Large Room Temperature Bulk DNP of ¹³C via P1 Centers in Diamond

Abstract

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 (nonspinning) 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 = ±1) and to blur some of the sharper features associated with the central manifold. The outer lines are best fit using a combination of the cross effect and the 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.

P1 Center Electron Spin Clusters Are Prevalent in Type Ib Diamonds

“…In order to understand what mechanisms are involved, we need to decompose the DNP frequency profile into different mechanisms. Based on the prior success of decomposing the DNP frequency profile at 3.34 T into SE, CE, tCE, and apparent Overhauser effect (OE) DNP mechanisms, we apply the same analysis to the DNP frequency profile acquired at 7 T Figure a,b displays the DNP spectrum at 3.34 and 7 T, respectively, measured at room temperature (in black) and the resulting fit (in green).…”

“…DNP is a widely used technique to enhance the intensity of the nuclear magnetic resonance (NMR) signal, but our interest in DNP stems from its sensitivity to the nature of interacting spins. 26,45 A recent DNP study of type-Ib diamond samples by Shimon discussed that the observed truncated cross effect 45 can be explained by the existence of coupled P1 spins in the sample in addition to the isolated P1 center, 26 but the fundamental properties of the coupling, whether they are driven by dipolar or exchange coupling, whether there is crosstalk between the different P1 populations, the population of clustered P1 centers, and the mechanistic origin of the proposed Overhauser DNP effect remained unknown. 46,47 In this study, we uncover the coexistence of clustered and isolated P1 centers in type-Ib diamonds using a combination of multifrequency DNP, pulsed high-frequency EPR, and ELDOR techniques.…”

“…One of the most widely studied defects is the nitrogen vacancy (NV) center, which consists of a substitutional nitrogen atom and an adjacent vacancy in the diamond lattice. − The NV center has been found to exhibit remarkable properties, including long spin coherence times, room temperature operation, and optical addressability. − As a result, it has been used in a range of applications, such as single-photon sources, nanoscale magnetic sensors, and quantum memories. − NV centers arise from P1 centers that are single substitutional nitrogen defects in the diamond lattice that also have a long coherence time, even at room temperature. , Studying the spatial distribution of the P1 and NV centers is critical for many reasons. For example, P1 centers are a major source of polarization for nearby carbon nuclei that can be used for quantum sensing applications. − P1 centers are also the main culprit for the relaxation of the NV center electron paramagnetic resonance (EPR) signal and coherence and hence may be the bottleneck in applications where long coherence times from the NV center are needed for sensing applications. ,− Knowledge of the microscopic spin distribution is essential for the investigation of exotic physics in disordered dipolar spin systems. , Diamond is also a promising polarization agent for dynamic nuclear polarization (DNP) for achieving high polarization of intrinsic 13 C nuclear spins for sensing or imaging applications ,− or of nuclear spins of external analytes …”

“…In our study, we focus on characterizing representative type Ib diamonds to test high-field DNP- and EPR-based methods to evaluate P1–P1 interactions. In fact, this study was inspired by a surprising observation reported in the literature that hinted toward the existence of nitrogen defect clusters in HPHT-made, type Ib diamonds …”

“…We turn to DNP and ELDOR spectroscopy to study the cross-talk between different P1 populations via the 13 C NMR probe frequency or directly via pump probe experiments, respectively. DNP is a widely used technique to enhance the intensity of the nuclear magnetic resonance (NMR) signal, but our interest in DNP stems from its sensitivity to the nature of interacting spins. , A recent DNP study of type-Ib diamond samples by Shimon discussed that the observed truncated cross effect can be explained by the existence of coupled P1 spins in the sample in addition to the isolated P1 center, but the fundamental properties of the coupling, whether they are driven by dipolar or exchange coupling, whether there is cross-talk between the different P1 populations, the population of clustered P1 centers, and the mechanistic origin of the proposed Overhauser DNP effect remained unknown. , …”