Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: The Intrinsic Origin of Spin Echoes in Dipolar Solids Generated by
  Strong Pi Pulses   In spectroscopy, it is conventional to treat pulses much stronger than the
linewidth as delta-functions. In NMR, this assumption leads to the prediction
that pi pulses do not refocus the dipolar coupling. However, NMR spin echo
measurements in dipolar solids defy these conventional expectations when more
than one pi pulse is used. Observed effects include a long tail in the CPMG
echo train for short delays between pi pulses, an even-odd asymmetry in the
echo amplitudes for long delays, an unusual fingerprint pattern for
intermediate delays, and a strong sensitivity to pi-pulse phase. Experiments
that set limits on possible extrinsic causes for the phenomena are reported. We
find that the action of the system's internal Hamiltonian during any real pulse
is sufficient to cause the effects. Exact numerical calculations, combined with
average Hamiltonian theory, identify novel terms that are sensitive to
parameters such as pulse phase, dipolar coupling, and system size.
Visualization of the entire density matrix shows a unique flow of quantum
coherence from non-observable to observable channels when applying repeated pi
pulses.

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The categories of this paper are: nlin
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The categories of this paper are: math-ph
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The categories of this paper are: physics
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The categories of this paper are: stat
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The categories of this paper are: hep-th
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The categories of this paper are: math
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The categories of this paper are: supr-con
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The categories of this paper are: eess
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The categories of this paper are: hep-ex
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The categories of this paper are: hep-ph
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The categories of this paper are: plasm-ph
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The categories of this paper are: econ
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The categories of this paper are: ao-sci
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The categories of this paper are: chao-dyn
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The categories of this paper are: patt-sol
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The categories of this paper are: alg-geom