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

Abstract

Path-integral Monte Carlo calculations were performed to study the adsorption of ${}^4$He atoms on $\alpha$-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.

5 pages, 4 figures, submitted to Physical Review Letters