Discovery of Magnetic Superatoms and Assessment of van der Waals Dispersion Effects in Cs_n Clusters

Abstract

We report the global minimum (GM) structures and electronic properties of Cs n 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 n are found to differ markedly from those of lighter alkali clusters like Na n . 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 n 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 n clusters are close to zero, a distinguishing feature of metallicity previously observed for Na n clusters. However, our predictions show that some clusters like Cs 10 or Cs 26 have sizable electric dipole moments. High spin multiplicities are observed for many different sizes. The spontaneous magnetization of pure Cs n 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.

Magnetism in Simple Metal and 4d Transition Metal Clusters

“…Electronic structure calculations that Aguado performed [58] on the Cs 13 cluster, in fact, support this conjecture. He performed several calculations on the icosahedron (I h ), C 2 and C s isomers.…”

“…This was in fact demonstrated for the first time in, what has by now become a cult paper in the area of cluster science, by Knight et al [3]. Na n clusters at sizes 8,18,20,40,58 and 92 were found to be more stable than the neighboring sizes. Analogy of the simple metal clusters was taken a step further when it was experimentally shown that clusters with one electron less than shell filling have large electron affinities (EA), like halogen atoms [4][5][6][7], and those with one electron more than shell filling have small ionization potentials (IP), like alkali atoms [8][9][10][11][12].…”

“…The idea is best explained with the example of the Cs 13 cluster as demonstrated by Aguado [58]. According to the shell model, electron configuration of Cs 13 (or other 13-atom alkali clusters) is 1S 2 1P 6 1D 5 .…”

“…If the JT distortion is large, one would have a single unpaired spin in the majority spin channel, whereas a large exchange splitting would give a high spin ground state. Aguado's calculations [58] showed that Cs 15 does undergo a JT distortion from the perfectly symmetric Kasper polyhedron of D 6d symmetry to a lower symmetry D 2 structure. The associated splittings of the orbitals in both spin channels are smaller than 0.02 eV.…”

“…Noting that S, P, D… orbitals can accommodate maximum of 2, 6, 10,… electrons, clusters with 2,8,18,20,34,40,58,… valence electrons will have filled electronic shells, and hence will be more stable than their neighbors. This was in fact demonstrated for the first time in, what has by now become a cult paper in the area of cluster science, by Knight et al [3].…”