Phase Diagram of 4He Film in 3D Nanopores of ZTC

In the study to examine effects of three-dimensional (3D) film connectivity on superfluid ⁴He film in the pore network, we have measured heat capacities of ⁴He film adsorbed in zeolite templated carbon (ZTC), which is an assembly of graphene fragments and has network of 1.2 nm pore with the 3D period of 1.4 nm, shorter than those of nanoporous materials studied so far. From the heat of desorption, the ⁴He film adsorbed in ZTC was observed to be formed up to about 1.4 atomic layers. Heat capacities of the ⁴He film are rather similar to those on SiO₂-based porous materials than those on graphite, except for large heat capacities of monolayer ⁴He film. At low coverages, the heat capacity rapidly decreases below a temperature T _L, suggesting a localized state of ⁴He at T<T _L. The T _L almost monotonically lowers with increasing the coverage. Heat capacity isotherms show maxima around 1.1 layers, which suggests quantum degenerate fluid (Bose fluid) above the coverage. From these results, the phase diagram of ⁴He film adsorbed in ZTC has been determined.