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Edited by Jack H. Freed, Cornell University, Ithaca, NY, and accepted by the Editorial Board April 16, 2014 (received for review August 20, 2013)
Significance
Nuclear spins are only slightly aligned even in the strong magnetic fields of superconducting magnets because the magnetic energy of nuclear spin is much smaller than thermal energy. This is the major reason for the low sensitivity of NMR spectroscopy. Using electron spins in thermal equilibrium, which have 660 times higher magnetic energy, the sensitivity can be enhanced by at most this factor through a method called dynamic nuclear polarization. Utilizing photo-excited nonthermalized electrons instead, we demonstrate an enhancement factor of 250,000 at room temperature, which can be applied to a wide range of fields including NMR, MRI, and physics.
Abstract
Dynamic nuclear polarization (DNP), a means of transferring spin polarization from electrons to nuclei, can enhance the nuclear spin polarization (hence the NMR sensitivity) in bulk materials at most 660 times for 1H spins, using electron spins in thermal equilibrium as polarizing agents. By using electron spins in photo-excited triplet states instead, DNP can overcome the above limit. We demonstrate a 1H spin polarization of 34%, which gives an enhancement factor of 250,000 in 0.40 T, while maintaining a bulk sample (∼0.6 mg, ∼0.7 × 0.7 × 1 mm3) containing >1019 1H spins at room temperature. Room temperature hyperpolarization achieved with DNP using photo-excited triplet electrons has potentials to be applied to a wide range of fields, including NMR spectroscopy and MRI as well as fundamental physics.
Footnotes
↵1K.T. and M.N. contributed equally to this work.
- ↵2To whom correspondence should be addressed. E-mail: negoro{at}ee.es.osaka-u.ac.jp.
↵3Present address: Department of Applied Chemistry, Faculty of Engineering, Aichi Institute of Technology, Toyota 470-0392, Japan.
Author contributions: K.T., M.N., Y.M., and M.K. designed research; K.T. and M.N. performed research; K.T., S.N., A.K., and Y.M. contributed new reagents/analytic tools; K.T. and M.N. analyzed data; and K.T., M.N., and M.K. wrote the paper.
The authors declare no conflict of interest.
This article is a PNAS Direct Submission. J.H.F. is a guest editor invited by the Editorial Board.
This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10.1073/pnas.1315778111/-/DCSupplemental.
Room temperature nuclear hyperpolarization
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