, so that the pressure needed is reduced.

The project also involved contributions from the University of Cambridge, and the work was funded through Dr Miriam Peña-Alvarez’s Future Leaders Fellowship.