Mott Insulator to Superfluid Quantum Phase Transition for Helium on Strained Graphene*

An exciting development in the field of correlated systems is the possibility of realizing two-dimensional (2D) superfluidity, particularly by adsorbing helium on novel 2D quantum materials, such as graphene. How superfluidity emerges in this system, and whether it could exist in the first, second, etc. layer, has been a topic of considerable controversy. We argue theoretically that under certain favorable external conditions where uniaxial stress is applied to graphene, 2D anisotropic superfluidity can form in the first layer. This result is based on preliminary large-scale ab initio quantum Monte Carlo simulations combined with a mapping of the problem to an effective Bose-Hubbard model. We show that a critical ratio of the onsite repulsion to hopping strength (U/t) can be achieved via strain, allowing for a quantum phase transition to a superfluid state below a critical value. Our analysis supports, for the first time, the existence of an unconventional first layer 2D anisotropic superfluid, possibly also exhibiting supersolid correlations reflecting the underlying graphene lattice structure.