Discovery of Magnetic Superatoms and Assessment of van der Waals Dispersion Effects in Cs_<i>n</i> Clusters

We report the global minimum (GM) structures and electronic properties of Cs_<i>n</i> clusters with up to 80 atoms, obtained employing a density functional theory method which accounts for van der Waals dispersion interactions (vdW-DFT). The GM structures of Cs_<i>n</i> are found to differ markedly from those of lighter alkali clusters like Na_<i>n</i>. Three main physical factors are invoked to interpret the differences: vdW attraction, sd hybridization, and spin polarization. vdW effects are found to modify the GM structures of many Cs_<i>n</i> clusters as compared to the predictions of local and semilocal DFT approximations and tend to favor compact structures. sd hybridization accounts for the enhanced stability of strained motifs such as icosahedra or Kasper polyhedra. Finally, spin polarization stabilizes highly symmetric structures with a high orbital degeneracy. The electric dipole moments of most Cs_<i>n</i> clusters are close to zero, a distinguishing feature of metallicity previously observed for Na_<i>n</i> clusters. However, our predictions show that some clusters like Cs₁₀ or Cs₂₆ have sizable electric dipole moments. High spin multiplicities are observed for many different sizes. The spontaneous magnetization of pure Cs_<i>n</i> clusters is intrinsic and exclusively due to delocalized electrons. This allows us to extend the concept of superatoms (multicenter systems that mimic the electronic behavior of elemental atoms) so that it encompasses also magnetic properties, without the need to introduce localized electrons or extrinsic impurities. The physical reasons for the stability of magnetic superatoms are identified and described