1 Decoherence window and electron - nuclear cross - relaxation in the molecular

Rabi oscillations in the V15 Single Molecule Magnet (SMM) embedded in the surfactant DODA have been studied at different microwave powers. An intense damping peak is observed when the Rabi frequency ΩR falls in the vicinity of the Larmor frequency of protons N ω , while the damping time R τ of oscillations reaches values 10 times shorter than the phase coherence time 2 τ measured at the same temperature. The experiments are interpreted by the N-spin model showing that R τ is directly associated with the decoherence via electronic/nuclear spin cross-relaxation in the rotating reference frame. It is shown that this decoherence is accompanied with energy dissipation in the range of the Rabi frequencies N e R N ω σ ω − < Ω < , where e σ is the mean super-hyperfine field (in frequency units) induced by protons at SMMs. Weaker damping without dissipation takes place outside this dissipation window. Simple local field estimations suggest that this rapid cross-relaxation in resonant microwave field observed for the first time in SMM V15 should take place in other SMMs like Fe8 and Mn12 containing protons, too. PACS numbers: 36.40.-c, 76.30.-v, 03.65.Yz ha l-0 04 95 05 5, v er si on 3 16 M ay 2 01 2