Search for ultralight axion dark matter in a side-band analysis of a 199Hg free-spin precession signal

Ultra-low-mass axions are a viable dark matter candidate and may form a coherently oscillating classical field. Nuclear spins in experiments on Earth might couple to this oscillating axion dark-matter field, when propagating on Earth’s trajectory through our Galaxy. This spin coupling resembles an oscillating pseudo-magnetic field which modulates the spin precession of nuclear spins. Here we report on the null result of a demonstration experiment searching for a frequency modulation of the free spin-precession signal of \magHg in a \SI{1}{\micro\tesla} magnetic field. Our search covers the axion mass range ${10}^{-16}\text{ eV}\lesssim m_a\lesssim {10}^{-13}\text{ eV}$ and achieves a peak sensitivity to the axion-nucleon coupling of $g_{aNN}\approx 3.5\times {10}^{-6}{\text{GeV}}^{-1}$.