Researchers reveal spin-orbit effects on exciton complexes in diamond

Diamonds have a range of industrial applications, such as in solid-state electronics. New technologies aim to provide high-purity synthetic crystals to act as semiconductors when doped with impurities as electron donors or acceptors of other elements. These extra electrons — or holes — do not participate in atomic bonding but sometimes bind to excitons in semiconductors and other condensed matter. Doping may cause physical changes, but how the exciton complex — a bound state of two positively charged holes and one negatively charged electron — manifests in diamonds doped with boron is unconfirmed, as two conflicting interpretations exist of the exciton’s structure.

Now, a team of researchers led by Kyoto University has determined the magnitude of the spin-orbit interaction in acceptor-bound excitons in a semiconductor. Team leader Nobuko Naka from Kyoto University said the researchers broke through the energy resolution limit of conventional luminescence measurements by observing the fine structure of bound excitons in boron-duped blue diamond, using optical absorption.

“We hypothesised that, in an exciton, two positively charged holes are more strongly bound than an electron-and-hole pair. This acceptor-bound exciton structure yielded two triplets separated by a spin-orbit splitting of 14.3 meV, supporting the hypothesis,” said first author Shinya Takahashi.

Luminescence resulting from thermal excitation can be used to observe high-energy states, but this measurement method broadens spectral lines and blurs ultra-fine splitting. Instead, the researchers cooled the diamond crystal to cryogenic temperatures, obtaining nine peaks on the deep-ultraviolet absorption spectrum, compared to the usual four using luminescence. The researchers also developed an analytical model including the spin-orbit effect to predict the energy positions and absorption intensities. Going forward, the researchers are considering the possibility of measuring absorption under external fields, leading to further line splitting and validation due to the changes in symmetry.

“Our results provide useful insights into spin-orbit interactions in systems beyond solid-state materials, such as atomic and nuclear physics. A deeper understanding of materials may improve the performance of diamond devices, such as light-emitting diodes, quantum emitters and radiation detectors,” Naka said.

Image caption: Highly precise optical absorption spectra of diamond reveal ultra-fine splitting. Image credit: KyotoU/Nobuko Naka.