Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: Manifestations of the absence of spin diffusion in multipulse NMR
  experiments on diluted dipolar solids   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. 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 pi 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. Sequences with two, three and four pi pulses after the pi/2 pulse
allow us to disentangle the contributions of the different echoes and show how
the stimulated echoes originate the even-odd asymmetry observed in both 29Si
and C60 polycrystalline samples.

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The categories of this paper are: nlin
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The categories of this paper are: chao-dyn
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The categories of this paper are: ao-sci
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The categories of this paper are: eess
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The categories of this paper are: alg-geom
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The categories of this paper are: q-fin
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The categories of this paper are: supr-con
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The categories of this paper are: nucl-th
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The categories of this paper are: dg-ga
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The categories of this paper are: physics
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The categories of this paper are: plasm-ph
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The categories of this paper are: math-ph
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The categories of this paper are: gr-qc
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The categories of this paper are: hep-ex
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The categories of this paper are: solv-int
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The categories of this paper are: nucl-ex