Demixing in symmetric supersolid mixtures

The droplet crystal phase of a symmetric binary mixture of soft-core bosons is studied by computer simulation. At high temperature each droplet comprises on average equal numbers of particles of either component, but the two components demix below the supersolid transition temperature, i.e., droplets mostly consist of particles of one component. Clustering of droplets of the same component is also observed. Demixing is driven by quantum tunneling of particles across droplets over the system and does not take place in an insulating crystal. This effect provides an unambiguous experimental signature of supersolidity.

Figure 1

Number of particles per species $N_{\alpha }$ ($\alpha =1,2$) as a function of simulation time at temperatures (a) $T=20{\epsilon }_{\circ }$, (b) $T=10{\epsilon }_{\circ }$, and (c) $T=5{\epsilon }_{\circ }$, for a system size ${(L/r_{\circ })}^2=1.92$. Phase equilibrium requires that the chemical potentials for both species be equal, with ${\mu }_1={\mu }_2$, chosen so the total number of particles is $N\approx 170$. In all cases the total simulation time consists of 7000 block averages, each block consisting of ${10}^4$ Monte Carlo moves. Note the light gray (blue online) curve corresponds to species $\alpha =1$, the dark gray (red online) corresponds to $\alpha =2$, whereas the dotted curves represent the equivalent simulation in the case of Boltzmann statistics.

Figure 2

(a) Demixing parameter $D$ as a function of temperature. (b) Corresponding superfluid fraction $f_s$ for component $a$. Parameters are $r_s/r_{\circ }=0.15$ and $a/r_{\circ }=0.27$. Where error bars are absent, statistical errors are smaller than the point size.