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

TL;DR

This study investigates how multiple dynamic nuclear polarization mechanisms interact in diamond, demonstrating that frequency modulation can selectively enhance or suppress specific mechanisms, leading to improved NMR signal enhancement.

Contribution

It introduces the use of frequency-modulated microwave excitation to control competing DNP mechanisms in diamond, enabling selective enhancement of NMR signals.

Findings

Frequency modulation suppresses certain DNP mechanisms.

Frequency modulation enhances overall NMR signal beyond monochromatic methods.

Sign change in DNP signal observed in powder samples but not in single crystals.

Abstract

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.