  We report polarization of a dense nuclear-spin ensemble in diamond and its
dependence on magnetic field and temperature. The polarization method is based
on the transfer of electron spin polarization of negatively charged nitrogen
vacancy color centers to the nuclear spins via the excited-state level
anti-crossing of the center. We polarize 90% of the 14N nuclear spins within
the NV centers, and 70% of the proximal 13C nuclear spins with hyperfine
interaction strength of 13-14 MHz. Magnetic-field dependence of the
polarization reveals sharp decrease in polarization at specific field values
corresponding to cross-relaxation with substitutional nitrogen centers, while
temperature dependence of the polarization reveals that high polarization
persists down to 50 K. This work enables polarization of the 13C in bulk
diamond, which is of interest in applications of nuclear magnetic resonance, in
quantum memories of hybrid quantum devices, and in sensing.
