Dynamic nuclear polarization (DNP) is widely used to enhance solid state nuclear magnetic resonance (NMR) sensitivity. Its efficiency as a generic signal-enhancing approach for liquid state NMR, however, decays rapidly with magnetic field B 0 , unless mediated by scalar interactions arising only in exceptional cases. This has prevented a more widespread use of DNP in structural and dynamical solution NMR analyses. This study introduces a potential solution to this problem, relying on biradicals with exchange couplings J ex of the order of the electron Larmor frequency ω E . Numerical and analytical calculations show that in such J ex ≈ ± ω E cases a phenomenon akin to that occurring in chemically induced DNP (CIDNP) happens, leading to different relaxation rates for the biradical singlet and triplet states which are hyperfine-coupled to the nuclear α or β states. Microwave irradiation can then generate a transient nuclear polarization build-up with high efficiency, at all magnetic fields that are relevant in contemporary NMR, and for all rotational diffusion correlation times that occur in small- and medium-sized molecules in conventional solvents.
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Optically pumped spin polarization as a probe of many-body thermalization
Disorder and many body interactions are known to impact transport and thermalization in competing ways, with the dominance of one or the other giving rise to fundamentally different dynamical phases. Here we investigate the spin diffusion dynamics of 13 C in diamond, which we dynamically polarize at room temperature via optical spin pumping of engineered color centers. We focus on low-abundance, strongly hyperfine-coupled nuclei, whose role in the polarization transport we expose through the integrated impact of variable radio-frequency excitation on the observable bulk 13 C magnetic resonance signal. Unexpectedly, we find good thermal contact throughout the nuclear spin bath, virtually independent of the hyperfine coupling strength, which we attribute to effective carbon-carbon interactions mediated by the electronic spin ensemble. In particular, observations across the full range of hyperfine couplings indicate the nuclear spin diffusion constant takes values up to two orders of magnitude greater than that expected from homo-nuclear spin couplings.
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- NSF-PAR ID:
- 10154634
- Date Published:
- Journal Name:
- Science Advances
- Volume:
- 6
- Issue:
- 18
- ISSN:
- 2375-2548
- Page Range / eLocation ID:
- eaaz6986
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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A broad effort is underway to improve the sensitivity of NMR through the use of dynamic nuclear polarization. Nitrogen vacancy (NV) centers in diamond offer an appealing platform because these paramagnetic defects can be optically polarized efficiently at room temperature. However, work thus far has been mainly limited to single crystals, because most polarization transfer protocols are sensitive to misalignment between the NV and magnetic field axes. Here we study the spin dynamics of NV−13C pairs in the simultaneous presence of optical excitation and microwave frequency sweeps at low magnetic fields. We show that a subtle interplay between illumination intensity, frequency sweep rate, and hyperfine coupling strength leads to efficient, sweep-direction-dependent13C spin polarization over a broad range of orientations of the magnetic field. In particular, our results strongly suggest that finely tuned, moderately coupled nuclear spins are key to the hyperpolarization process, which makes this mechanism distinct from other known dynamic polarization channels. These findings pave the route to applications where powders are intrinsically advantageous, including the hyperpolarization of target fluids in contact with the diamond surface or the use of hyperpolarized particles as contrast agents for in vivo imaging.
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Abstract 167 Er 3+ doped solids are a promising platform for quantum technology due to erbium’s telecom C-band optical transition and its long hyperfine coherence times. We experimentally study the spin Hamiltonian and dynamics of 167 Er 3+ spins in Y 2 O 3 using electron paramagnetic resonance (EPR) spectroscopy. The anisotropic electron Zeeman, hyperfine and nuclear quadrupole matrices are fitted using data obtained by X-band (9.5 GHz) EPR spectroscopy. We perform pulsed EPR spectroscopy to measure spin relaxation time T 1 and coherence time T 2 for the 3 principal axes of an anisotropic g tensor. Long electronic spin coherence time up to 24.4 μ s is measured for lowest g transition at 4 K, exceeding previously reported values at much lower temperatures. Measurements of decoherence mechanism indicates T 2 limited by spectral diffusion and instantaneous diffusion. Long spin coherence times, along with a strong anisotropic hyperfine interaction makes 167 Er 3+ :Y 2 O 3 a rich system and an excellent candidate for spin-based quantum technologies.more » « less
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Color-center–hosting semiconductors are emerging as promising source materials for low-field dynamic nuclear polarization (DNP) at or near room temperature, but hyperfine broadening, susceptibility to magnetic field heterogeneity, and nuclear spin relaxation induced by other paramagnetic defects set practical constraints difficult to circumvent. Here, we explore an alternate route to color-center–assisted DNP using nitrogen-vacancy (NV) centers in diamond coupled to substitutional nitrogen impurities, the so-called P1 centers. Working near the level anticrossing condition—where the P1 Zeeman splitting matches one of the NV spin transitions—we demonstrate efficient microwave-free 13 C DNP through the use of consecutive magnetic field sweeps and continuous optical excitation. The amplitude and sign of the polarization can be controlled by adjusting the low-to-high and high-to-low magnetic field sweep rates in each cycle so that one is much faster than the other. By comparing the 13 C DNP response for different crystal orientations, we show that the process is robust to magnetic field/NV misalignment, a feature that makes the present technique suitable to diamond powders and settings where the field is heterogeneous. Applications to shallow NVs could capitalize on the greater physical proximity between surface paramagnetic defects and outer nuclei to efficiently polarize target samples in contact with the diamond crystal.more » « less
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Abstract The composition of fluorescent polymer nanoparticles, commonly referred to as carbon dots, synthesized by microwave‐assisted reaction of citric acid and ethylenediamine was investigated by13C,13C{1H},1H─13C,13C{14N}, and15N solid‐state nuclear magnetic resonance (NMR) experiments.13C NMR with spectral editing provided no evidence for significant condensed aromatic or diamondoid carbon phases.15N NMR showed that the nanoparticle matrix has been polymerized by amide and some imide formation. Five small, resolved13C NMR peaks, including an unusual ═CH signal at 84 ppm (1H chemical shift of 5.8 ppm) and ═CN2at 155 ppm, and two distinctive15N NMR resonances near 80 and 160 ppm proved the presence of 5‐oxo‐1,2,3,5‐tetrahydroimidazo[1,2‐
a ]pyridine‐7‐carboxylic acid (IPCA) or its derivatives. This molecular fluorophore with conjugated double bonds, formed by a double cyclization reaction of citric acid and ethylenediamine as first shown by Y. Song, B. Yang, and coworkers in 2015, accounts for the fluorescence of the carbon dots. Cross‐peaks in a1H─13C HETCOR spectrum with brief1H spin diffusion proved that IPCA is finely dispersed in the polyamide matrix. From quantitative13C and15N NMR spectra, a high concentration (18 ± 2 wt%) of IPCA in the carbon dots was determined. A pronounced gradient in13C chemical‐shift perturbations and peak widths, with the broadest lines near the COO group of IPCA, indicated at least partial transformation of the carboxylic acid of IPCA by amide or ester formation.
- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1126/sciadv.aaz6986
