We use microwave-induced dynamic nuclear polarization (DNP) of the substitutional nitrogen defects (P1 centers) in diamond to hyperpolarize bulk 13C 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 13C 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 (mI= 0) manifold of the 14N 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 (mI= ±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.
Bibliographical noteFunding Information:
The authors thank Hailey Mullen for helping acquire the DNP spectrum of the diamond crystal and Element 6 for the donation of the powder samples used in these experiments. We thank Professor David Cory (Waterloo) for the 500 mW IMPATT source. This work was partially supported by funding from the National Science Foundation under cooperative agreement OIA-1921199. S.T. acknowledges support from the U.S. National Science Foundation (Grants DMR-1508661, CHE-1611134, and CHE-2004252 with partial cofunding from the Quantum Information Science program in the Division of Physics).
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