title: Dynamic nuclear polarization in the hyperfine-field-dominant region.
abstract: Dynamic nuclear polarization (DNP) allows measuring enhanced nuclear magnetic resonance (NMR) signals. Though the efficiency of DNP has been known to increase at low fields, the usefulness of DNP has not been throughly investigated yet. Here, using a superconducting quantum interference device-based NMR system, we performed a series of DNP experiments with a nitroxide radical and measured DNP spectra at several magnetic fields down to sub-microtesla. In the DNP spectra, the large overlap of two peaks having opposite signs results in net enhancement factors, which are significantly lower than theoretical expectations and nearly invariant with respect to magnetic fields below the Earth's field. The numerical analysis based on the radical's Hamiltonian provides qualitative explanations of such features. The net enhancement factor reached 325 at maximum experimentally, but our analysis reveals that the local enhancement factor at the center of the rf coil is 575, which is unaffected by detection schemes. We conclude that DNP in the hyperfine-field-dominant region yields sufficiently enhanced NMR signals at magnetic fields above 1 μT.
authors.affiliation: Center for Biosignals, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, Republic of Korea. Electronic address: sj.lee@kriss.re.kr.
authors.forename: Seong-Joo
authors.initials: SJ
authors.lastname: Lee
authors.affiliation: Center for Biosignals, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, Republic of Korea.
authors.forename: Jeong Hyun
authors.initials: JH
authors.lastname: Shim
authors.affiliation: Center for Biosignals, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, Republic of Korea.
authors.forename: Kiwoong
authors.initials: K
authors.lastname: Kim
authors.affiliation: Center for Biosignals, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, Republic of Korea.
authors.forename: Kwon Kyu
authors.initials: KK
authors.lastname: Yu
authors.affiliation: Center for Biosignals, Korea Research Institute of Standards and Science (KRISS), 267 Gajeong-ro, Yuseong-gu, Daejeon 305-340, Republic of Korea.
authors.forename: Seong-min
authors.initials: SM
authors.lastname: Hwang
keywords: Dynamic nuclear polarization; Hyperfine-field-dominant region; SQUID detection; Ultra-low field NMR