Henshaw Jacob, Pagliero Daniela, Zangara Pablo R, Franzoni María B, Ajoy Ashok, Acosta Rodolfo H, Reimer Jeffrey A, Pines Alexander, Meriles Carlos A
Department of Physics, City College of New York, City University of New York, New York, NY 10031.
Facultad de Matemática, Astronomía, Física y Computación, Universidad Nacional de Córdoba, CP X5000HUA Córdoba, Argentina.
Proc Natl Acad Sci U S A. 2019 Sep 10;116(37):18334-18340. doi: 10.1073/pnas.1908780116. Epub 2019 Aug 26.
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 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 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.
含色心的半导体正成为在室温或接近室温下进行低场动态核极化（DNP）的有前途的源材料，但超精细展宽、对磁场不均匀性的敏感性以及由其他顺磁缺陷引起的核自旋弛豫带来了难以规避的实际限制。在此，我们探索了一条利用金刚石中的氮空位（NV）中心与替代氮杂质（即所谓的P1中心）耦合来实现色心辅助DNP的替代途径。在接近能级反交叉条件下工作——即P1塞曼分裂与NV自旋跃迁之一相匹配——我们通过连续的磁场扫描和连续的光激发展示了高效的无微波碳DNP。通过在每个周期中调整低到高和高到低的磁场扫描速率，使得其中一个比另一个快得多，可以控制极化的幅度和符号。通过比较不同晶体取向的碳DNP响应，我们表明该过程对磁场/NV失准具有鲁棒性，这一特性使得本技术适用于金刚石粉末以及磁场不均匀的环境。应用于浅NVs可以利用表面顺磁缺陷与外层原子核之间更近的物理距离，有效地极化与金刚石晶体接触的目标样品。
