Nuclear magnetic resonance studies of tunneling in quantum solids at very low temperatures

A review is given of recent NMR experiments at ultra-low temperatures that explore properties of quantum fluids and solids in regimes where unusual dynamics characterizes the low temperature behavior. It is shown how careful analysis of the NMR spin-spin and spin-lattice relaxation rates and spectral properties can determine fundamental thermodynamic features that are otherwise difficult to observe using standard thermodynamic methods. The review focuses on the observation of the diffusion of vacancies and isotopic impurities in solid hydrogen and solid helium by quantum tunneling at low temperatures.