Interlayer correlation between two He 4 monolayers adsorbed on both sides of α-graphyne

Path-integral Monte Carlo calculations have been performed to study the He4 adsorption on both sides of a single α-graphyne sheet. For investigation of the interlayer correlation between the upper and the lower monolayers of He4 adatoms, the He4-substrate interaction is described by the sum of the He4-C interatomic pair potentials for which we use both Lennard-Jones and Yukawa-6 anisotropic potentials. When the lower He4 layer is a C4/3 commensurate solid, the upper-layer He4 atoms are found to form a kagome lattice structure at a Mott-insulating density of 0.0706-2 and a commensurate solid at an areal density of 0.0941-2 for both substrate potentials. The correlation between upper- and lower-layer pseudospins, which were introduced in Kwon [Phys. Rev. B 88, 201403(R) (2013)]PRBMDO1098-012110.1103/PhysRevB.88.201403 for two degenerate configurations of three He4 atoms in a hexagonal cell, depends on the substrate potential used; with the substrate potential based on the anisotropic Yukawa-6 pair potentials, the Ising pseudospins of both He4 layers are found to be antiparallel to each other whereas the parallel and antiparallel pseudospin alignments between the two He4 layers are nearly degenerate with the Lennard-Jones potentials. This is attributed to the difference in the interlayer distance, which is ∼4 with the Yukawa-6 substrate potential but as large as ∼4.8 with the Lennard-Jones potential.