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arXiv:2104.09144 · Physical review. B./Physical review. B

Photoionization of negatively charged NV centers in diamond: Theory and ab initio calculations

Lukas Razinkovas, M. Maciaszek, Friedemann Reinhard, Marcus W. Doherty, Audrius Alkauskas

Published Dec 6, 2021• 58 citations
arXivHTMLPDF

Abstract

A nitrogen-vacancy center in diamond is a defect that has become a key platform for testing and eventually implementing various quantum technologies. During optical illumination, the center can change its charge state via a photoionization process. In this work, the authors present $a\phantom{\rule{0}{0ex}}b$ $i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o$ calculations of photoionization thresholds and cross sections of the nitrogen-vacancy center. The results explain several puzzling experimental findings.

DOI
10.1103/physrevb.104.235301

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A nitrogen-vacancy center in diamond is a defect that has become a key platform for testing and eventually implementing various quantum technologies. During optical illumination, the center can change its charge state via a photoionization process. In this work, the authors present a\phantom{\rule{0}{0ex}}b i\phantom{\rule{0}{0ex}}n\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}t\phantom{\rule{0}{0ex}}i\phantom{\rule{0}{0ex}}o calculations of photoionization thresholds and cross sections of the nitrogen-vacancy center. The results explain several puzzling experimental findings.
Nitrogen-vacancy (NV) complex in diamond is one of the most prominent solid state defects as the negatively charged NV defect (NV−) is a leading contender for quantum technologies. In quantum information processing applications, NV− is photoexcited that often leads to photoionization to neutral NV defect, NV0, and re-ionization back to NV− should occur to control the S=1 spin of NV−. As a consequence, understanding the photophysics of NV0 is crucial for controlling NV−. Furthermore, recent studies have shown that the S=1/2 electron spin of NV0 can also be initialized and read out at certain conditions that turns single NV0 a potential quantum bit. Quantum optics protocols rest on detailed knowledge on the electronic structure of the given system, which is obviously missing for NV0 in diamond. In this study, we combine the group theory and density functional theory calculations toward exploring the nature of the ground and excited states of NV0. We show that the effective three-electron system of NV0 leads to high correlation effects that make this system very challenging for ab initio simulations.
The negatively charged nitrogen-vacancy defect (NV)−^-− in diamond has attracted much attention in recent years in qubit and biological applications. The negative charge is donated from nearby nitrogen donors that could limit or stem the successful application of (NV)−^-−. In this Letter, we unambiguously identify the \emph{neutral} nitrogen-vacancy defect (NV0^00) by \emph{ab initio} supercell calculations. Our analysis shows that i) the spin state can be \emph{selectively} occupied optically, ii) the electron spin state can be manipulated by time-varying magnetic field, and iii) the spin state may be read out optically. Based on this NV0^00 is a new hope for realizing qubit in diamond \emph{without} the need of nitrogen donors.
The charge state stability of nitrogen-vacancy (NV) centers critically affects their application as quantum sensors and qubits. Understanding charge state conversion and equilibration is important not only for NV centers in diamond but also for defects and impurities in wide-bandgap materials in general. The mechanisms by which these centers change charge state upon optical or electronic excitation without the presence of mobile carriers remain unclear. Here, we elucidate this issue for the case of photoionization of NV center ensembles. Using pump-probe spectroscopy, we ionize negatively charged NV centers and monitor the recovery of NV^{-} on timescales of up to several seconds. We find that the recovery rate depends strongly on the concentration of surrounding nitrogen donors. Remarkably, the equilibration dynamics exhibit no discernible dependence on temperature, ruling out thermally activated processes. We develop a multiphonon-assisted electron-tunneling model, supported by density-functional calculations, that explains the measurements and identifies tunneling as the equilibration mechanism. Because tunneling rates depend exponentially on donor separation, the random distribution of nitrogen donors spreads recovery times from nanoseconds to hours or longer.
The negatively charged nitrogen-vacancy defect $({\text{NV}}^{\ensuremath{-}})$ in diamond has attracted much attention in recent years in qubit and biological applications. The negative charge is donated from nearby nitrogen donors that could limit or stem the successful application of ${\text{NV}}^{\ensuremath{-}}$. In this study, we identify the neutral nitrogen-vacancy defect (NV0)({\text{NV}}^{0})(NV0) by ab initio supercell calculations through the comparison of the measured and calculated hyperfine tensors of the 4A2{^{4}A}_{2}4A2​ excited state. Our analysis shows that (i) the spin state can be selectively occupied optically, (ii) the electron spin state can be manipulated by time-varying magnetic field, and (iii) the spin state may be read out optically. Based on this NV0{\text{NV}}^{0}NV0 is a hope for realizing qubit in diamond without the need of nitrogen donors. In addition, we propose that NV0{\text{NV}}^{0}NV0 may be more sensitive magnetometer than the ultrasensitive ${\text{NV}}^{\ensuremath{-}}$.
The negatively charged nitrogen-vacancy center in diamond is highly suited to many quantum information processing applications. Although the center has been comprehensively observed experimentally, there still remains contention regarding some of the key aspects of the current theoretical model of the center. In this article, the explicit development of the defect-molecule model of the center and the implications of the accumulated ab initio results are discussed. The aspects of the model that require further ab initio investigation are clearly identified and a possible Hartree-Fock extension is motivated.
The positively charged nitrogen vacancy (NV+) center in diamond has been traditionally treated as a dark state due to the experimental lack of an optical signature. Recent computational studies have shown that it is possible for the NV+ defect to have an excited state transition equivalent to that of the negatively charged (NV−) center, but no photoluminescence (PL) predictions have been reported so far. We report the first ab initio calculation showing that the NV+ center presents quantum emission, with zero phonon line at 765 nm and a non-zero transition dipole moment, approximately one quarter of the transition dipole moment of NV−. We calculate the energy levels of the multielectron states under the time-dependent density functional theory (singlet and triplet E states), and using our recently developed frequency cutoff method, we predict the full PL spectrum. Our results suggest that this state cannot be considered intrinsically “dark” and charge specific quenching mechanisms should be investigated as the cause of the lack of optical activity in experimental characterizations.