Using muonium to probe point defects in diamond
06 Nov 2026 - Peter Hurrell
Researchers from ISIS and the National Institutes for Quantum Science and Technology (QST) in Japan have shown that muonium can be used to probe point defects in diamond, including nitrogen-vacancy (NV) centres. For the first time in diamond, the team modelled how the different muonium states convert into one another, which allowed them to work out how muonium interacts with two different point defects.
Point defects, such as a missing or foreign atom in a crystal, strongly affect the properties of semiconductors, and NV centres are now key building blocks of quantum technologies. The findings offer scientists a new way to study such defects using muons.
Point defects
The researchers used type Ib diamond, which contains nitrogen as single atoms scattered through the crystal. Type Ib diamond occurs rarely in nature but can be grown artificially. The team were interested in two point defects caused by the nitrogen atoms. The first is substitutional nitrogen, where a nitrogen atom replaces a carbon atom in the lattice; it carries one unpaired electron, which makes it magnetic.
The second is the NV centre, where a substitutional nitrogen atom sits next to a vacancy, a place where a carbon atom is missing. In its negatively charged form, the NV centre holds a pair of electrons whose spins keep their quantum state for an exceptionally long time and can be read out with light. This makes NV centres a leading platform for quantum information processing and nanoscale quantum sensors. The team measured the diamond as grown, and again after NV centres had been created in it.
Separating the components
Muonium is an exotic atom made of a positive muon and an electron. It behaves like a light version of hydrogen and can move rapidly through a crystal, interacting with any defects it meets. In diamond, muonium can exist in more than one state; one sits in the open spaces between carbon atoms and moves rapidly, while another sits in the middle of a carbon–carbon bond and barely moves. These states also convert into one another, so the measured signal mixes contributions from all of them. The challenge for the researchers was to isolate the signal from the fast-moving muonium, which interacts with the defects.
The researchers used the EMU spectrometer at ISIS to measure how the muon spin relaxes in both samples at 290 K (room temperature) and 20 K, under a range of magnetic fields.
Type Ib diamond made an ideal test suject. Muonium interacts mainly with the two nitrogen-related defects, whose concentrations are approximately known. By fitting a model of the exchanging muonium states to the data, the team separated the signal into its components and measured the rate of each process.
The fast-moving muonium behaves very differently with the two defects. When it meets a substitutional nitrogen atom, the two swap electron spins. When it meets a negatively charged NV centre, it is captured and becomes non-magnetic, so the NV centres act as traps for muonium. This trapping was about three times faster at 20 K than at room temperature.
The same approach could potentially be applied to other industrially important semiconductors, such as silicon and silicon carbide, where this fast-moving muonium state is also known to exist.
Related publication: Muonium as a probe of point defects in type-Ib diamond | Phys. Rev. Research
