Long-lived heteronuclear spin-singlet states in liquids at a zero magnetic field

We report an observation of long-lived spin-singlet states in a $$spin pair in a zero magnetic field. In $$-labeled formic acid, we observe spin-singlet lifetimes as long as 37 s, about a factor of 3 longer than the $$lifetime of dipole polarization in the triplet state. In contrast to common high-field experiments, the observed coherence is a singlet-triplet coherence with a lifetime $$longer than the $$lifetime of dipole polarization in the triplet manifold. Moreover, we demonstrate that heteronuclear singlet states formed between a $$and a $$nucleus can exhibit longer lifetimes than the respective triplet states even in the presence of additional spins that couple to the spin pair of interest. Although long-lived homonuclear spin-singlet states have been extensively studied, this is the first experimental observation of analogous singlet states in heteronuclear spin pairs.