CONICET | Buscador de Institutos y Recursos Humanos

In the last few years, Nuclear Magnetic Resonance experiments in C60, 29Si and Y2O3 with unusual behaviors were reported: The spin-spin decay time (T2) measured through a multipulse sequence like CPMG results to be very long (order of seconds) compared to T2HE obtained through a Hahn Echo experiment (order milliseconds), although from analytic calculations it is well known that both sequences should give the same result.After a careful experimental and analytical study we were able to establish the necessary conditions for the long tails in the magnetization decays, i) an rf field inhomogeneity or a highly inhomogeneous line able to produce different tilting angles in different sites of the sample; ii) The absence of spin diffusion (noneffective flip-flop interactions). Because of condition (ii), the different tilting angles across the sample can not be averaged out, then different sites evolvedifferently leading to magnetization storage in the direction of the external magnetic field (z polarization), which then is brought back to the plane and observed as a long tail. So the long tails do not indicate long decoherence times.As we realized that the long tails are originated on some kind of disorder in the samples which can not be averaged out by the dipolar dynamics, we wonder if in analogy with the localization phenomena, there is a transition between tails -not tails behavior. For this purpose, we developed two strategies: To study the tails (a) as a function of the flip-flop effectivity as affected by time windows or special cancelling protocols, or (b) as a function of spin dilution inthe sample. In the present work, we used the first approach. Our results show that by allowing enough time for the flip-flop to become operative, the long tails tend to smoothly disappear. This was also confirmed through numerical calculations inspired on the C60 sample. The experiments performed so far do not show any transition, instead, a smooth crossover is observed.