The divacancy in silicon carbide (SiC) is a prominent solid state defect quantum bit that bears a relatively strong fluorescence and optically detected magnetic resonance contrast (ODMR) at room temperature. These properties exemplify it for quantum sensing of biological molecules. To this end, we previously developed a top-down method to create divacancies in cubic SiC nanoparticles (NPs) as non-perturbative ODMR biomarkers. In this process, large SiC particles are synthesized and then stain etched to form porous SiC and then ultrasonication and filtering are applied to the solution to extract few nanometer diameter SiC NPs. We called this process no-photon exciton generation chemistry (NPEGEC). We showed that by adding aluminum to carbon and silicon in the synthesis process of cubic SiC, one can engineer divacancy defects in SiC NPs by NPEGEC. An alternative traditional way to introduce vacancies to the SiC lattice is irradiation. Here, we compare the fluorescence spectra of divacancies as created by neutron irradiation in porous cubic SiC and NPEGEC technique in SiC NPs, and the results are analyzed in detail by means of first principles calculations. We find that the irradiation technique produces a larger shift in the fluorescence spectrum with residual background fluorescence than that for divacancies in SiC NPs, which is most likely caused by the parasitic defects left after irradiation and annealing in the former sample. These results imply that the chemistry technique applied to prepare divacancies in few nanometer SiC NPs may preserve the bulk-like quality of divacancy quantum bits near the surface.

Divacancy in its neutral charge state (VCVSi) in 4H silicon carbide (SiC) is a leading quantum bit (qubit) contender. Owing to the lattice structure of 4H SiC, four different VCVSi configurations can be formed. The ground and the optically accessible excited states of VCVSi configurations exhibit a high-spin state, and the corresponding optical transition energies are around ≈ 1.1 eV falling in the near-infrared wavelength region. Recently, photoluminescence (PL) quenching has been experimentally observed for all VCVSi configurations in 4H SiC at cryogenic temperatures. It has been shown that VCVSi is converted to VCVSi and it remains in this shelving dark state at cryogenic temperatures until photoexcitation with the threshold energies or above is applied to convert VCVSi back to VCVSi. In this study, we demonstrate both in experiments and theory that the threshold energy for reionization is temperature dependent. We carry out density functional theory (DFT) calculations in order to investigate the temperature dependent reionization spectrum, i.e., the spectrum of the VCVSi → VCVSi process. We find that simultaneous optical reionization and qubit manipulation can be carried out at room temperature with photoexcitation at the typical excitation wavelength used for readout of the divacancy qubits in 4H SiC, in agreement with our experimental data. We also provide the analysis of the PL spectrum of VCVSi, characteristic for each VCVSi configuration in 4H SiC, using the Huang-Rhys theory, and find that one configuration in 4H SiC stands out in terms of the strength of coherent emission among the four configurations. 1 ar X iv :2 10 7. 01 97 1v 2 [ co nd -m at .m es -h al l] 2 7 Fe b 20 22

Divacancy in its neutral charge state (V_CV_Si^0) in 4H silicon carbide (SiC) is a leading quantum bit (qubit) contender. Owing to the lattice structure of 4H SiC four different V_CV_Si configurations can be formed. Ground and optical excited states of V_CV_Si^0 exhibit S=1 spintriplet state and the corresponding transition energies are around ≈ 1.1 eV falling in the near-infrared wavelength region. Recently, photoluminescence (PL) quenching has been experimentally observed for all V_CV_Si configurations in 4H SiC, i.e. the corresponding zero-phonon lines (ZPLs) appear only at higher-than-ZPL photoexcitation energies (threshold energies). It has been shown that V_CV_Si^0 is converted to V_CV_Si^- upon photoexcitation below the correspoding excitation threshold energies at cryogenic temperature, i.e. V_CV_Si^- is the so-called dark state. In this study we demonstrate that the threshold energy for reinozation is temperature dependent. We further carry out density functional theory (DFT) calculations in order to investigate the temperature dependent reionization spectrum, i.e. the spectrum of the V_CV_Si^- → V_CV_Si^0 process and found that simultaneous reionization and qubit manipulation can be carried out at around room temperature (≈300 K) by using the usually applied excitation wavelength. We also investigate the PL lineshape of V_CV_Si^0 by using the Huang-Rhys theory.

In this review paper, we list the basic properties of the most relevant point defect quantum bits or quantum bit candidates that have been harnessed to realize single photon sources and quantum devices, or they can be potentially employed to this end. This review demonstrates that joint efforts of ab initio simulations and experiments were very fruitful to achieve ground breaking results in thisfield.

The negatively charged silicon vacancy [VSi(−)] in silicon carbide (SiC) is a paramagnetic and optically active defect in hexagonal SiC. VSi(−) defect possesses S = 3/2 spin with long spin coherence time and can be optically manipulated even at room temperature. Recently, electron spin resonance signals have been observed besides the signals associated with the VSi(−) defects in the 4H polytype of SiC. The corresponding centers share akin properties to those of the VSi(−) defects and thus they may be promising candidates for quantum technology applications. However, the exact origin of the new signals is unknown. In this paper we report VSi(−)-related pair defect models as possible candidates for the unknown centers. We determine the corresponding electronic structures and magneto-optical properties as obtained by density functional theory (DFT) calculations. We propose models for the recently observed electron paramagnetic resonance centers with predicting their optical signals for identification in future experiments.

Paramagnetic point defects in solids may exhibit a rich set of interesting and not yet fully resolved physics. In particular, character of wave functions and electron-phonon coupling in these defects may highly influence their interaction with external magnetic fields. Complex interplay among the electronic orbitals, phonons, and electron spin determines the effective pseudospin of the system that we demonstrate on vanadium and molybdenum defects in hexagonal silicon carbide by means of ab initio calculations. In this Rapid Communication, we find a giant anisotropy in the g tensor of these defects with Kramers doublet spin ground state, resulting in reduced and vanishing interaction with the magnetic field in parallel and transverse directions, respectively. The consequences of our finding in the application of these defects for quantum information processing are briefly discussed.

In this work, quenching effect in the photoluminescence (PL) spectrum of divacancy defects in 4H SiC is investigated. Quenching in PL occurs when photoexcitation with an energy below a certain threshold is applied. In order to understand this phenomenon, we carried out Kohn-Sham density functional theory (DFT) calculations. In accordance with recent experimental results, we propose a physical model which explains the quenching phenomenon.

We study the optical properties of tetravalent vanadium impurities in 4H silicon carbide (4H SiC). Emission from two crystalline sites is observed at wavelengths of 1.28 \mum and 1.33 \mum, with optical lifetimes of 163 ns and 43 ns. Group theory and ab initio density functional supercell calculations enable unequivocal site assignment and shed light on the spectral features of the defects. We conclude with a brief outlook on applications in quantum photonics.

We investigate the quenching of the photoluminescence (PL) from the divacancy defect in 4H-SiC consisting of a nearest-neighbor silicon and carbon vacancies. The quenching occurs only when the PL is excited below certain photon energies (thresholds), which differ for the four different inequivalent divacancy configurations in 4H-SiC. An accurate theoretical ab initio calculation for the charge-transfer levels of the divacancy shows very good agreement between the position of the (0/-) level with respect to the conduction band for each divacancy configuration and the corresponding experimentally observed threshold, allowing us to associate the PL decay with conversion of the divacancy from neutral to negative charge state due to capture of electrons photoionized from other defects (traps) by the excitation. Electron paramagnetic resonance measurements are conducted in the dark and under excitation similar to that used in the PL experiments and shed light on the possible origin of traps in the different samples. A simple model built on this concept agrees well with the experimentally observed decay curves.

As a novel procedure for determining dislocation density, a software was improved with which data obtained by Scanning Electron Microscope (SEM) measurements can be collected and the value of superficial dislocation density can be calculated. Applying this method we investigated cold rolled lath martensitic steel samples. Besides dislocation density values, microstructure mapped by Electron Backscatter Diffraction (EBSD) will be discussed.

Fluorescent paramagnetic defects in solids have become attractive systems for quantum information processing in the recent years. One of the leading contenders is the negatively charged nitrogen-vacancy defect in diamond with visible emission but alternative solution in technologically mature host is an immediate quest for many applications in this field. It has been recently found that various polytypes of silicon carbide (SiC), that are standard semiconductors with wafer scale technology, can host nitrogen-vacancy defect (NV) that could be an alternative qubit candidate with emission in the near infrared region. However, it is much less known about this defect than its counterpart in diamond. The inequivalent sites within a polytype and the polytype variations offer a family of NV defects. However, there is an insufficient knowledge on the magneto-optical properties of these configurations. Here we carry out density functional theory calculations, in order to characterize the numerous forms of NV defects in the most common polytypes of SiC including 3C, 4H and 6H, and we also provide new experimental data in 4H SiC. Our calculations mediate the identification of individual NV qubits in SiC polytypes. In addition, we discuss the formation of NV defects in SiC with providing detailed ionization energies of NV defect in SiC which reveals the critical optical excitation energies for ionizing this qubits in SiC. Our calculations unravel the challenges to produce NV defects in SiC with a desirable spin bath.

Paramagnetic defects in solids have become attractive systems for quantum computing as well as magnetometry in recent years. One of the leading contenders is the negatively charged nitrogen-vacancy defect (NV center) in diamond proposed to be highly promising with respect the afore-mentioned applications. In our study we investigate the NCVSi defect in 3C, 4H and 6H SiC as alternative choices with superior properties. Electronic structure of NV center in SiC exhibits S = 1 triplet ground state with the possibility of optical spin polarization. On the other hand, our results obtained by density functional theory calculations may contribute to unambiguously identify the possible defect configurations.

The microstructure and the dislocation density in as-quenched ferrous lath martensite were studied by different methods. The blocks, packets and variants formed due to martensitic transformation were identified and their sizes were determined by electron backscatter diffraction (EBSD). Concomitant transmission electron microscopy (TEM) investigation revealed that the laths contain subgrains with the size between 50 and 100nm. A novel evaluation procedure of EBSD images was elaborated for the determination of the density and the space distribution of geometrically necessary dislocations from the misorientation distribution. The total dislocation density obtained by X-ray diffraction line profile analysis was in good agreement with the value determined by EBSD, indicating that the majority of dislocations formed due to martensitic transformation during quenching are geometrically necessary dislocations.

The outstanding magneto-optical properties of the nitrogen-vacancy (NV) center in diamond have stimulated the search for similar systems. We show here that NV triplet centers can also be generated in all the main SiC polytypes. We have identified by electron paramagnetic resonance spectroscopy and first-principles calculations the axial NV- pairs in 3C, 4H, and 6H SiC, showing polytype and lattice site-specific magnetic and optical properties. We demonstrate very efficient room-temperature spin polarization of the ground state upon near infrared optical excitation for the NV center in 3C SiC and axial NV centers in the hexagonal (4H, 6H) polytypes; the signals of basal pairs are much lower in intensity. Axial NV centers in hexagonal SiC polytypes and thus constitute unidirectional ensembles which may be useful in nanosensing applications.

We investigated Molybdenum (Mo) defects in 4H silicon carbide (SiC). This system can be suitable candidate for in vivo biomarker applications since it shows photoluminescence (PL) in the near-infrared (NIR) region. In order to reveal the origin of this Mo-related PL center we carried out ab initio density functional theory (DFT) calculations on two microscopic models.

From the lattice orientation of a sample, elements of the Nye-tensor can be determined. With the help of Nye’s tensor, dislocation density can be calculated for the certain sample. Since the measures were carried out with scanning electronmicroscope (SEM), just superficial orientations can be measured. Hence the Nye-tensor is an incomplete matrix, with five elements. Because of the absence of the other four elements just a quasi-dislocation density can be obtained. The algorithm of the calculation was programmed on the language C#.