Size-dependent concentration of N⁰ paramagnetic centres in HPHT nanodiamonds

Size-calibrated commercial nanodiamonds synthesized by high-pressure high-temperature (HPHT) technique were studied by high-frequency W- and conventional X-band electron paramagnetic resonance (EPR) spectroscopy. The numbers of spins in the studied samples were estimated. The core-shell model of the HPHT nanodiamonds was proposed to explain the observed dependence of the concentration of the N0 paramagnetic centers. Two other observed paramagnetic centers are attributed to the two types of structures in the nanodiamond shell

Hyperfine and nuclear quadrupole splitting of the NV- ground state in 4H -SiC

Optically addressable spin-triplet defects in silicon carbide, such as divacancies and negatively charged nitrogen vacancy (NV-) allow to develop modern quantum technologies operating in the near-infrared range based on the well-developed semiconductor material. Here, by means of both high-frequency (94 GHz) pulsed electron paramagnetic resonance (EPR) and electron-nuclear double Rresonance (ENDOR) techniques the ground state properties of the negatively charged NV- defect in 4H-SiC were studied. We experimentally determined the ordering of the ground state spin sublevels and established the sign of the zero-field splitting to be positive as predicted by theory. Analysis of nuclear magnetic resonance transitions in ENDOR spectra allowed to determine the sign, symmetry, and absolute values of the hyperfine interaction of the NV- defect electron spin with N14 nuclear spin as A∥=-1.142MHz and A⊥=-1.184MHz. The absolute value of the nuclear quadrupole interaction constant reflecting an interaction between the N14 nuclear electric quadrupole moment with the electric field gradient was determined to be |Cq|=2.44MHz. This large value is compatible with a threefold coordinated N14 nucleus with uniaxial symmetry and proves conclusively the existence of a nearestneighbor NCVSi pair in the material. For this NV- defect, an ensemble (Hahn-echo) coherence time of T2=49μs was measured, a value which is in the range previously reported for silicon vacancy spin ensembles and slightly longer than T2=40μs measured here on the divacancy spin ensemble

Electron nuclear interactions in spin-3/2 color centers in silicon carbide: A high-field pulse EPR and ENDOR study

High-frequency pulsed electron paramagnetic resonance (EPR) and electron nuclear double resonance (ENDOR) were used to determine electron nuclear interactions on remote ligand shells of silicon and carbon in spin-3/2 color centers with an optically induced high-temperature spin alignment in hexagonal -, -, and rhombic -silicon carbide (SiC) polytypes. The EPR and ENDOR experimental data relate unambiguously to spin-3/2 centers in which the optically induced alignment of the spin-level populations occurs. The identification is based on resolved ligand hyperfine interactions with carbon and silicon nearest, next-nearest, and the more distant neighbors and on the determination of the unpaired electron spin densities. The hyperfine interactions with and nuclei are unambiguously separated due to the selective population of the fine-structure levels with certain values of . The signs of these interactions and, as a result, the signs of oscillating spin density at and nuclei, are determined. On the basis of the EPR and optically induced ENDOR measurements, signs of the fine-structure splitting for all the centers were demonstrated, which made it possible to establish the character of optically induced spin alignment, including the inverse populations of the spin levels for these centers. The values of hyperfine interaction with and nuclei, including those remote from the localization of the spin-3/2 center, are tabulated, which can be used by a number of algorithms in quantum information processing as long-term memory

Creation of negatively charged boron vacancies in hexagonal boron nitride crystal by electron irradiation and mechanism of inhomogeneous broadening of boron vacancy-related spin resonance lines

Optically addressable high-spin states (S ≥ 1) of defects in semiconductors are the basis for the development of solid-state quantum technologies. Recently, one such defect has been found in hexagonal boron nitride (hBN) and identified as a negatively charged boron vacancy (V−B ). To explore and utilize the properties of this defect, one needs to design a robust way for its creation in an hBN crystal. We investigate the possibility of creating V−B centers in an hBN single crystal by means of irradiation with a high-energy (E = 2 MeV) electron flux. Optical excitation of the irradiated sample induces fluorescence in the near-infrared range together with the electron spin resonance (ESR) spectrum of the triplet centers with a zero-field splitting value of D = 3.6 GHz, manifesting an optically induced population inversion of the ground state spin sublevels. These observations are the signatures of the V−B centers and demonstrate that electron irradiation can be reliably used to create these centers in hBN. Exploration of the V−B spin resonance line shape allowed us to establish the source of the line broadening, which occurs due to the slight deviation in orientation of the two-dimensional B-N atomic plains being exactly parallel relative to each other. The results of the analysis of the broadening mechanism can be used for the crystalline quality control of the 2D materials, using the V−B spin embedded in the hBN as a probe

Relaxation processes and high-field coherent spin manipulation in color center ensembles in 6 H-SiC

Coherent spin manipulations of spin-32 color center ensembles in 6H-SiC crystal have been studied in high magnetic fields using methods of pulsed electron paramagnetic resonance, Rabi oscillations, and pulsed electron-electron double resonance under optical alignment conditions of the spin level populations. Rabi oscillation experiments show room temperature coherent control of these spin-32 color center ensembles in strong magnetic fields. A sharp decrease of the spin-lattice relaxation time T1, ∼40 times, was observed in 6H-SiC at magnetic field of ∼3.5 T with increasing temperature from 100 to 300 K, while the spin-spin relaxation time T2 is only shortened by ∼1.3 times. With an increase in the magnetic field, the times T1 and T2 were shown to decrease. The relaxation time T1 in the case of magnetic field directed along the axis of the spin-32 center is ∼2 times longer than T1 in magnetic field perpendicular to this axis. Relaxation times of the spin center in crystal grown with a reduced concentration of an isotope Si29 are significantly longer than crystal, with the natural content of isotopes. With a decrease in the Si29 content in our experiments by a factor of ∼5, the effective nuclear spin bath in SiC is reduced by a factor of ∼2. In a zero magnetic field resonance, transitions are allowed as magnetic dipole transitions with frequency ω0 which correspond to the zero-field splitting. In zero magnetic field and in fixed magnetic fields, the Rabi frequency was shown, using so-called "Feynman-Vernon-Hellwarth transformation,"to be ωR=|γ|B1. In pulsed electron-electron double resonance experiments, a change in the intensity of the electron spin echo signal corresponding to one of the spin-allowed fine structure transitions is recorded depending on the sweep of the second frequency. The experiments show the possibility to coherently detect the optical spin alignment between MS=±32 via optically pumped silent MS=±12 sublevels of the spin-32 color centers, including a detection of Rabi oscillations

Radiation-induced stable radicals in calcium phosphates: Results of multifrequency epr, ednmr, eseem, and endor studies

This article presents the results of a study of radiation-induced defects in various synthetic calcium phosphate (CP) powder materials (hydroxyapatite—HA and octacalcium phosphate—OCP) by electron paramagnetic resonance (EPR) spectroscopy at the X, Q, and W-bands (9, 34, 95 GHz for the microwave frequencies, respectively). Currently, CP materials are widely used in orthopedics and dentistry owing to their high biocompatibility and physico-chemical similarity with human hard tissue. It is shown that in addition to the classical EPR techniques, other experimental approaches such as ELDOR-detected NMR (EDNMR), electron spin echo envelope modulation (ESEEM), and electronnuclear double resonance (ENDOR) can be used to analyze the electron–nuclear interactions of CP powders. We demonstrated that the value and angular dependence of the quadrupole interaction for14 N nuclei of a nitrate radical can be determined by the EDNMR method at room temperature. The ESEEM technique has allowed for a rapid analysis of the nuclear environment and estimation of the structural positions of radiation-induced centers in various crystal matrices. ENDOR spectra can provide information about the distribution of the nitrate radicals in the OCP structure