Zero-field magnetometry using hyperfine-biased nitrogen-vacancy centers near diamond surfaces

Shallow nitrogen-vacancy (NV) centers in diamond are promising for nanomagnetometry, they can be placed proximate to targets. To study the intrinsic magnetic properties, zero-field magnetometry is desirable. However, shallow NV under zero field, strain near surfaces would cause level anticrossing between spin states, leading clock transitions whose frequencies insensitive signals. Furthermore, charge noises from induce extra decoherence and hence reduce sensitivity. Here, we demonstrate that relatively strong hyperfine coupling (130 MHz) a first-shell $^{13}\mathrm{C}$ nuclear provide an effective bias field center so clock-transition condition broken suppressed. The enhances dc sensitivity by factor of 22 our setup. With suppressed ac also reaches limit set due bath. In addition, 130 MHz splitting allows relaxometry simultaneously at two well-separated (\ensuremath{\sim}2.870 \ifmmode\pm\else\textpm\fi{} 0.065 GHz), providing (low-resolution) spectral information high-frequency field. hyperfine-bias-enhanced combined with dynamical decoupling enhance single-molecule resonance spectroscopy improve frequency resolution nanoscale imaging.