Phase transitions in two-dimensional fluids with internal quantum states

A model fluid whose molecules have classical degrees of freedom in 2D and two internal quantum states is studied by path-integral Monte Carlo simulation. We obtain a rich phase diagram including phase coexistences and tricritical and triple points with high precision by combining for the first time block-analysis finite-size scaling ideas with quantum Monte Carlo simulations. We find a square lattice solid phase in coexistence with gas and/or liquid phases. Compressibilities and susceptibilities are compared to mean-field predictions and low-density expansions.