Electron-Nuclear Coherent Coupling and Nuclear Spin Readout through Optically Polarized VB- Spin States in hBN

Coherent coupling of defect spins with surrounding nuclei along with the endowment to read out the latter are basic requirements for an application in quantum technologies. We show that negatively charged boron vacancies (V_B^-) in hexagonal boron nitride (hBN) meet these prerequisites. We demonstrate Hahn-echo coherence of the V_B^- spin with a characteristic decay time T_coh = 15 μs, close to the theoretically predicted limit of 18 μs for defects in hBN. Elongation of the coherence time up to 36 μs is demonstrated by means of the Carr-Purcell-Meiboom-Gill decoupling technique. Modulation of the Hahn-echo decay is shown to be induced by coherent coupling of the V_B^- spin with the three nearest ¹⁴N nuclei via a nuclear quadrupole interaction of 2.11 MHz. DFT calculation confirms that the electron-nuclear coupling is confined to the defective layer and stays almost unchanged with a transition from the bulk to the single layer.

Study of Electron-Nuclear Interactions in Doped Calcium Phosphates by Various Pulsed EPR Spectroscopy Techniques

Substituted calcium phosphates (CaPs) are vital materials for the treatment of bone diseases and repairing and replacement of defects in human hard tissues. In this paper, we present some applications of the rarely used pulsed electron paramagnetic resonance (EPR) and hyperfine interaction spectroscopy approaches [namely, electron spin-echo envelope modulation (ESEEM) and electron-electron double-resonance detected nuclear magnetic resonance (EDNMR)] to investigate synthetic CaPs (hydroxyapatite, tricalcium, and octacalcium phosphate) doped with various cations (Li+, Na+, Mn2+, Cu2+, Fe3+, and Ba2+). These resonance techniques provide reliable tools to obtain unique information about the presence and localization of impurity centers and values of hyperfine and quadrupole tensors. We show that revealed in CaPs by EPR techniques, radiation-induced stable nitrogen-containing species and carbonate radicals can serve as sensitive paramagnetic probes to follow CaPs' structural changes caused by cation doping. The most pulsed EPR, ESEEM, and EDNMR spectra can be detected at room temperature, reducing the costs of the measurements and facilitating the usage of pulsed EPR techniques for CaP characterization.

X-Ray Diffraction and Multifrequency EPR Study of Radiation-Induced Room Temperature Stable Radicals in Octacalcium Phosphate

Octacalcium phosphate (OCP) {Ca8H2(PO4)6×5H2O] has attracted increasing attention over the last decade as a transient intermediate to the biogenic apatite for bone engineering and in studies involving the processes of pathological calcification. In this work, OCP powders obtained by hydrolysis of dicalcium phosphate dehydrate were subjected to X- and γ-ray irradiation and studied by means of stationary and pulsed electron paramagnetic resonance at 9, 36 and 94 GHz microwave frequencies. Several types of paramagnetic centers were observed in the investigated samples. Their spectroscopic parameters (components of the g and hyperfine tensors) were determined. Based on the extracted parameters, the induced centers were ascribed to H0, CO33-, CO2- and nitrogen-centered (presumably NO32-) radicals. The spectroscopic parameters of the nitrogen-centered stable radical in OCP powders were found to be markedly different from those in hydroxyapatite. According to X-ray diffraction data, γ-ray irradiation allowed the phase composition of calcium phosphates to change; all minor phases with the exception of OCP and hydroxyapatite disappeared, while the OCP crystal lattice parameters changed after irradiation. The obtained results could be used for the tracing of mineralization processes from their initiation to completion of the final product, identification of the OCP phase, and to follow the influence of radiation processes on phase composition of calcium phosphates.

[Calcification of atherosclerotic plaques and assessment of their stability]

Presented herein is a review of the literature concerning mechanisms of calcification of atherosclerotic plaques (ASP), showing molecular mechanisms of interaction of processes of calcification with the factors inducing instability of ASPs (anti-inflammatory cytokines, neoangiogenesis, increased level of matrix metalloproteinases, etc.), also describing the effect of the value of volume of scope of calcification on stability of ASPs, followed by discussing the problems related to the role of biominerals (hydroxyapatite calcium phosphate) and Mn2+ in calcification of ASPs and their impact upon stability of the plaque.

High-frequency pulsed ENDOR spectroscopy of the NV(-) centre in the commercial HPHT diamond

This work reports direct 94GHz ENDOR spectroscopy of the (14)N nuclei in the NV(-) centre in single-crystal diamond. Roadmaps of ENDOR frequencies were measured and hyperfine/quadrupole interaction parameters were obtained, with AX,Y=-2.7MHz, AZ=-2.2MHz and P=-4.8MHz. The sign and value of each parameter was calculated using spin Hamiltonian matrix diagonalization, first and second order perturbation theory and confirmed experimentally. Magnetic field magnitude was measured by (13)C ENDOR signal with 0.02% precision or 0.5mT. The orientation of quadrupole, hyperfine and fine structure tensors are the same within error of experiment, g-factor is isotropic.

Identification of Fe3+-Li+ complexes in ZnO by means of high-frequency EPR/ENDOR spectroscopy

Theoretical prediction of a high Curie temperature in ZnO doped with Mn, Fe, and other transition metals has stimulated the investigation of these materials by many research groups. Although charge-compensated Fe(3+) centers in ZnO:Fe have been observed by means of EPR and have been known for decades, conclusions on the chemical nature of these defects are still contradictory. Originally, these centers were treated as Fe(3+)-Li(+) complexes with both ions occupying adjacent cationic sites. Recently, however, the centers were interpreted as a substitutional Fe(3+) ion with a vacancy at an adjacent zinc or oxygen site (Fe-VZn or Fe-VO). In order to determine the chemical nature of the impurity associated with Fe(3+), electron-nuclear double resonance (ENDOR) spectroscopy was used. ENDOR measurements reveal NMR transitions corresponding to nuclei with g-factor gN=2.171 and spin I=3/2. This unambiguously shows presence of Li as a charge compensator and also resolves contradictions with the theoretical prediction of the Fe-VO formation energy. The electric field gradients at the (7)Li nuclei (within the Fe(3+)-Li(+) complexes) were estimated to be significantly lower than the gradient at undistorted Zn sites.

[Manganese in atherogenesis: detection, origin, and role]

The role of transition metal ions in atherogenesis is controversial; they can participate in the hydroxyl radical generation and catalyze the reactive oxygen species neutralization reaction as cofactors of antioxidant enzymes. Using EPR spectroscopy, we revealed that 70% of the samples of aorta with atherosclerotic lesions possessed superoxide dismutase activity, 100% of the samples initiated Fenton reaction and demonstrated the presence of manganese paramagnetic centers. The sodA gene encoding manganese-dependent bacterial superoxide dismutase was not found in the samples of atherosclerotic plaques by PCR using degenerate primers. The data obtained indicates the perspectives of manganese analysis as a marker element in the express diagnostics of atherosclerosis.