Cracking the case of the mysterious conducting surface of NiS2
15 Sep 2026 - Rosie de Laune
Combining neutrons, microscopy and theory, an international collaboration has uncovered the magnetic structure and the microscopic mechanism at the basis of the conductive edge state in the bulk insulator NiS2.
This material shows some very unusual electronic and magnetic behaviour. Despite being an insulator, its surface shows metallic conductivity. As well as this, it takes on multiple different magnetic phases upon changing the temperature, with the ground state magnetic structure being a topic of debate in literature.
In this study, published in Nature Communications, an international research collaboration has finally got to the bottom of both puzzles. Using neutron diffraction, they were able to characterise the magnetic phases in NiS2, including the previously debated ground state. With these results and with a careful modelling of the band structure and surface edge states, they were able to interpret their high-resolution scanning tunnelling microscopy and spectroscopy (STM/STS) measurements, finally understanding its mysterious conducting surface.
By carrying out neutron diffraction using WISH, they were able to use group theory calculations to determine the magnetic ground state of NiS2. Their solution also addresses the previously unexplained distortions observed using synchrotron X-ray and thermal expansion techniques.
“Neutron diffraction on WISH was pivotal in determining the magnetic symmetry of NiS2, which was then used to calculate the surface state and eventually understand the STM measurement,” explains ISIS beamline scientist Fabio Orlandi.
“We found that NiS₂ hosts robust edge states that explain its mysterious conducting surface” explains Maia Vergniory, from the Université de Sherbrooke, Canada. “By combining neutron scattering, STM/STS, symmetry analysis, and ab initio calculations, we could uncover the new magnetic ground state and identify obstructed topology as the origin of these states.”
This study is a good example of how insight into symmetry and topology can guide experiments towards beautiful physics in real materials
Miguel Ugeda, Donostia International Physics Center
“Our group at the Donostia International Physics Center was able to visualise robust one-dimensional conducting channels at the step edges of NiS₂, whose conducting surface had remained puzzling for decades. This was a great collaboration combining neutron scattering, atomic-scale spectroscopy, symmetry analysis and ab initio calculations,” explains Miguel Ugeda, from DIPC in Spain.
Their research sheds new light on the physics of NiS2 and establishes a solid experimental and theoretical foundation for exploring the interplay between topology and electronic correlations.
“It is a good example of how insight into symmetry and topology can guide experiments towards beautiful physics in real materials,” adds Miguel.
The full paper can be found at DOI: 10.1038/s41467-026-76126-x
