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

Abstract

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.