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We apply density functional theory to investigate interactions between electronic and vibrational states in crystal defects with multi-mode dynamical Jahn-Teller (JT) systems. Our focus is on transitions between orbital singlet and degenerate orbital doublet characterized by $E \otimes (e \oplus e \oplus \cdots)$ JT coupling, which frequently occurs in crystal defects that are investigated for applications in quantum information science. We utilize a recently developed methodology to model the photoluminescence (PL) spectrum of the negatively charged split nickel-vacancy center (NiV$^{-}$) in diamond, where JT-active modes significantly influence electron-phonon interactions. Our results validate the effectiveness of the methodology in accurately reproducing the observed 1.4 eV PL lineshape. The strong agreement between our theoretical predictions and experimental observations reinforces the identification of the 1.4 eV PL center with the NiV$^{-}$ complex. This study highlights the critical role of JT-active modes in affecting optical lineshapes and demonstrates the power of advanced techniques for modeling optical properties in complex systems with multiple JT-active frequencies., Comment: 13 pages, 7 figures

Single photon emitters in hexagonal boron nitride offer a gateway to the future of quantum technologies, yet their identification remains challenging and subject to ongoing debate. We demonstrate through ab initio calculations that the optical properties of a carbon chain tetramer are in excellent agreement with the characteristics of a blue quantum emitter in hexagonal boron nitride emitting at 435 nm. Its calculated zero-phonon line energy (2.77 eV) and radiative lifetime (1.6 ns) perfectly align with experimental observations. The relatively weak electron--phonon coupling (Huang-Rhys factor of 1.5) indicates intense emission at the zero-phonon line. Despite the absence of an inversion center in the carbon tetramer, we demonstrate that it exhibits a negligible linear Stark effect, consistent with experimental findings. Additionally, our hypothesis explains the experimental observation that the formation of blue emitters is only possible in samples containing numerous ultraviolet emitters, which are likely identical to carbon dimers., Comment: 7 pages, 3 figures

The nitrogen-vacancy (NV) center in diamond is a heavily studied defect due to its potential applications to quantum metrology and computation, particularly in ambient conditions. The key mechanism to using the NV in any application lies in the ability to read out the spin state of the defect which is typically done optically. The optical contrast is then the key metric for electron spin readout fidelity and one of the key limiting factors in the NV's overall performance. We present a new mechanism for high contrast readout using the spin-to-charge conversion (SCC) mechanism in conjunction with an electrode to improve the spin contrast by altering the NV energy levels relative to the diamond conduction band. Theoretical modelling predicts an optical spin contrast at 42% which would be the highest optical contrast for the NV at room temperature and the technique opens up a range of alternative research pathways for the NV which are discussed., Comment: 6 pages, 5 figures + 4 pages and 3 figures in the appendix

We investigate the optical properties of silicon-vacancy (SiV) and germanium-vacancy (GeV) color centers in nanodiamonds under hydrostatic pressure up to 180 GPa. The nanodiamonds were synthetized by Si or Ge-doped plasma assisted chemical vapor deposition and, for our experiment, pressurized in a diamond anvil cell. Under hydrostatic pressure we observe blue-shifts of the SiV and GeV zero-phonon lines by 17 THz (70 meV) and 78 THz (320 meV), respectively. These measured pressure induced shifts are in good agreement with ab initio calculations that take into account the lattice compression based on the equation of state of diamond and that are extended to the case of the tin-vacancy (SnV) center. This work provides guidance on the use of group-IV-vacancy centers as quantum sensors under extreme pressures that will exploit their specific optical and spin properties induced by their intrinsic inversion-symmetric structure., Comment: 7 pages, 4 figures

We present a first-principles computational study of the thermodynamics of carbon defects in hexagonal boron nitride (hBN). The defects considered are carbon monomers, dimers, trimers, and larger carbon clusters, as well as complexes of carbon with vacancies, antisites, and substitutional oxygen. Our calculations show that monomers ($\text{C}_{\text{B}}$, $\text{C}_{\text{B}}$), dimers, trimers, and $\text{C}_{\text{N}}\text{O}_{\text{N}}$ pairs are the most prevalent species under most growth conditions. Compared to these defects, larger carbon clusters, as well as complexes of carbon with vacancies and antisites, occur at much smaller concentrations. Our results are discussed in view of the relevance of carbon defects in single-photon emission in hBN., Comment: 13 pages, 6 figures and supplemental material

We present ab-initio calculations of photoionization thresholds and cross sections of the negatively charged nitrogen-vacancy (NV) center in diamond from the ground $^{3}\!A_2$ and the excited $^{3}\!E$ states. We show that after the ionization from the $^{3}\!E$ level the NV center transitions into the metastable $^{4}\!A_2$ electronic state of the neutral defect. We reveal how spin polarization of $\mathrm{NV}^{-}$ gives rise to spin polarization of the $^{4}\!A_2$ state, providing an explanation of electron spin resonance experiments. We obtain smooth photoionization cross sections by employing dense $k$-point meshes for the Brillouin zone integration together with the band unfolding technique to rectify the distortions of the band structure induced by artificial periodicity of the supercell approach. Our calculations provide a comprehensive picture of photoionization mechanisms of $\mathrm{NV}^{-}$. They will be useful in interpreting and designing experiments on charge-state dynamics at NV centers. In particular, we offer a consistent explanation of recent results of spin-to-charge conversion of NV centers., Comment: 14 pages, 9 figures

We present a theoretical study of vibrational and vibronic properties of a point defect in the dilute limit by means of first-principles density functional theory calculations. As an exemplar we choose the negatively charged nitrogen-vacancy center, a solid-state system that has served as a testbed for many protocols of quantum technology. We achieve low effective concentrations of defects by constructing dynamical matrices of large supercells containing tens of thousands of atoms. The main goal of the paper is to calculate luminescence and absorption lineshapes due to coupling to vibrational degrees of freedom. The coupling to symmetric $a_1$ modes is computed via the Huang-Rhys theory. Importantly, to include a nontrivial contribution of $e$ modes we develop an effective methodology to solve the multi-mode $E \otimes e$ Jahn-Teller problem. Our results show that for NV centers in diamond a proper treatment of $e$ modes is particularly important for absorption. We obtain good agreement with experiment for both luminescence and absorption. Finally, the remaining shortcomings of the theoretical approach are critically reviewed. The presented theoretical approach will benefit identification and future studies of point defects in solids., Comment: 21 pages, 17 figures

Silicon-vacancy (SiV) center in diamond is a photoluminescence (PL) center with a characteristic zero-phonon line energy at 1.681 eV that acts as a solid-state single photon source and, potentially, as a quantum bit. The majority of the luminescence intensity appears in the zero-phonon line; nevertheless, about 30\% of the intensity manifests in the phonon sideband. Since phonons play an essential role in the operation of this system, it is of importance to understand the vibrational properties of the SiV center in detail. To this end, we carry out density functional theory calculations of dilute SiV centers by embedding the defect in supercells of a size of a few thousand atoms. We find that there exist two well-pronounced quasi-local vibrational modes (resonances) with $A_{2u}$ and $E_u$ symmetries, corresponding to the vibration of the Si atom along and perpendicular to the defect symmetry axis, respectively. Isotopic shifts of these modes explain the isotopic shifts of prominent vibronic features in the experimental SiV PL spectrum. Moreover, calculations show that the vibrational frequency of the $A_{2u}$ mode increases by about 30\% in the excited state with respect to the ground state, while the frequency of the $E_u$ mode increases by about 5\%. These changes explain experimentally observed isotopic shifts of the zero-phonon line energy. We also emphasize possible dangers of extracting isotopic shifts of vibrational resonances from finite-size supercell calculations, and instead propose a method to do this correctly., Comment: 9 pages, 6 figures, 2 tables