⁴He Fluid in Extremely Narrow 1D Channels 1.5 nm in Diameter

Abstract

To examine whether one-dimensional (1D) helium quantum fluid is realized in narrower channels than those studied previously, we have measured heat capacities of ⁴He adsorbed in nanoporous material FSM with straight 1D channels 1.5 nm in diameter. From the heat of desorption for adsorbed ⁴He, the coverage n _f, up to which ⁴He film grows in the channels, is determined to be 15.4 μmol/m² using the Frenkel-Halsey-Hill model. At coverages sufficiently below n _f, the temperature dependence of the ⁴He heat capacity has a shoulder, above which adsorbed ⁴He is delocalized from the substrate. On the other hand, the depression of the heat capacity indicating quantum effects has not been observed up to n _f, which suggests that ⁴He film in the channels remains amorphous-like normal fluid. Just above n _f, the quantum effect is observed in ⁴He adatoms on the grain surface of FSM powder, which indicates that 1.5 nm channels are slightly below the limit required to realize quantum effect in the inside ⁴He fluid.