  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.
