New Simulation Method Shows Silicon Carbide Defects Lose More Energy to Atomic Vibrations Than Expected

Tiny flaws in crystals such as diamond and silicon carbide are being studied as building blocks for quantum computers and sensors. Researchers at the University of Chicago Pritzker School of Molecular Engineering (UChicago PME) now report a computational method that models how these defects respond to light more completely than before. In doing so, they found that one energy-loss pathway in silicon carbide matters much more than scientists had assumed.

The work, published in Physical Review Letters, focuses on so-called spin defects. When a laser excites one of these defects, it can return to its lower-energy state in several ways. It can release a photon, switch between states with different spins, or hand its energy to vibrations of the surrounding atoms. That last route is called internal conversion.

According to the researchers, internal conversion has been especially hard to model. The basic equations of quantum mechanics are known, but applying them is difficult. "You need to account for complex interactions between the electrons and surrounding atoms, and that's challenging in realistic solids represented by models with hundreds or thousands of atoms," said Stefano Villani, a UChicago PME postdoctoral researcher and the study's first author.

To get around this, Giulia Galli, the study's senior author, Villani and Yu Jin combined several computational techniques developed by Galli's group over many years. Jin, a former graduate student in the group, is now a postdoctoral scholar at the Flatiron Institute in New York. The team used mathematical descriptions called many-body wavefunctions to represent the excited states of the defects. Through exact mathematical derivations, they worked out how strongly each excited state interacts with the motions of nearby atoms.

That step let them calculate how individual electronic states connect to all possible vibrational configurations of hundreds of surrounding atoms, rather than analyzing each configuration one by one. The researchers built the framework into WEST, an open-source simulation program optimized for graphics processing units. Galli's group has developed WEST over the past decade through the Midwest Integrated Center for Computational Materials, which the U.S. Department of Energy's Office of Basic Energy Sciences has funded since 2015.

As a test, the team first examined a nitrogen-vacancy center in diamond, a widely studied quantum defect. Their calculations closely matched recent spectroscopy measurements. For triplet states, internal conversion contributed relatively little to energy loss, while in singlet states it was the main mechanism for releasing energy. Both results agreed with what scientists had previously expected.

Silicon carbide told a different story. For a divacancy defect in that material, the calculations showed internal conversion has a much greater influence on the lifetime of the excited triplet state than earlier models had accounted for. The researchers say this underestimated contribution helps explain a long-standing disagreement between theoretical predictions and experimental observations.

"We were expecting to see an effect, but of a smaller magnitude," Galli said. "What we found is that this internal conversion process turns out to be more important than we thought."

The researchers say a better grasp of this energy-loss pathway could help scientists predict the behavior of silicon carbide defects more accurately and improve their potential use as qubits in future quantum devices. Because internal conversion also appears in processes relevant to energy technologies, the method may prove useful for studying materials involved in renewable energy conversion and storage.

There are limits. Galli noted that the simulations still demand considerable computing power. The team is exploring ways to lower those demands, including machine learning. "We are working on ways to integrate machine learning into excited states calculations to further improve the efficiency of our method," Villani said.

The group is also extending the approach to spin defects in silicon and to energy materials where internal conversion shapes how energy is transferred or lost. With the framework available through WEST, researchers at other institutions can begin applying it to their own work.

"We provided the missing piece of the puzzle needed to completely describe the optical cycle of spin defects," Galli said.