  Characterizing the local internal environment surrounding solid-state spin
defects is crucial to harnessing them as nanoscale sensors of external fields.
This is especially germane to the case of defect ensembles which can exhibit a
complex interplay between interactions, internal fields and lattice strain.
Working with the nitrogen-vacancy (NV) center in diamond, we demonstrate that
local electric fields dominate the magnetic resonance behavior of NV ensembles
at low magnetic field. We introduce a simple microscopic model that
quantitatively captures the observed spectra for samples with NV concentrations
spanning over two orders of magnitude. Motivated by this understanding, we
propose and implement a novel method for the nanoscale localization of
individual charges within the diamond lattice; our approach relies upon the
fact that the charge induces an NV dark state which depends on the electric
field orientation.
