Mott-insulator to commensurate-solid transition in a 4He layer on α-graphyne: Pseudo-spin symmetry breaking under a particle-induced pseudo-magnetic field

Path-integral Monte Carlo calculations were performed to study the adsorption of $^4$He atoms on $α$-graphyne. We find that one $^4$He atom can be embedded onto the in-plane center of each hexagon of the graphyne. In the first $^4$He layer above the $^4$He-embedded graphyne surface, a Mott insulating state was observed at the areal density of 0.0706 Å$^{-2}$ with three $^4$He atoms occupying each hexagonal cell while the helium atoms form a commensurate triangular solid at a density of 0.0941 Å$^{-2}$. Here we show that the Ising pseudo-spin symmetry introduced for two degenerate configurations of three $^4$He atoms in a hexagonal cell can be broken by additional $^4$He atoms placed at the hexagon vertices and the Mott-insulator to commensurate-solid transition is a transition from a nonmagnetic spin liquid of frustrated antiferromagnets to a spin-aligned ferromagnet under a particle-induced pseudo-magnetic field.

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