  We measure the lifetime of long-lived nuclear spin singlet states as a
function of the strength of the RF spin-locking field and present a simple
theoretical model that agrees well with our measurements, including the
low-RF-power regime. We also measure the lifetime of a long-lived coherence
between singlet and triplet states that does not require a spin-locking field
for preservation. Our results indicate that for many molecules, singlet states
can be created using weak RF spin-locking fields: more than two orders of
magnitude lower RF power than in previous studies. Our findings suggest that in
many biomolecules, singlets and related states with enhanced lifetimes might be
achievable in vivo with safe levels of RF power.
