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Quantum Interference Phenomena in the Local Polarization Dynamics of Mesoscopic Systems: An NMR Observation

It was predicted that local spin polarization in a ring of five dipolar coupled spins should present a particular fingerprint of quantum interferences reflecting both the discrete and finite nature of the system [Phys. Rev. Lett. 75 (1995) 4310]. We report its observation for the proton system of a (C$_5$H$_5$)$_2$Fe molecule using a rare $^{13}$C as {\it local probe}. Novel high frequency ($\simeq 60k$Hz) polarization oscillations appear because incomplete $^{13}$C-$^1$H cross-polarization transfer {\it splits} the polarization state, in a portion that wanders in the proton system and one that remains in the $^{13}$ C. They interfere with each other after rejoining.