Atomic Motion in Low-Coverage Helium Films Adsorbed in FSM Nanochannels

摘要

~4He and ~3He films adsorbed in nanoporous silicates have shown similar heat capacities until the quantum-fluid layer appears at coverages over the first-layer completion n_1. To obtain information on dynamics of adatoms at low coverages below the quantum fluid region, we have done pulsed-NMR experiment at 3.3 MHz for ~3He films adsorbed in straight 2.4 nm nanochannels of FSM silicates. The spin-lattice and spin-spin relaxation times T_1 and T_2 observed at 0.54-7 K were well described by the two-dimensional version of the Bloembergen-Purcell-Pound model. At coverages 0.4-1.7n_1, minima of T_1, indicating the spin correlation time τ_c of 4.8 × 10~(-8) sec, were observed at temperatures between 6 and 3 K. With decreasing temperature, changes in T_1 and T_2 become small below about 1.5 K, suggesting crossover from thermally-activated motion to quantum tunneling. In contrast to large variations below n_1, both relaxation times above n_1 are almost independent of coverage, which is likely to indicate that τ_c is determined by interlayer exchange of adatoms. Below n_1, onsets for localization of adatoms were suggested by a decrease in T_2.