The tiny difference between hard pi pulses and their delta-function approximation can be exploited to control coherence. Variants on the magic echo that work despite a large spread in resonance offsets are demonstrated using the zeroth- and first-order average Hamiltonian terms, for 13-C NMR in C60. The 29-Si NMR linewidth of Silicon has been reduced by a factor of about 70,000 using this approach, which also has potential applications in magnetic resonance microscopy and imaging of solids.
@article{arxiv.0804.3087,
title = {Controlling coherence using the internal structure of hard pi pulses},
author = {Yanqun Dong and R. G. Ramos and Dale Li and S. E. Barrett},
journal= {arXiv preprint arXiv:0804.3087},
year = {2009}
}