The quantum states in metal clusters are grouped into bunches of close-lying eigenvalues, termed electronic shells, similar to those of atoms. Filling of the electronic shells with paired electrons results in local minima in energy to give stable species called magic clusters. This led to the realization that selected clusters mimic chemical properties of elemental atoms on the periodic table and can be classified as superatoms. So far the work on superatoms has focused on non-magnetic species. Here we propose a framework for magnetic superatoms by invoking systems that have both localized and delocalized electronic states, in which localized electrons stabilize magnetic moments and filled nearly-free electron shells lead to stable species. An isolated VCs(8) and a ligated MnAu(24)(SH)(18) are shown to be such magnetic superatoms. The magnetic superatoms' assemblies could be ideal for molecular electronic devices, as the coupling could be altered by charging or weak fields.

We study the two orbital double-exchange model in two dimensions in the presence of antiferromagnetic (AF) superexchange, strong Jahn-Teller coupling, and substitutional disorder. At hole doping x = 0.5 we explore the 'bicritical' regime where the energy of a ferromagnetic metal and a charge and orbital ordered (CO-OO) CE state are closely balanced, and compare the impact of weak homogeneous disorder to that of a low density of strong scatterers. Even moderate homogeneous disorder suppresses the CE-CO-OO phase and leads to a glass with nanoscale correlations. Dilute strong scatterers of comparable strength, however, convert the CE-CO-OO phase to a phase separated state with ferromagnetic and AF-CO-OO clusters. We provide the first spatial description of these phenomena and compare our results in detail to experiments on the half-doped manganites.The manganese oxides of the form A 1−x A' x MnO 3 involve a remarkable interplay of charge, spin, lattice, and orbital degrees of freedom [1]. This cross coupling is most striking in the half doped (x = 0.5) manganites many of which have a charge and orbital ordered insulating (CO-OO-I) ground state with 'CE' magnetic order -a zigzag pattern of ferromagnetic chains with antiferromagnetic (AF) coupling between them. The CE-CO-OO-I phase shows up in manganites with low mean cation radius (r A ) while systems with large r A are ferromagnetic metals (FM-M). The variation of r A leads to a 'bicritical' phase diagram [2] with a first order boundary between the FM-M and the CE-CO-OO-I phases.Disorder has a remarkable effect on the bicriticality. Even moderate 'alloy' disorder, due to random location of A and A' ions at the rare earth site, converts the CO-OO-CE phase to a short range correlated glass, but has only limited impact on the ferromagnet [2,3,4]. The asymmetric suppression of spatial order by cation disorder and the emergence of a charge-orbital-spin glass at low r A are one set of intriguing issues in these materials. Unusually, while alloy type randomness on the A site leads to a homogeneous glassy phase, the substitution of a few percent of Mn (the 'B site') by Cr [5,6] leads to phase separation of the system [7,8,9, 10] into FM-M and AF-CO-OO-I domains. The difference between A and B site disorder holds the key to the much discussed phase coexistence and spatial inhomogeneity in the manganites.In this paper we provide the first results on the relative effects of A and B type substitutional disorder on phase competetion in a manganite model. We study weak 'alloy' disorder and dilute strongly repulsive scatterers. Our main results are: (i) Alloy disorder indeed leads to asymmetric suppression of long range order; moderate disorder converts long range CE-CO-OO to an insulating glass with nanoscale inhomogeneities, while FM order is only weakened. (ii) A low density, > ∼ 4%, of strong scatterers in the CE phase leads to cluster coexistence of AF-CO-OO and FM regions and the ground state is a poor metal.(iii) The impact of strong scatterers depends crucially on whether...

Using density functional theory with generalized gradient approximation, we have performed a systematic study of the structure and properties of neutral and charged trioxides (MO(3)) and tetraoxides (MO(4)) of the 3d-metal atoms. The results of our calculations revealed a number of interesting features when moving along the 3d-metal series. (1) Geometrical configurations of the lowest total energy states of neutral and charged trioxides and tetraoxides are composed of oxo and∕or peroxo groups, except for CuO(3)(-) and ZnO(3)(-) which possess a superoxo group, CuO(4)(+) and ZnO(4)(+) which possess two superoxo groups, and CuO(3)(+), ZnO(3)(+), and ZnO(4)(-) which possess an ozonide group. While peroxo groups are found in the early and late transition metals, all oxygen atoms bind chemically to the metal atom in the middle of the series. (2) Attachment or detachment of an electron to∕from an oxide often leads to a change in the geometry. In some cases, two dissociatively attached oxygen atoms combine and form a peroxo group or a peroxo group transforms into a superoxo group and vice versa. (3) The adiabatic electron affinity of as many as two trioxides (VO(3) and CoO(3)) and four tetraoxides (TiO(4), CrO(4), MnO(4), and FeO(4)) are larger than the electron affinity of halogen atoms. All these oxides are hence superhalogens although only VO(3) and MnO(4) satisfy the general superhalogen formula.

Recent work has shown that BO(2) which is a superhalogen with an electron affinity of 4.46 eV, can be used as building block of a new class of molecules/clusters whose electron affinities can exceed that of BO(2). This class of molecules was named hyperhalogens and the concept was illustrated by focusing on Au(BO(2))(2). Here we explore other superhalogens besides BO(2) to see if they too can be used to form hyperhalogens. We have chosen to focus on AlO(2) which is valence isoelectronic with BO(2) as well as VO(3) which involves a transition metal atom. The results obtained using density functional theory show unexpected behavior: Although AlO(2) and VO(3) are both superhalogens such as BO(2), only Na(BO(2))(2) is a hyperhalogen while Na(AlO(2))(2) and Na(VO(3))(2) are not. The origin of this anomalous result is traced to the large binding energy of the dimers of AlO(2) and VO(3).

A systematic density functional theory based study of the structure and spectroscopic properties of neutral and negatively charged MX(n) clusters formed by a transition metal atom M (M=Sc,Ti,V) and up to seven halogen atoms X (X=F,Cl,Br) has revealed a number of interesting features: (1) Halogen atoms are bound chemically to Sc, Ti, and V for n≤n(max), where the maximal valence n(max) equals to 3, 4, and 5 for Sc, Ti, and V, respectively. For n>n(max), two halogen atoms became dimerized in the neutral species, while dimerization begins at n=5, 6, and 7 for negatively charged clusters containing Sc, Ti, and V. (2) Magnetic moments of the transition metal atoms depend strongly on the number of halogen atoms in a cluster and the cluster charge. (3) The number of halogen atoms that can be attached to a metal atom exceeds the maximal formal valence of the metal atom. (4) The electron affinities of the neutral clusters abruptly rise at n=n(max), reaching values as high as 7 eV. The corresponding anions could be used in the synthesis of new salts, once appropriate counterions are identified.