A charge transfer mechanism for optically addressable solid-state spin pairs

TL;DR

This paper introduces a universal charge transfer model for optically addressable spin pairs in hexagonal boron nitride, explaining their ODMR features through defect pair interactions inspired by spin chemistry.

Contribution

It proposes the optical-spin defect pair (OSDP) model, combining experimental measurements and first-principle calculations to explain ODMR phenomena in solid-state defects.

Findings

The OSDP model accounts for all key experimental ODMR features.

Charge transfer creates a metastable spin pair with observable ODMR.

Simple defect pairs of common carbon defects are plausible microscopic sources.

Abstract

Optically detected magnetic resonance (ODMR) with no resolvable zero-field splitting has been observed from emitters in hexagonal boron nitride across a broad range of wavelengths, but so far an understanding of their microscopic structure and the physical origin of ODMR has been lacking. Here we perform comprehensive measurements and modelling of the spin-resolved photodynamics of ensembles and single emitters, and uncover a universal model that accounts, and provides an intuitive physical explanation, for all key experimental features. The model, inspired by the radical-pair mechanism from spin chemistry, assumes a pair of nearby point defects -- a primary optically active defect and a secondary defect. Charge transfer between the two defects creates a metastable weakly coupled spin pair with ODMR naturally arising from selection rules. Using first-principle calculations, we show that simple defect pairs made of common carbon defects provide a plausible microscopic explanation. Our optical-spin defect pair (OSDP) model resolves several previously open questions including the asymmetric envelope of the Rabi oscillations, the large variability in ODMR contrast amplitude and sign, and the wide spread in emission wavelength. It may also explain similar phenomena observed in other wide bandgap semiconductors such as GaN. The presented framework will be instrumental in guiding future theoretical and experimental efforts to study and engineer solid-state spin pairs.