Color centers in hexagonal boron nitride monolayers: A group theory and ab-initio analysis (1709.05414v2)

Abstract: We theoretically study physical properties of the most promising color center candidates for the recently observed single-photon emissions in hexagonal boron nitride (h-BN) monolayers. Through our group theory analysis combined with density functional theory (DFT) calculations we provide several pieces of evidence that the electronic properties of the color centers match the characters of the experimentally observed emitters. We calculate the symmetry-adapted multi-electron wavefunctions of the defects using group theory methods and analyze the spin-orbit and spin-spin interactions in detail. We also identify the radiative and non-radiative transition channels for each color center. An advanced ab-initio DFT method is then used to compute energy levels of the color centers and their zero-phonon-line (ZPL) emissions. The computed ZPLs, the profile of excitation and emission dipole polarizations, and the competing relaxation processes are discussed and matched with the observed emission lines. By providing evidence for the relation between single-photon emitters and local defects in h-BN, this work provides the first steps towards harnessing quantum dynamics of these color centers.

Analysis of Color Centers in Hexagonal Boron Nitride Monolayers

The paper "Color centers in hexagonal boron nitride monolayers: A group theory and ab initio analysis" presents a comprehensive theoretical investigation of the physical properties of color centers in hexagonal boron nitride (h-BN) monolayers. This paper is an integration of group theory analysis and ab initio density functional theory (DFT) calculations aimed at identifying the relationship between electronic characteristics of these color centers and the single-photon emissions observed in experiments.

In recent years, two-dimensional (2D) materials, particularly h-BN, have garnered significant interest in the context of quantum technologies. These materials offer promising attributes for applications ranging from quantum nanophotonics to quantum information processing. The discovery of single-photon emissions from h-BN monolayers has driven efforts to pinpoint the origins of these emissions, believed to be linked to defect-derived color centers.

The paper highlights several key theoretical predictions:

1. Electronic Structure of Defects: The authors focus on two primary defects in h-BN: the neutral complex anti-site $\mathrm{V_NN_B}$ and the negatively charged boron vacancy $\mathrm{V_B}^{-}$. Their group theoretical analysis, coupled with advanced DFT calculations, explores the symmetry-adapted multi-electron wavefunctions and comprehensively details the spin-orbit and spin-spin interactions relevant to these color centers.
2. Transition Channels and Energy Levels: Radiative and non-radiative transition channels were identified, offering insights into the observed emission lines. Specifically, the zero-phonon-line (ZPL) emissions and excitation/emission dipole polarizations are calculated, providing a match to experimental data. For example, the calculated ZPL energies for $\mathrm{V_NN_B}$ and $\mathrm{V_B}^{-}$ are approximately $2.05$ eV and $1.92$ eV, respectively.
3. Comparison with Experimental Observations: The paper correlates its findings with two types of experimentally observed single-photon emitters in h-BN: ones with broader emission line shapes and in-plane polarization symmetries, and those with sharper emission lines and distinct polarization patterns between excitation and emission. These observations are grounded in the calculations for the identified defect candidates.
4. Defect Stability and Charge States: The paper discusses the relative stabilities of different charge states for $\mathrm{V_B}$ and $\mathrm{V_NN_B}$ defects under varying growth conditions (N-rich vs. B-rich). This is crucial for determining the prevalence and role of these defects in h-BN under experimental growth conditions.
5. Potential Applications: Beyond understanding the origins of emissions, this work suggests that the properties of these defects can be harnessed for practical quantum applications, particularly leveraging their spin properties for spin-photon interfaces.

By presenting a rigorous theoretical framework, this paper enhances our understanding of quantum emitters in h-BN monolayers, setting the stage for future experimental validations and potential advancements in quantum material applications. The integration of group theory and DFT methodologies provides a robust pathway for predicting electronic structures and transition dynamics of defects in 2D materials, further contributing valuable insights into the field of quantum optics and materials science.