Bright oxygen- and vacancy-derived spin-singlet diamond color centers with metastable spin triplets: OV$^{2+}$ and VOV$^{2+}$

TL;DR

This study uses advanced quantum mechanical calculations to identify the atomic structure of the ST1 diamond color center, revealing it as a V$_C$O$_C$V$_C^{2+}$ defect with specific spin states and optical properties.

Contribution

The paper provides the first detailed theoretical identification of the ST1 center as a V$_C$O$_C$V$_C^{2+}$ defect with matching optical excitation energies.

Findings

V$_C$O$_C$V$_C^{2+}$ has a spin-singlet ground state and metastable triplet state.

Calculated excitation energies closely match experimental zero phonon line.

O$_C$V$_C^{2+}$ absorbs at higher energy (2.8 eV) compared to ST1.

Abstract

The ST1 diamond color center was experimentally demonstrated to involve a substitutional oxygen atom (O$_C$) and carbon vacancy (V$_C$), has a spin singlet ground-state, and a metastable electron spin ancilla: a triplet. ST1's structure was left unsolved for more than a decade. With embedded multiconfigurational quantum mechanical theory, we investigate O$_C$-V$_C$-derived diamond defects, specifically both 0 and +2-charged coupled O$_C$V$_C$, and O$_C$ surrounded by V$_C$s along the [110] axis (V$_C$O$_C$V$_C$). We found both O$_C$V$_{C}^{2+}$ (C$_{3v}$) and V$_C$O$_C$V$_{C}^{2+}$ (C$_{2v}$) to have a spin-singlet ground state (1$^1$A$_1$) and metastable spin triplets. We demonstrate ST1 to be V$_C$O$_C$V$_{C}^{2+}$. The calculated vertical excitation energies of V$_C$O$_C$V$_{C}^{2+}$'s first (1$^1$B$_2$) and second (2$^1$A$_1$) bright spin-singlet excited states closely match ST1's experimental zero phonon line (2.2-2.3 eV). O$_C$V$_{C}^{2+}$ ($^1$E) absorbs much higher (2.8 eV). The two O lone pairs favor V$_C$O$_C$V$_C$ over O$_C$V$_C$, in a similar manner as the single N lone pair favors formation of N$_C$V$_C$ centers.