Two-level systems in hydrogenated amorphous silicon: NMR studies

A low-temperature deviation from the spin-lattice relaxation of bonded hydrogen by molecular hydrogen is observed in a-Si:H. Two likely causes for this deviation, namely, relaxation by paramagnetic electronic spins and by two-level systems (TLS), are both shown to be negligible. In ruling out TLS or tunneling modes, a second NMR experiment is performed that allows a low limit to be placed on the number or excursion of H atoms involved in a tunneling mode. These results indicate that the H-atom motion is not involved to any significant degree in the atomic motion associated with a TLS.