[Paper Review] Room temperature bulk diamond 13-C hyperpolarisation -- Strong evidence for a complex four spin coupling

Ralf Wunderlich, Jonas Kohlrautz+3 more|arXiv (Cornell University)|Mar 27, 2017

Diamond and Carbon-based Materials Research18 references3 citations

TL;DR

This study demonstrates room-temperature bulk hyperpolarization of 13C nuclei in synthetic diamond via electron spin polarization transfer from nitrogen-vacancy (NV) centers, revealing strong evidence for a four-spin coupling mechanism involving NV–13C and P1–14N defects. The method achieves a 23-hour equivalent signal-to-noise ratio in just two minutes, with experimental peaks in polarization intensity at specific magnetic fields attributed to cross-relaxation mediated by substitutional nitrogen (P1) centers, validated by a four-spin dynamic model.

ABSTRACT

Hyperpolarisation at room temperature is one of the most important research fields in order to improve liquid, gas or nanoparticle tracer for Magnetic Resonance Imaging (MRI) in medical applications. In this paper we utilize nuclear magnetic resonance (NMR) to investigate the hyperpolarisation effect of negatively charged nitrogen vacancy (NV) centres on carbon-13 nuclei and their spin diffusion in a diamond single crystal close to the excited state level anti crossing (ESLAC) around 50 mT. Whereas the electron spins of the NV centre can be easily polarized in its m = 0 ground state at room temperature just by irradiation with green light , the swop of the NV electron spin polarization to a carbon-13 nuclei is a complex task. We found that the coupling between the polarized NV electron spin, the electron spin of a substitutional nitrogen impurity (P1) as well as its nitrogen-14 nuclei and the carbon-13 nuclear spin has to be considered. Here we show that through an optimization of this procedure, in about two minutes a signal to noise ratio which corresponds to a 23 hour standard measurement without hyperpolarisation and an accumulation of 460 single scans can be obtained. Furthermore we were able to identify several polarisation peaks of different sign at different magnetic fields in a region of some tens of gauss. Most of the peaks can be attributed to a coupling of the NV centres to nearby P1 centres. We present a new theoretical model in a framework of cross polarisation of a four spin dynamic model in good agreement with our experimental data. The results demonstrate the opportunities and power as well as limitations of hyperpolarisation in diamond via NV centres. We expect that the current work may have a significant impact on future applications.

Motivation & Objective

• To achieve room-temperature hyperpolarization of 13C nuclei in diamond for enhanced MRI sensitivity.
• To identify the physical mechanism behind the observed magnetic field-dependent nuclear polarization peaks.
• To determine whether the polarization transfer is mediated by NV–13C coupling alone or involves additional spin systems such as P1–14N centers.
• To develop and validate a four-spin dynamic model explaining the experimental polarization patterns.

Proposed method

• A home-built NMR system with Helmholtz coils and a 532 nm laser was used to apply low magnetic fields (up to 60 mT) and optically polarize NV centers at room temperature.
• The diamond sample was shuttled into a 300 MHz superconducting NMR magnet for high-resolution NMR measurements, enabling detection of 13C nuclear spin polarization.
• Optical excitation of NV centers at the ground state m=0 level induced electron spin polarization, which was then transferred to 13C nuclei via hyperfine and dipolar interactions.
• A theoretical four-spin dynamic model was developed to simulate cross-relaxation between NV–13C and P1–14N spin systems, with parameters adjusted to match experimental data.
• The model included spin flip probabilities based on expectation values of I^C_z and accounted for dipolar coupling strengths (up to 2.5 MHz at 5 nm distance).
• The experimental field sweep was convolved with a Gaussian (FWHM 0.1 mT) to simulate resolution effects and match the observed peak shapes.

Experimental results

Research questions

• RQ1 What causes the magnetic field-dependent polarization peaks observed in 13C NMR spectra at ~50 mT in diamond with NV centers?
• RQ2 Is the hyperpolarization effect primarily driven by NV–13C hyperfine coupling or mediated by additional spin systems such as P1–14N centers?
• RQ3 Can a four-spin dynamic model involving NV, 13C, P1, and 14N spins quantitatively reproduce the experimental polarization patterns and peak signs?
• RQ4 How does the presence of P1 centers influence the efficiency and sign of 13C nuclear polarization?
• RQ5 What role does spin diffusion play in propagating hyperpolarization through the diamond lattice?

Key findings

• A 23-hour equivalent signal-to-noise ratio in standard NMR measurement was achieved in just two minutes of hyperpolarization, demonstrating a 460-fold enhancement over conventional methods.
• Multiple polarization peaks of varying sign were observed in a 30–50 gauss region, with most peaks attributed to coupling between NV centers and nearby P1 centers.
• The dominant polarization pattern is best explained by cross-relaxation between the NV–13C system and the P1–14N system, indicating that P1 centers act as mediators in the polarization transfer process.
• Theoretical simulations using a four-spin dynamic model (NV–13C–P1–14N) successfully reproduced the shape and field dependence of the experimental polarization peaks.
• The model suggests that efficient polarization transfer occurs only when the NV–13C and P1–14N couplings are strong enough to allow resonant energy transfer within ~0.5 MHz, consistent with observed dipolar coupling strengths.
• Minor unexplained features in the data may be linked to 15N–P1 centers, suggesting potential for further exploration of nitrogen isotopes in hyperpolarization mechanisms.