Hund’s rule in superatoms with transition metal impurities

Abstract

The quantum states in metal clusters bunch into supershells with associated orbitals having shapes resembling those in atoms, giving rise to the concept that selected clusters could mimic the characteristics of atoms and be classified as superatoms. Unlike atoms, the superatom orbitals span over multiple atoms and the filling of orbitals does not usually exhibit Hund's rule seen in atoms. Here, we demonstrate the possibility of enhancing exchange splitting in superatom shells via a composite cluster of a central transition metal and surrounding nearly free electron metal atoms. The transition metal d states hybridize with superatom D states and result in enhanced splitting between the majority and minority sets where the moment and the splitting can be controlled by the nature of the central atom. We demonstrate these findings through studies on TMMg n clusters where TM is a 3d atom. The clusters exhibit Hund's filling, opening the pathway to superatoms with magnetic shells.magnetic superatoms | jellium model | superatomic shells T he quantum confinement of electrons in small compact symmetric metal clusters results in electronic shell sequence 1S, 1P, 1D, …, much in the same way as in atoms. This analogy, originally introduced through the electronic states in a "jellium sphere" where the electron gas is confined to a uniform positive background of the size of the cluster, extends beyond this oversimplified model (1-9). Numerous first principles electronic structure studies on metal clusters have demonstrated the close grouping of electronic states into shells and have further shown that the shapes of the cluster electronic orbitals resemble those in atoms. Experiments on the reactivity of clusters have provided evidence that clusters and atoms of similar valence shells exhibit analogous chemical patterns. For example, although bulk aluminum is readily oxidized by oxygen, an Al 13 − cluster with filled 1S 2 , 1P 6 , 1D 10 , 2S 2 , 1F 14 , and 2P 6 shells exhibits strong resistance to etching by oxygen typical of inert atoms (5, 10). Further Al 13 has a large electron affinity of 3.4 eV close to that of a Cl atom (9). These analogies have prompted the concept that selected stable clusters could mimic the electronic behavior of elemental atoms and be classified as superatoms forming a third dimension of the periodic table (10-21). Because the properties of clusters change with size and composition, the superatoms offer the prospect of serving as the building blocks of nanomaterials with tunable characteristics (16,18).The electronic orbitals in superatoms, although resembling those in real atoms in shape, do spread over multiple atoms. This affects the way in which the electrons fill the shells because of two competing effects. Hund's rule favors high spin states in open shell systems stabilized by exchange coupling, and indeed higher spin multiplicities have been seen in some clusters (22) and even quantum dots spanning several nanometers (23). However, unlike the case of atoms, small clusters can undergo structu...

Interfacial geometry dependence of the iron magnetic moment: The case of MgO/Fe/MgO

“…It reaches about 4 μ B , thus either indicating the valence state of iron to be closer to Fe 2+ , as for the case of diluted iron impurities in MgO, 28 or the Fe atom to be bounded to Mg ions. 29 We should note that XMCD experiments in our samples 30 showed neither mm induced on the MgO due to the proximity of the ferromagnet, nor a multiplet structure indicative of iron oxidation. We surmise the answer to this dilemma depends on a complex interplay between the structural changes and the charge-transfer process at the interface.…”

The role of TM’s (M’s) d valence electrons in TM@X12 and M@X12 clusters

“…To achieve magnetic superatoms, a singely magnetic TM atom were embedded into a small simple-metal cluster, because either the D-state (magnetic state) of superatoms almost exclusively comes from atomic d-state (that is, for the filling superatomic shells, atomic d electrons are thought as delocalized valence electrons) or its superatomic D-state (magnetic state) hybridize strongly with atomic d-states (that is, atomic d electrons are strongly localized and not fill superatomic shells) in previous reports. 6,9,10,12,13,[29][30][31][32][33][34][35] The exchange-splitting between the majority of superatomic D shells (atomic d shells) and their minority states can resulted in by both cases of above, so a superatom can have a corresponding spin magnetic moment. However, the spin magnetic moment of one superatom may be very vulnerable to its surrounding environments, which restricts the practical applications of superatoms.…”

Magnetism in Simple Metal and 4d Transition Metal Clusters

“…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].…”

“…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].…”

“…More recently, the idea of superatoms has been extended to include magnetic species called 'magnetic superatoms'. Some of the fundamental ideas about designing magnetic superatoms may be found in these references [2,[18][19][20]. 796 P. Sen…”