Binary cluster crystals formed by ultrasoft particles: Classical density functional theory

We provide evidence of the existence of cluster crystals formed by abinary mixture of ultrasoft, cluster-forming particles. In an effortto reduce the high dimensionality of the parameter space of a generalbinary mixture we have investigated a binary, size-symmetric,equimolar mixture of these particles: here, the like-particleinteractions are identical, while the cross interaction betweenparticles is scaled by a factor $\zeta$, a parameter which plays apivotal role in our investigations. Applying classical densityfunctional -- DFT -- (with a mean-field format and a sphericallysymmetric Gaussian shape for the density profiles) we identify via anunbiased, unrestricted optimization of the functional with respect tothe parameters of the density profile and among ``all possible latticestructures' two ordered cluster crystal phases, a BCC-type and atetragonal lattice. The phase diagram provides evidence that the BCCphase is able to form stable cluster crystals in a wedge-shaped areain the temperature ($T$) vs. density ($\rho$) plane up to a thresholdvalue $\zeta_{\rm th}$, beyond which only the tetragonal latticeremains as the only ordered cluster crystal. A thorough discussion ofthe characteristic parameters of the system -- such as the occupancyof the clusters or the width of the density profile -- as we vary theexternal parameters $T$, $\rho$, and $\zeta$ provides a comprehensiveand exhaustive insight into the properties of this remarkable phase. Abrief account is given to ongoing computer simulations which provide astringent assessment of the simplifying assumptions that were imposedby the DFT-approach.