Low-Temperature Quantum Relaxation in a System of Magnetic Nanomolecules

We argue that to explain recent resonant tunneling experiments on crystals of Mn₁₂ and Fe₈, particularly in the low-T limit, one must invoke dynamic nuclear spin and dipolar interactions. The low- T, short-time relaxation will then have a √t/τ form, where τ depends on the nuclear T₂, on the tunneling matrix element Δ₁₀ between the two lowest levels, and on the initial distribution N(ξ) of internal fields in the sample, which depends strongly on sample shape. We also give results for the long-time relaxation.