Decoherence Window and Electron-Nuclear Cross Relaxation in the Molecular Magnet V-15

Rabi oscillations in the V15 single molecule magnet embedded in the surfactant (CH3)2 X [CH3(CH2)16CH2]2N+ 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 ωN. The experiments are interpreted by a model showing that the damping (or Rabi) time τR is directly associated with decoherence caused by electron-nuclear cross relaxation in the rotating reference frame. This decoherence induces energy dissipation in the range ωN - σe < ΩR < ωN, where σe is the mean superhyperfine field induced by protons at V15. Weaker decoherence without dissipation takes place outside this window. Specific estimations suggest that this rapid cross relaxation in a resonant microwave field, observed for the first time in V15, should also take place, e.g., in Fe8 and Mn12.