Nitrogen Substitutions Aggregation and Clustering in Diamonds as Revealed by High-Field Electron Paramagnetic Resonance

Abstract

Diamonds have been shown to be an excellent platform for quantum computing and quantum sensing applications. These applications are enabled by the presence of defects in the lattice, which are also known as color centers. The most common nitrogen-based defect in synthetic diamonds is the paramagnetic nitrogen substitution (P1) center. While the majority of quantum applications rely on nitrogen-vacancy (NV) centers, the properties of the latter are heavily influenced by the presence and the spatial distribution of the P1 centers. Hence, understanding the spatial distribution and mutual interactions of P1 centers is crucial for the successful development of diamond-based quantum devices. Unlike NV centers, P1 centers do not have a spin-dependent optical signature, and their spin-related properties, therefore, have to be detected and characterized using magnetic resonance methods. We show that using high-field (6.9 and 13.8 T) pulsed electron paramagnetic resonance (EPR) and dynamic nuclear polarization (DNP) experiments, we can distinguish and quantify three distinct populations of P1 centers: isolated P1 centers, weakly interacting ones, and exchange-coupled ones that are clustered together. While such clustering was suggested before, these clusters were never detected directly and unambiguously. Moreover, by using electron−electron double resonance (ELDOR) pump−probe experiments, we demonstrate that the latter clustered population does not exist in isolation but coexists with the more weakly interacting P1 centers throughout the diamond lattice. Its presence thus strongly affects the quantum properties of the diamond. We also show that the existence of this population can explain recent hyperpolarization results in type Ib high-pressure, high-temperature (HPHT) diamonds. We propose a combination of high-field pulsed EPR, ELDOR, and DNP as a tool for probing the aggregation state and interactions among different populations of nitrogen substitution centers.

P1 Center Electron Spin Clusters Are Prevalent in Type Ib Diamonds

“…The clustered P1 centers are likely a mixture of coupled spins that have 1/2 and 1 or greater quantum numbers. In fact, a paper published concurrently with this study found the coexistence of dipolar-coupled and exchange-coupled P1 centers with dipolar coupling not exceeding tens of MHz and exchange coupling of the order of 100–200 MHz in HPHT-type 1B single crystal diamonds …”

“…Remarkably, this property of a strongly heterogeneous P1 center distribution is independent of the manufacturer and nominal P1 concentrations in type Ib diamonds synthesized by HPHT. Another publication that is concurrently coming out with this paper presents the same discovery of a significant fraction of exchange coupled P1 centers in the type Ib single-crystal diamond . Whether strong clustering of P1 centers occurs with CVD diamonds remains to be seen.…”

“…Critically, when exchange coupling is involved, the relationship between broadening and distance is no longer straightforward. Hence, further clarifications on the nature of the P1 clusters need to rely on measurements of EPR line shape of single crystals or at higher magnetic field where different paramagnetic species are better resolved …”

“…The origin of a frequency shift can be a strong dipolar coupling of P1 centers with g anisotropy and/or the presence of strong exchange coupling exceeding 80 MHz (corresponding to the 13 C nuclear Larmor frequency) with a positive or negative sign. However, the precise analysis of such features would benefit from DNP and ELDOR measurements at even higher magnetic fields than 3.34 and 7 T or using single crystal diamond samples, as presented in a study published concurrently …”

Dynamic Nuclear Polarization Using Electron Spin Cluster

“…While the orientations of the two electron spin packets that overlap might end up having matching, fast electron spin–lattice relaxation times, it is the spin packets with their dipolar coupling vector orientation giving rise to EPR frequency shifts with respect to the center frequency of the isolated electron spin by the nuclear larmor frequency that matter for ESC-DNP by resonance matching. In fact, a sharp contrast in spin–lattice relaxation times between the sensitizer populations and the clustered radical populations can give rise to a positive or negative absorptive central DNP profile feature via the truncated Cross Effect (tCE) mechanism. − Such an absorptive feature is advantageous for NMR signal amplification under MAS where the positive and negative dispersive features could otherwise cancel out due to oscillatory broadband irradiation of the entire EPR spectrum unless special pulse sequences and irradiation schemes and timings are employed. These qualities give ESC-DNP even further design scope and potential for efficient enhancement at high fields with low power.…”