  We use field-cycling-assisted dynamic nuclear polarization and continuous
radio-frequency (RF) driving over a broad spectral range to demonstrate
magnetic-field-dependent activation of nuclear spin transport from
strongly-hyperfine-coupled 13C sites in diamond. We interpret our observations
with the help of a theoretical framework where nuclear spin interactions are
mediated by electron spins. In particular, we build on the results from a
4-spin toy model to show how otherwise localized nuclear spins must thermalize
as they are brought in contact with a larger ancilla spin network. Further, by
probing the system response to a variable driving field amplitude, we witness
stark changes in the RF-absorption spectrum, which we interpret as partly due
to contributions from heterogeneous multi-spin sets, whose 'zero-quantum'
transitions become RF active thanks to the hybrid electron-nuclear nature of
the system. These findings could prove relevant in applications to dynamic
nuclear polarization, spin-based quantum information processing, and nanoscale
sensing.
