FRANZONI MARIA BELEN
Congresos y reuniones científicas
Título:
Spin Coherence and Pseudo-coherence in Magnetically Diluted Systems: A Manifestation of Localization in Nuclear Magnetic Resonance Experiments
Autor/es:
FRANZONI M.B, LEVSTEIN P.R.
Lugar:
Buenos Aires, Argentina
Reunión:
Workshop; Eight Giambiagi Winter School; 2006
Resumen:
Puzzling anomalies previously observed in multipulse NMR experiments in natural abundance 29Si [A.E. Dementyev, D. Li, K. MacLean, and S.E. Barrett, Phys. Rev. B 68, 153302 (2003)] such as long-lived spin echoes and even-odd asymmetries, are also found in polycrystalline C60. Further experiments controlling the phases and tilting angles of the pulse trains, as well as analytical and numerical calculations allowed us to explain the origin of these anomalies [M. B. Franzoni and P. R. Levstein Phys. Rev. B 72, 235410 (2005)]. We prove that the observation of long magnetization tails requires two conditions: i) an rf field inhomogeneity able to produce different tilting angles in different sites of the sample and ii) the absence of spin diffusion (non-effective flip-flop interactions). The last requirement is easily satisfied in diluted dipolar solids, where the frequency differences between sites, caused by disorder or other sources, are usually at least one order of magnitude larger than the dipolar couplings. Both conditions lead to the generation of stimulated echoes in Carr-Purcell (CP) and Carr-Purcell-Meiboom-Gill (CPMG) pulse trains. We show, both experimentally and theoretically, that the stimulated echoes interfere constructively or destructively with the normal (Hahn) echoes depending on the alternation or not of the p pulse phases in the CP and the CPMG sequences. Constructive interferences occur for the CP and CPMG sequences with and without phase alternation respectively, which are the cases where long magnetization tails are observed. In the present work, we have also studied how long time (Tl) magnetizations depend on the interpulse separation. As the interpulse separation increases, the dipolar interaction between magnetic spins becomes operative with a consequent decrease in Tl. Using the fact that the stimulated and normal echoes interfere constructively or destructively depending on the pulse phases of the sequences, we designed a sequence based on phase cycling to obtain the decay time of the stimulated echo. T. D. Ladd et al [Phys. Rev. B 71, 014401 (2005)] reported similar NMR experiments but using decoupling techniques that show coherence times for 29Si which are four orders of magnitude longer than those observed previously. The same kind of experiments (homodecoupling using multipulse sequences) are performed in C60. We find that the magnetization persists for long times, order of seconds, but we realize that for these sequences the same effect due to the presence of stimulated echoes is manifested. Thus, these long decay times cannot be interpreted as the true coherence times. As expected, the behavior and decay time of the stimulated echoes are independent on the presence of the decoupling multipulse sequence.
