Internal conversion resolves silicon carbide divacancy lifetime discrepancy

A new computational framework models internal conversion alongside photon emission and spin transitions to describe how spin defects relax after laser excitation. Researchers in the Galli Group at the University of Chicago Pritzker School of Molecular Engineering built the framework, which captures the full optical cycle of spin defects. When a spin defect is excited with a laser, it can relax via photon emission, spin-state jumping, or internal conversion. Internal conversion involves energy dissipation as atomic vibrations rather than light. Physicists have used quantum mechanics to describe photon emission and spin-state jumping, but internal conversion was harder to predict.

Method overcomes modeling challenges with many-body wavefunctions

By using many-body wavefunctions and exact derivations to couple electronic states to atomic vibrations across hundreds of atoms, the framework enables accurate prediction of internal conversion in realistic solids. The difficulty in modeling internal conversion arises from needing to account for complex electron-atom interactions in realistic solids with hundreds or thousands of atoms. Stefano Villani, a UChicago PME postdoctoral researcher and the study's first author, said, "Although the equations are known, it's hard to describe the process accurately because of computational difficulties." The method calculates coupling to all vibrational configurations of hundreds of nearby atoms simultaneously.

The framework relies on the WEST code, an open-source, GPU-optimized simulation program developed by Galli’s group over the past decade. Funding for the WEST code came from the U.S. Office of Basic Energy Sciences through the Midwest Integrated Center for Computational Materials (MICCoM). The framework’s success depended on a complete mathematical derivation and efficient algorithms for implementation.

Results show internal conversion dominates in silicon carbide divacancies

Applied to silicon carbide divacancies, the framework revealed internal conversion as a substantial, previously overlooked contributor to triplet excited-state lifetime. The result resolves a long-standing discrepancy between theoretical predictions and experimental measurements of lifetime in silicon carbide divacancies. This finding challenges existing assumptions about silicon carbide divacancies as potential qubits. The silicon carbide results were particularly revealing compared to diamond nitrogen-vacancy centers.

For triplet states in diamond NV centers, internal conversion plays little role in energy loss. When applied to a nitrogen-vacancy center in diamond, the framework's predictions matched recent spectroscopy measurements, confirming established understanding of singlet and triplet state behavior. Giulia Galli, the Liew Family professor of electronic structure and simulations and the study's senior author, noted the team expected an effect but of a smaller magnitude.

Framework validated and enables broader use

The framework is now available in the open-source WEST code for other researchers to apply to quantum and energy materials. Yu Jin, a former graduate student in the Galli group and now a postdoctoral scholar at the Flatiron Institute, said the work can now be used to make more accurate predictions about many materials of interest for quantum technologies. Galli cautioned that the framework requires substantial computational resources.

The team is working to integrate machine learning into excited-state calculations to improve efficiency. They are focused on reducing computational demands and broadening applicability to other materials. Giulia Galli said, "This internal conversion process is not only relevant to spin defects-it's also related to processes of interest to energy technologies." The framework could be applied to renewable energy conversion and storage systems. The team is extending the framework to spin defects in silicon and to energy materials where internal conversion matters.