Exchange-induced crystallization of soft-core bosons

We study the phase diagram of a two-dimensional assembly of bosons interacting via a soft-core repulsive pair potential of varying strength, and compare it to that of the equivalent system in which particles are regarded as distinguishable. We show that quantum-mechanical exchanges stabilize a 'cluster crystal' phase in a wider region of parameter space than predicted by calculations in which exchanges are neglected. This physical effect is diametrically opposite to that which takes place in hard-core Bose systems such as He-4, wherein exchanges strengthen the fluid phase. This is underlain in the cluster crystal phase of soft-core bosons by the free energy gain associated with the formation of local Bose-Einstein condensates.