Summary
A Physical Review Letters study presents a first-principles framework for calculating internal conversion in optically active spin defects. The authors report quantitative agreement with experiments for the NV− centre in diamond and identify a previously overlooked nonradiative channel affecting a divacancy triplet lifetime in silicon carbide.
Researchers have developed a first-principles framework for calculating how quickly optically active spin defects lose electronic excitation without emitting light. In a Physical Review Letters paper published on 14 October 2026, Stefano Paolo Villani, Yu Jin and Giulia Galli report that their method agrees quantitatively with experiments on the negatively charged nitrogen-vacancy (NV−) centre in diamond. They also identify a previously overlooked nonradiative pathway affecting the triplet lifetime of a divacancy in silicon carbide (SiC).
Accounting for electronic and atomic motion
A spin defect is a disturbance in a crystal that can create electronic states with useful spin and optical properties. These defects are studied as components for quantum technologies. After a defect absorbs energy, it can return to a lower-energy state by emitting a photon or through a nonradiative process in which energy is transferred to the surrounding atomic motion.
The latter process is called internal conversion. Its rate matters because it helps determine how long an excited state lasts and how the defect’s optical behaviour can be predicted. The authors note that commonly used approximations can underestimate internal-conversion rates by orders of magnitude.
Their framework combines two features intended to improve those calculations. It incorporates multiconfigurational effects using many-body wave functions within time-dependent density functional theory (TDDFT), a computational approach for describing electronic excitations. It also includes contributions from all phonon modes through analytical nonadiabatic couplings. Phonons are quantised vibrations of a crystal; nonadiabatic couplings describe how atomic motion can connect different electronic states.
Tests in diamond and silicon carbide
For the NV− centre in diamond, the researchers report quantitative agreement between their calculated rates and experimental results. That comparison addresses a central problem with earlier approximations: if internal conversion is underestimated, calculations can misrepresent how an excited defect relaxes.
The team also applied the framework to the triplet lifetime of a divacancy in SiC. A divacancy consists of a neighbouring missing silicon atom and missing carbon atom in the crystal. The calculations revealed a nonradiative channel that had not previously been accounted for in that lifetime.
Together, the results show how a more complete treatment of electronic structure and crystal vibrations can change predictions of spin-defect dynamics. The work is computational, with its reported agreement against experiment for the NV− centre providing a key test of the approach.