P1 Centers Research Articles

New data on the N1 nitrogen paramagnetic center in brownish type IaAB diamonds from MIR pipe

In this work, two brownish crystals from the Mir pipe attributed to type IaAB have been examined by a complex of spectroscopic methods: electron paramagnetic resonance, infrared, and photoluminescence spectroscopies. A combination of features such as brownish color, optical system 490.7 nm, and paramagnetic centers W7 and 490.7 points out to plastic deformation of the crystals. The W7 is known to be formed as a result of destruction of A-aggregates during plastic deformation while part of the N3V centrers can be formed due to the disruption of the B-aggregates. The narrow-line EPR spectra from the nitrogen-related N3V centers and the P1 centers indicate that the crystals were annealed after plastic deformation. Another feature of the crystals studied is the observation of the well-known paramagnetic N1 center with only two magnetically inequivalent positions (i.e. with two magnetically inequivalent directions of the C1-N1 fragments) instead of the previously reported four. Possible transformation pathways of the W7 center (N1-C1-C2-N2+) into the N1 center (N1-C-N2+) during the post-deformation annealing are considered.

Preferential Placement of Aligned Nitrogen Vacancy Centers in Chemical Vapor Deposition Overgrown Diamond Microstructures

The usefulness of nitrogen vacancy (NV) centers in diamond is augmented by a low defect and impurity density in the surrounding host material, and applications benefit from the ability to control the position of the NV centers. Herein, a process to create NV centers on single‐crystalline diamond microstructures by chemical vapor deposition (CVD) is presented. Pyramidal structures with {111} side facets are formed during the intrinsic overgrowth of dry chemically etched cylindrical pillars on a substrate with {100} surface orientation. A thin nitrogen‐doped epitaxial layer is deposited on top of the pyramids resulting in the creation of NV centers exclusively on the {111} pyramid side faces. Optically detected magnetic resonance (ODMR) and spin echo measurements reveal preferential alignment of the NV centers in a single {111} direction and a time of . The time of the NV centers is limited by the surrounding substitutional nitrogen (P1 center) concentration of . A low density of other paramagnetic spin noise is detected by double‐electron electron resonance (DEER) measurements.

Weak Electron Irradiation Suppresses the Anomalous Magnetization of N‐Doped Diamond Crystals

Several diamond bulk crystals with a concentration of electrically neutral single substitutional nitrogen atoms of ≲80 ppm, the so‐called C or P1 centers, are irradiated with electrons at 10 MeV energy and low fluence. The results show a complete suppression of the irreversible behavior in field and temperature of the magnetization below 30 K, after a decrease in ≲40 ppm in the concentration of C centers produced by the electron irradiation. This result indicates that magnetic C centers are at the origin of the large hysteretic behavior found recently in nitrogen‐doped diamond crystals. This is remarkable because of the relatively low density of C centers, stressing the extraordinary role of the C centers in triggering those phenomena in diamond at relatively high temperatures. After annealing the samples at high temperatures in vacuum, the hysteretic behavior is partially recovered.

Entanglement of dark electron-nuclear spin defects in diamond

A promising approach for multi-qubit quantum registers is to use optically addressable spins to control multiple dark electron-spin defects in the environment. While recent experiments have observed signatures of coherent interactions with such dark spins, it is an open challenge to realize the individual control required for quantum information processing. Here, we demonstrate the heralded initialisation, control and entanglement of individual dark spins associated to multiple P1 centers, which are part of a spin bath surrounding a nitrogen-vacancy center in diamond. We realize projective measurements to prepare the multiple degrees of freedom of P1 centers—their Jahn-Teller axis, nuclear spin and charge state—and exploit these to selectively access multiple P1s in the bath. We develop control and single-shot readout of the nuclear and electron spin, and use this to demonstrate an entangled state of two P1 centers. These results provide a proof-of-principle towards using dark electron-nuclear spin defects as qubits for quantum sensing, computation and networks.

Exceptional Point and Cross-Relaxation Effect in a Hybrid Quantum System

Exceptional points (EPs) are exotic degeneracies of non-Hermitian systems, where the eigenvalues and the corresponding eigenvectors simultaneously coalesce in parameter space, and these degeneracies are sensitive to tiny perturbations on the system. Here we report an experimental observation of the EP in a hybrid quantum system consisting of dense nitrogen (P1) centers in diamond coupled to a coplanar-waveguide resonator. These P1 centers can be divided into three subensembles of spins, and cross relaxation occurs among them. As a new method to demonstrate this EP, we pump a given spin subensemble with a drive field to tune the magnon-photon coupling in a wide range. We observe the EP in the middle spin subensemble coupled to the resonator mode, irrespective of which spin subensemble is actually driven. This robustness of the EP against pumping reveals the key role of the cross relaxation in P1 centers. It offers a novel way to convincingly prove the existence of the cross-relaxation effect via the EP.

Paramagnetic Intrinsic Point Defects in Alkali Phosphate Glasses: Unraveling the P3 Center Origin and Local Environment Effects

In this work, we carry out a first-principles investigation of intrinsic paramagnetic point defects in P2O5 and in Na2O–P2O5 glasses as a representative of alkali phosphate glasses. Glass models are generated by combining classical molecular dynamics and Monte Carlo simulations and validated by comparing their corresponding structure factors with the available X-ray and neutron scattering experiments. We use density functional theory to calculate the electron paramagnetic resonance parameters for a large set of paramagnetic oxygen-vacancy configurations. Our investigation, also by unveiling the effect of the local environment and disorder on the hyperfine tensor, enables us to propose a new model for the much debated P3 center. In particular, we establish the occurrence of two variants, which we name P3a and P3b centers, that are instrumental to explaining the experimental shifts of the hyperfine splittings observed in alkali phosphate glasses as a function of the alkali content x in the phosphate glass. Our scenario predicts that for low to intermediate alkali contents (0 < x < 50%), a mixture of P1 and P3a centers should be generated under irradiation. For x > 50%, essentially only P3a and P3b centers would be generated, while P1 will be absent. Therefore, our findings, by providing an improved mapping of P centers in phosphate glasses, pave the way for fine-controlling/tuning the optical absorption in a wide range of technological applications.

Photoluminescence at the ground-state level anticrossing of the nitrogen-vacancy center in diamond: A comprehensive study

The nitrogen-vacancy center (NV center) in diamond at magnetic fields corresponding to the ground state level anticrossing (GSLAC) region gives rise to rich photoluminescence (PL) signals due to the vanishing energy gap between the electron spin states, which enables to have an effect on the NV center's luminescence for a broad variety of environmental couplings. In this article we report on the GSLAC photoluminescence signature of NV ensembles in different spin environments at various external fields. We investigate the effects of transverse electric and magnetic fields, P1 centers, NV centers, and the $^{13}$C nuclear spins, each of which gives rise to a unique PL signature at the GSLAC. The comprehensive analysis of the couplings and related optical signal at the GSLAC provides a solid ground for advancing various microwave-free applications at the GSLAC, including but not limited to magnetometry, spectroscopy, dynamic nuclear polarization (DNP), and nuclear magnetic resonance (NMR) detection. We demonstrate that not only the most abundant $^{14}$NV center but the $^{15}$NV can also be utilized in such applications and that nuclear spins coupled to P1 centers can be polarized directly by the NV center at the GSLAC, through a giant effective nuclear $g$-factor arising from the NV center-P1 center-nuclear spin coupling. We report on new alternative for measuring defect concentration in the vicinity of NV centers and on the optical signatures of interacting, mutually aligned NV centers.

Electron spin relaxation of P1 centers in synthetic diamonds with potential as B1 standards for DNP enhanced NMR

The microwave magnetic field, B1, in the non-resonant structures typically used for DNP-enhanced NMR is relatively small, so calibration via continuous wave (CW) power saturation requires a sample with longer spin lattice relaxation times than the samples used as CW standards in X-band cavities. HPHT diamonds have strong, easily observed EPR signals from P1 centers (nitrogen defects), and are indefinitely stable. This makes HPHT diamonds attractive as secondary standards for calibration of electron B1 field strength in a variety of experimental arrangements. The concentrations of P1 centers is also typically in the 30–200 ppm range, or equivalently 10–60 mM, and therefore the EPR relaxation observed is relevant to DNP enhanced NMR employing free radical polarizing agents at similar concentrations. Pulsed and CW saturation relaxation measurements T1 and T2 are compared at X-band. Under CW conditions the relevant T1T2 product of time constants in our samples at room temperature is found to be dominated by electron–electron spin diffusion, and the product is large enough that saturation will be possible with the B1 of typical DNP systems. The similarity of T1 and T2 values obtained by pulse measurements at X-band and Q-band suggests that the X-band results can be extrapolated to the higher EPR frequencies used for DNP experiments. The electron spin dynamics observed here in HPHT diamond samples identify them as useful model systems to better delineate the interplay of electron spin relaxation, magic angle spinning, and inhomogeneous microwave irradiation as they affect DNP enhancement.

Detection of narrow lines in the inhomogeneously broadened line of P1 centers in diamond by double modulation EPR spectroscopy

The study of double modulation continuous wave (CW) electron spin resonance (DM-EPR) is reported to detect narrow “spin-packet”-like lines within an inhomogeneous CW-EPR line of low concentration P1 (Ns0) centers ([Ns0] &amp;lt; 1 ppm) in diamond. These narrow lines appear as peaks at multiples of modulation frequency in the double modulation spectrum. It is shown that the respective line shape can be fitted assuming two Lorentzian line shapes. The linewidth of the narrower Lorentzian is comparable with the linewidth of the Lorentzian obtained from the T2 relaxation time δ1=1/πT2 estimated from the Hahn echo pulsed-EPR experiment. The broader component δ2 correlates with the T2* relaxation time obtained from the free induction decay measurement of the inhomogeneous spectral line shape of the P1 center. The ratio δ2/δ1∼18 derived from DM-EPR measurements approaches within the experimental error the value T2/T2*∼16 derived from pulsed-EPR. Sensitivity to the small shift of the magnetic field (∼0.6 μT) was shown to be detectable from the broadening behavior of the δ2 component. The results promote DM-EPR as a method complementary to pulsed-EPR for studying coherence times of low density paramagnetic centers in a diamond lattice like nitrogen-vacancy (NV−) center, since it exhibits similar properties to the here investigated P1 center in terms of long relaxation time at a monitored low concentration.

Efficient conversion of nitrogen to nitrogen-vacancy centers in diamond particles with high-temperature electron irradiation

Fluorescent nanodiamonds containing negatively-charged nitrogen-vacancy (NV−) centers are promising for a wide range of applications, such as for sensing, as fluorescence biomarkers, or to hyperpolarize nuclear spins. NV− centers are formed from substitutional nitrogen (P1 centers) defects and vacancies in the diamond lattice. Maximizing the concentration of NVs is most beneficial, which justifies the search for methods with a high yield of conversion from P1 to NV−. We report here the characterization of surface cleaned fluorescent micro- and nanodiamonds, obtained by irradiation of commercial diamond powder with high-energy (10 MeV) electrons and simultaneous annealing at 800 ∘C. Using this technique and increasing the irradiation dose, we demonstrate the creation of NV− with up to 25% conversion yield. Finally, we monitor the creation of irradiation-induced spin-1 defects in microdiamond particles, which we associate with W16 and W33 centers, and investigate the effects of irradiation dose and particle size on the coherence time of NV−.

Reduction of surface spin-induced electron spin relaxations in nanodiamonds

Nanodiamonds (NDs) hosting nitrogen-vacancy (NV) centers are promising for applications of quantum sensing. Long spin relaxation times (T1 and T2) are critical for high sensitivity in quantum applications. It has been shown that fluctuations of magnetic fields due to surface spins strongly influence T1 and T2 in NDs. However, their relaxation mechanisms have yet to be fully understood. In this paper, we investigate the relation between surface spins and T1 and T2 of single-substitutional nitrogen impurity (P1) centers in NDs. The P1 centers located typically in the vicinity of NV centers are a great model system to study the spin relaxation processes of the NV centers. By employing high-frequency electron paramagnetic resonance spectroscopy, we verify that air annealing removes surface spins efficiently and significantly reduces their contribution to T1.

Hyperpolarized relaxometry based nuclear T1 noise spectroscopy in diamond

The origins of spin lifetimes in quantum systems is a matter of importance in several areas of quantum information. Spectrally mapping spin relaxation processes provides insight into their origin and motivates methods to mitigate them. In this paper, we map nuclear relaxation in a prototypical system of {}^{13}{rm{C}} nuclei in diamond coupled to Nitrogen Vacancy (NV) centers over a wide field range (1 mT-7 T). Nuclear hyperpolarization through optically pumped NV electrons allows signal measurement savings exceeding million-fold over conventional methods. Through a systematic study with varying substitutional electron (P1 center) and {}^{13}{rm{C}} concentrations, we identify the operational relaxation channels for the nuclei at different fields as well as the dominant role played by {}^{13}{rm{C}} coupling to the interacting P1 electronic spin bath. These results motivate quantum control techniques for dissipation engineering to boost spin lifetimes in diamond, with applications including engineered quantum memories and hyperpolarized {}^{13}{rm{C}} imaging.

Carbon-13 dynamic nuclear polarization in diamond via a microwave-free integrated cross effect

Color-center-hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center-assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition-where the P1 Zeeman splitting matches one of the NV spin transitions-we demonstrate efficient microwave-free 13C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with the diamond crystal.

Understanding the Linewidth of the ESR Spectrum Detected by a Single NV Center in Diamond

Spectral analysis of electron spin resonance (ESR) is a powerful technique for various investigations including characterization of spin systems, measurements of spin concentration, and probing spin dynamics. The nitrogen-vacancy (NV) center in diamond is a promising magnetic sensor enabling improvement of ESR sensitivity to the level of a single spin. Therefore, understanding the nature of the NV-detected ESR (NV-ESR) spectrum is critical for applications to nanoscale ESR. Within this work, we investigate the linewidth of NV-ESR from single substitutional nitrogen centers (called P1 centers). NV-ESR is detected by a double electron-electron resonance (DEER) technique. By studying the dependence of the DEER excitation bandwidth on the NV-ESR linewidth, we find that the spectral resolution is improved significantly and eventually limited by inhomogeneous broadening of the detected P1 ESR. Moreover, we show that the NV-ESR linewidth can be as narrow as 0.3 MHz.

Nanoscale zero-field electron spin resonance spectroscopy

Electron spin resonance (ESR) spectroscopy has broad applications in physics, chemistry, and biology. As a complementary tool, zero-field ESR (ZF-ESR) spectroscopy has been proposed for decades and shown its own benefits for investigating the electron fine and hyperfine interaction. However, the ZF-ESR method has been rarely used due to the low sensitivity and the requirement of much larger samples than conventional ESR. In this work, we present a method for deploying ZF-ESR spectroscopy at the nanoscale by using a highly sensitive quantum sensor, the nitrogen vacancy center in diamond. We also measure the nanoscale ZF-ESR spectrum of a few P1 centers in diamond, and show that the hyperfine coupling constant can be directly extracted from the spectrum. This method opens the door to practical applications of ZF-ESR spectroscopy, such as investigation of the structure and polarity information in spin-modified organic and biological systems.

The Effect of Aggregation of Impurity Nitrogen on Diamond X-Ray Luminescence

Two samplings of 65 diamond crystals divided by the intensity of a slow X-ray luminescence component are studied from the Arkhangel’skaya and Karpinskogo-1 pipes. IR and EPR spectroscopies revealed a relationship between the nitrogen A and P2 centers of the diamonds and the presence of a slow X-ray luminescence component. Its absence in most diamonds with high content of P1 (C) centers is explained by the low number of A and P2 centers.

Continuous wave protocol for simultaneous polarization and optical detection of P1-center electron spin resonance

The ready optical detection and manipulation of bright nitrogen vacancy center spins in diamond plays a key role in contemporary quantum information science and quantum metrology. Other optically dark defects such as substitutional nitrogen atoms (`P1 centers') could also become potentially useful in this context if they could be as easily optically detected and manipulated. We develop a relatively straightforward continuous wave protocol that takes advantage of the dipolar coupling between nitrogen vacancy and P1 centers in type 1b diamond to detect and polarize the dark P1 spins. By combining mutual spin flip transitions with radio frequency driving, we demonstrate the simultaneous optical polarization and detection of the electron spin resonance of the P1 center. This technique should be applicable to detecting and manipulating a broad range of dark spin populations that couple to the nitrogen vacancy center via dipolar fields, allowing for quantum metrology using these spin populations.

Intrapulpal Temperature Increases Caused by 445-nm Diode Laser-Assisted Debonding of Self-Ligating Ceramic Brackets During Simulated Pulpal Fluid Circulation.

This study investigated temperature increases in dental pulp resulting from laser-assisted debonding of ceramic brackets using a 445-nm diode laser. Eighteen ceramic brackets were bonded in standardized manner to 18 caries-free human third molars. Pulpal fluid circulation was simulated by pumping distilled water at 37°C through the pulp chamber. The brackets were irradiated with a 445-nm diode laser. Temperatures were measured using a thermal camera at points P1 (center of the pulp) and P2 (in the hard dental tissue) at the baseline (T0), at the start and end of laser application (T1 and T2), and the maximum during the sequence (Tmax). Significant differences in the temperatures measured at P1 and P2 were observed among T0, T1, T2, and Tmax. Significant increases in temperature were noted at points P1 and P2, between T1 and T2, T1 and Tmax, and T2 and Tmax. The maximum P2 values were significantly higher than at P1. The maximum temperature increase measured in the pulp was 2.23°C, lower than the critical threshold of 5.5°C. On the basis of the laser settings used, there is no risk to the vitality of dental pulp during laser-assisted debonding of ceramic brackets with a 445-nm diode laser.

Multi-step phase-cycling in a free-electron laser-powered pulsed electron paramagnetic resonance spectrometer.

Electron paramagnetic resonance (EPR) is a powerful tool for research in chemistry, biology, physics and materials science, which can benefit significantly from moving to frequencies above 100 GHz. In pulsed EPR spectrometers driven by powerful sub-THz oscillators, such as the free electron laser (FEL)-powered EPR spectrometer at UCSB, control of the duration, power and relative phases of the pulses in a sequence must be performed at the frequency and power level of the oscillator. Here we report on the implementation of an all-quasioptical four-step phase cycling procedure carried out directly at the kW power level of the 240 GHz pulses used in the FEL-powered EPR spectrometer. Phase shifts are introduced by modifying the optical path length of a 240 GHz pulse with precision-machined dielectric plates in a procedure we call phase cycling with optomechanical phase shifters (POPS), while numerical receiver phase cycling is implemented in post-processing. The POPS scheme was successfully used to reduce experimental dead times, enabling pulsed EPR of fast-relaxing spin systems such as gadolinium complexes at temperatures above 190 K. Coherence transfer pathway selection with POPS was used to perform spin echo relaxation experiments to measure the phase memory time of P1 centers in diamond in the presence of a strong unwanted FID signal in the background. The large excitation bandwidth of FEL-EPR, together with phase cycling, enabled the quantitative measurement of instantaneous electron spectral diffusion, from which the P1 center concentration was estimated to within 10%. Finally, phase cycling enabled saturation-recovery measurements of T1 in a trityl-water solution at room temperature - the first FEL-EPR measurement of electron T1.

Optically induced cross relaxation via nitrogen-related defects for bulk diamond C13 hyperpolarization

In this Rapid Communication we utilize nuclear magnetic resonance to investigate the hyperpolarization effect of negatively charged nitrogen vacancy (NV) centers on bulk $^{13}\mathrm{C}$ nuclei in a diamond single crystal. We were able to identify several polarization peaks of a different sign at different magnetic fields in a region of some tens of Gauss centered around $50\phantom{\rule{0.28em}{0ex}}\mathrm{mT}$. The bulk $^{13}\mathrm{C}$ hyperpolarization in the investigated field range is usually attributed to the excited state level anticrossing of the NV center. However, we found that this bulk hyperpolarization is caused by optically induced cross relaxation and that it takes place in the NV center ground state. The four-spin coupling between the polarized NV electron spin, the electron spin of a substitutional nitrogen impurity (P1), as well as its $^{14}\mathrm{N}$ nuclei and the $^{13}\mathrm{C}$ nuclear spin have to be considered. We introduce a simple theoretical model which completely fits with the experimental data and which clearly shows that the P1 centers are involved in the polarization process. We expect that the current work has a significant impact on future NV-based polarization applications.