Proximity effects in cold atom artificial graphene

TL;DR

This paper explores the phase transitions and magnetic properties of bilayer cold atom artificial graphene systems with different lattice geometries, revealing potential for experimental realization and novel quantum phases.

Contribution

It introduces a bilayer model of artificial graphene with proximity effects, analyzing phase transitions and magnetic order using mean-field and quantum Monte Carlo methods.

Findings

Phase transition from semi-metal to band insulator in non-interacting fermions.

Competition between semi-metal and superfluid phases with attractive interactions.

Formation of a valence-bond crystal in the Mott phase with strong repulsive interactions.

Abstract

Cold atoms in an optical lattice with brick-wall geometry have been used to mimic graphene, a two-dimensional material with characteristic Dirac excitations. Here we propose to bring such artificial graphene into the proximity of a second atomic layer with a square lattice geometry. For non-interacting fermions, we find that such bilayer system undergoes a phase transition from a graphene-like semi-metal phase, characterized by a band structure with Dirac points, to a gapped band insulator phase. In the presence of attractive interactions between fermions with pseudospin-1/2 degree of freedom, a competition between semi-metal and superfluid behavior is found at the mean-field level. Using the quantum Monte Carlo method, we also investigate the case of strong repulsive interactions. In the Mott phase, each layer exhibits a different amount of long-range magnetic order. Upon coupling both layers, a valence-bond crystal is formed at a critical coupling strength. Finally, we discuss how these bilayer systems could be realized in existing cold atom experiments.