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NMR spin echo measurements of ^13C in C₆₀, ^89Y in Y₂O₃, and ^29Si in silicon are shown to defy conventional expectations when more than one pulse is used. Multiple -pulse echo trains may either freeze out or accelerate the decay of the signal, depending on the -pulse phase. Average Hamiltonian theory, combined with exact quantum calculations, reveals an intrinsic cause for these coherent phenomena: the dipolar coupling has a many-body effect during any real, finite pulse.
Li et al. (Thu,) studied this question.