arXiv subjects

Daphna Shimon

Publications and source records attributed to Daphna Shimon.

5 recordsLinked to original sources

arXiv2025

Exploring Overlapping Mechanisms of Dynamic Nuclear Polarization in Type 1b HPHT Diamond

The inhomogeneous distribution of P1 centers in type 1b HPHT diamond samples allows multiple DNP mechanisms to occur within the same crystal, resulting in complex DNP spectra. At some crystal orientations, different DNP mechanisms can compete to drive hyperpolarization with different signs at the same applied microwave frequency. We perform microwave-irradiated DNP using both monochromatic and frequency-modulated microwave excitation to explore the competition between these DNP mechanisms in diamond at room temperature. We demonstrate that frequency-modulated DNP is a tool for suppressing certain DNP mechanisms while enhancing others in a single-crystal diamond sample. Frequency modulation also enables higher enhancement of the NMR signal beyond traditional monochromatic DNP under some conditions. In a powder sample, competing enhancement mechanisms can also arise from different crystallite orientations in the powder. We observe that at certain microwave frequencies the DNP signal changes sign during the polarization build-up, even with monochromatic microwave irradiation. We do not observe this phenomenon in any single-crystal spectrum. We discuss both methods of investigating competing mechanisms of DNP as a means of selectively enhancing different DNP mechanisms driving $^{13}$C NMR signal enhancement.

physics.chem-ph↗

arXiv2023

P1 center electron spin clusters are prevalent in type Ib diamond

Understanding the spatial distribution of P1 centers is crucial for diamond-based sensors and quantum devices. P1 centers serve as a polarization source for DNP quantum sensing and play a significant role in the relaxation of NV centers. Additionally, the distribution of NV centers correlates with the distribution of P1 centers, as NV centers are formed through the conversion of P1 centers. We utilized dynamic nuclear polarization (DNP) and pulsed electron paramagnetic resonance (EPR) techniques that revealed strong clustering of a significant population of P1 centers that exhibit exchange coupling and produce asymmetric lineshapes. The $^{13}$C DNP frequency profile at high magnetic field revealed a pattern that requires an asymmetric EPR lineshape of the P1 clusters with electron-electron (e-e) coupling strengths exceeding the $^{13}$C nuclear Larmor frequency. EPR and DNP characterization at high magnetic fields was necessary to resolve energy contributions from different e-e couplings. We employed a two-frequency pump-probe pulsed Electron Double Resonance (ELDOR) technique to show crosstalk between the isolated and clustered P1 centers. This finding implies that the clustered P1 centers affect all P1 populations. Direct observation of clustered P1 centers and their asymmetric lineshape is a novel and crucial insight into understanding magnetic noise sources for quantum information applications of diamonds and for designing diamond-based polarizing agents with optimized DNP efficiency for $^{13}$C and other nuclear spins of analytes. We propose that room temperature $^{13}$C DNP at high field, achievable through straightforward modifications to existing solution-state NMR systems, is a potent tool for evaluating and controlling diamond defects.

physics.chem-ph↗

arXiv2022

Room Temperature DNP of Diamond Powder Using Frequency Modulation

Dynamic nuclear polarization (DNP) is a method of enhancing NMR signals via the transfer of polarization from electron spins to nuclear spins using on-resonance microwave (MW) irradiation. In most cases, monochromatic continuous-wave (MCW) MW irradiation is used. Recently, several groups have shown that the use of frequency modulation of the MW irradiation can result in an additional increase in DNP enhancement above that obtained with MCW. The effect of frequency modulation on the solid effect (SE) and the cross effect (CE) has previously been studied using the stable organic radical 4-hydroxy TEMPO (TEMPOL) at temperatures under 20 K. Here, in addition to the SE and CE, we discuss the effect of frequency modulation on the Overhauser effect (OE) and the truncated CE (tCE) in the room-temperature $^{13}$C-DNP of diamond powders. We recently showed that diamond powders can exhibit multiple DNP mechanisms simultaneously due to the heterogeneity of P1 (substitutional nitrogen) environments within diamond crystallites. We explore the enhancement obtained via the two most important parameters for frequency modulation 1) Modulation frequency, $f_{m}$ (how fast the modulation frequency is varied) and 2) Modulation amplitude, $Δω$ (the magnitude of the change in microwave frequency) influence the enhancement obtained via each mechanism. Frequency modulation during DNP not only allows us to improve DNP enhancement, but also gives us a way to control which DNP mechanism is most active. By choosing the appropriate modulation parameters, we can selectively enhance some mechanisms while simultaneously suppressing others.

physics.chem-ph↗

arXiv2022

Large Room Temperature Bulk DNP of $^{13}$C via P1 Centers in Diamond

We use microwave-induced dynamic nuclear polarization (DNP) of the substitutional nitrogen defects (P1 centers) in diamond to hyperpolarize bulk $^{13}$C nuclei in both single crystal and powder samples at room temperature at 3.34 T. The large ($>100$-fold) enhancements demonstrated correspond to a greater than 10,000 fold improvement in terms of signal averaging of the 1\% abundant $^{13}$C spins. The DNP was performed using low-power solid state sources under static (non-spinning) conditions. The DNP spectrum (DNP enhancement as a function of microwave frequency) of diamond powder shows features that broadly correlate with the EPR spectrum. A well-defined negative Overhauser peak and two solid effect peaks are observed for the central ($m_I=0$) manifold of the $^{14}$N spins. Previous low temperature measurements in diamond had measured a positive Overhauser enhancement in this manifold. Frequency-chirped millimeter-wave excitation of the electron spins is seen to significantly improve the enhancements for the two outer nuclear spin manifolds ($m_I = \pm 1$) and to blur some of the sharper features associated with the central manifolds. The outer lines are best fit using a combination of the cross effect and a truncated cross effect -- which is known to mimic features of an Overhauser effect. Similar features are also observed in experiments on single crystal samples. The observation of all of these mechanisms in a single material system under the same experimental conditions is likely due to the significant heterogeneity of the high pressure, high temperature (HPHT) type Ib diamond samples used. Large room temperature DNP enhancements at fields above a few Tesla enable spectroscopic studies with better chemical shift resolution under ambient conditions.

cond-mat.mtrl-sci↗

arXiv2019

DNP-NMR of surface hydrogen on silicon microparticles

Dynamic nuclear polarization (DNP) enhanced nuclear magnetic resonance (NMR) offers a promising route to studying local atomic environments at the surface of both crystalline and amorphous materials. We take advantage of unpaired electrons due to defects close to the surface of the silicon microparticles to hyperpolarize adjacent $^{1}$H nuclei. At 3.3 T and 4.2 K, we observe the presence of two proton peaks, each with a linewidth on the order of 5 kHz. Echo experiments indicate a homogeneous linewidth of $\sim 150-300$ Hz for both peaks, indicative of a sparse distribution of protons in both environments. The downfield peak at 10 ppm lies within the typical chemical shift range for proton NMR, and was found to be relatively stable over repeated measurements. The upfield peak was found to vary in position between -19 and -37 ppm, well outside the range of typical proton NMR shifts, and indicative of a high-degree of chemical shielding. The upfield peak was also found to vary significantly in intensity across different experimental runs, suggesting a weakly-bound species. These results suggest that the hydrogen is located in two distinct microscopic environments on the surface of these Si particles.

cond-mat.mtrl-sci↗