Bonding Character as a Descriptor for Huang-Rhys Factors in Optically Active Defects

Abstract: The electron phonon coupling of a defect characterized by its Huang Rhys (HR) factor is a crucial metric determining its excited-state dynamics, relevant to defect applications as qubits and quantum emitters. However, HR factors remain challenging to calculate from first principles, complicated by convergence issues in excited-state relaxation and time consuming phonon calculations. Even when calculated, HR factors lack a rational design principle. Here we show that an orbital-based descriptor can be used to rationalize and efficiently estimate HR factors. Combining this descriptor with a ground state deformation technique allows circumventing both excited state relaxation and full phonon calculations. Specifically, our descriptor for HR factors is constructed using bonding character differences obtained from ground state density functional theory, measured using crystal orbital Hamilton populations. We demonstrate this descriptor for prototypical hBN defects and the diamond NV center. This orbital-based descriptor can be potentially used in high throughput computational screening to identify ideal candidates of spin qubits and SPEs.