Excitations in Quantum Crystals (A Survey of NMR Experiments in Solid Helium)

This article gives a systematic review of the many NMR experiments in solid helium to date, emphasizing the view-point that the results may be interpreted primarily in terms of the effects of three fundamental excitations in the solid: the vacancy waves; the $^{3}\mathrm{He}$ tunneling interaction or exchange; and the $^{3}\mathrm{He}$-$^{4}\mathrm{He}$ tunneling interaction or mass fluctuation waves.