Generating Unexpected Spin Echoes in Dipolar Solids with<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mi>π</mml:mi></mml:math>Pulses

Abstract

NMR spin echo measurements of $^{13}\mathrm{C}$ in ${\mathrm{C}}_{60}$, $^{89}\mathrm{Y}$ in ${\mathrm{Y}}_{2}{\mathrm{O}}_{3}$, and $^{29}\mathrm{Si}$ in silicon are shown to defy conventional expectations when more than one $\ensuremath{\pi}$ pulse is used. Multiple $\ensuremath{\pi}$-pulse echo trains may either freeze out or accelerate the decay of the signal, depending on the $\ensuremath{\pi}$-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.